Context. A correlation has been reported between the arrival directions of high-energy IceCube events and γ-ray blazars classified as intermediate- and high-synchrotron-peaked BL Lacs. Subsequent studies have investigated the optical properties of these sources, compiled and analyzed public multiwavelength data, and constrained their individual neutrino emission based on public IceCube point-source data. Aims. We provide a theoretical interpretation of public multiwavelength and neutrino point source data for the 32 BL Lac objects in the sample previously associated with an IceCube alert event. We combined the individual source results to draw conclusions regarding the multimesssenger properties of the sample and the required power in relativistic protons. Methods. We performed particle interaction modeling using open-source numerical simulation software. We constrained the model parameters using a novel and unique approach that simultaneously describes the host galaxy contribution, the observed synchrotron peak properties, the average multiwavelength fluxes, and, where possible, the IceCube point source constraints. Results. We show that a single-zone leptohadronic model can describe the multiwavelength broadband fluxes from all 32 IceCube candidates. In some cases, the model suggests that hadronic emission may contribute a considerable fraction of the γ-ray flux. The required power in relativistic protons ranges from a few percent to a factor of ten of the Eddington luminosity, which is energetically less demanding compared to other leptohadronic blazar models in recent literature. The model can describe the 68% confidence level IceCube flux for a large fraction of the masquerading BL Lacs in the sample, including TXS 0506+056; whereas, for true BL Lacs, the model predicts a low neutrino flux in the IceCube sensitivity range. Physically, this distinction is due to the presence of photons from broad line emission in masquerading BL Lacs, which increase the efficiency of hadronic interactions. The predicted neutrino flux peaks between a few petaelectronvolt and 100 PeV and scales positively with the flux in the gigaelectronvolt, megaelectronvolt, X-ray, and optical bands. Based on these results, we provide a list of the brightest neutrino emitters, which can be used for future searches targeting the 10–100 PeV regime.

A fraction of the active supermassive black holes at the centers of galaxies in our Universe are capable of launching extreme kiloparsec-long relativistic jets. These jets are known multiband (radio to γ-ray) and multimessenger (neutrino) emitters, and some of them have been monitored over decades at all accessible wavelengths. However, several open questions remain unanswered about the processes powering these highly energetic phenomena. These jets intrinsically produce soft-to-hard X-ray emission that extends from E>0.1keV up to E>100keV, and simultaneous broadband X-ray coverage, combined with excellent timing and imaging capabilities, is required to uncover the physics of jets. Indeed, truly simultaneous soft-to-hard X-ray coverage, in synergy with current and upcoming high-energy facilities (such as IXPE, COSI, CTAO, etc.) and neutrino detectors (e.g., IceCube), would enable us to disentangle the particle population responsible for the high-energy radiation from these jets. A sensitive hard X-ray survey (F20−80keV<10−15ergcm−2s−1) could unveil the bulk of their population in the early Universe. Acceleration and radiative processes responsible for the majority of their X-ray emission would be pinned down by microsecond timing capabilities at both soft and hard X-rays. Furthermore, imaging jet structures for the first time in the hard X-ray regime could unravel the origin of their high-energy emission. The proposed Probe-class mission concept High Energy X-ray Probe (HEX-P) combines all these required capabilities, making it the crucial next-generation X-ray telescope in the multi-messenger, time-domain era. HEX-P will be the ideal mission to unravel the science behind the most powerful accelerators in the Universe.

The "twin birth" of a positron and an electron by a photon in the presence of a nucleus, known as Bethe-Heitler pair production, is a key process in astroparticle physics. The Bethe-Heitler process offers a way of channeling energy stored in a population of relativistic protons (or nuclei) to relativistic pairs with extended distributions. Contrary to accelerated leptons, whose maximum energy is limited by radiative losses, the maximal energy of pairs is determined by the kinematics of the process and can be as high as the parent proton energy. We take a closer look at the features of the injected pair distribution, and provide a novel empirical function that describes the spectrum of pairs produced by interactions of single-energy protons with single-energy photons. The function is the kernel of the Bethe-Heitler pair production spectrum that replaces a double numerical integration involving the complex differential cross section of the process, and can be easily implemented in numerical codes. We further examine under which conditions Bethe-Heitler pairs produced in blazar jets can emit γ-ray photons via synchrotron radiation, thus providing an alternative to the inverse Compton scattering process for high-energy emission in jetted active galactic nuclei. For this purpose, we create 36 numerical models using the code ATHEνA optimized so that the Bethe-Heitler synchrotron emission dominates their γ-ray emission. After taking into consideration the broadband spectral characteristics of the source, the jet energetics, and the properties of radiation fields present in the blazar environment, we conclude that γ-rays in high-synchrotron-peaked blazars are unlikely to be produced by Bethe-Heitler pairs, because the emitting region is found to be opaque in photon-photon pair production at photon energies ≳ 10 GeV. On the contrary, γ-ray spectra of low-synchrotron-peaked blazars may arise from Bethe-Heitler pairs in regions of the jet with typical transverse size ∼ 1015 – 1016 cm and co-moving magnetic field 50 – 500 G. For such cases, an external thermal target photon field with temperatures T ∼ 4 · 102– 6 · 103 K is needed. The latter values could point to the dusty torus of the AGN. Interestingly, a Bethe-Heitler-dominated high-energy component is mostly found in models of intermediate-synchrotron peaked blazars, for a wide range of magnetic fields and source radii.

Context. Very high-energy (VHE, E > 100 GeV) observations of the blazar Mrk 501 with MAGIC in 2014 provided evidence for an unusual narrow spectral feature at about 3 TeV during an extreme X-ray flaring activity. The one-zone synchrotron-self Compton scenario, widely used in blazar broadband spectral modeling, fails to explain the narrow TeV component. Aims: Motivated by this rare observation, we propose an alternative model for the production of narrow features in the VHE spectra of flaring blazars. These spectral features may result from the decay of neutral pions (π0 bumps) that are in turn produced via interactions of protons (of tens of TeV energy) with energetic photons, whose density increases during hard X-ray flares. Methods: We explored the conditions needed for the emergence of narrow π0 bumps in VHE blazar spectra during X-ray flares reaching synchrotron energies ∼100 keV using time-dependent radiative transfer calculations. We focused on high-synchrotron peaked (HSP) blazars, which comprise the majority of VHE-detected extragalactic sources. Results: We find that synchrotron-dominated flares with peak energies ≳100 keV can be ideal periods for the search of π0 bumps in the VHE spectra of HSP blazars. The flaring region is optically thin to photopion production, its energy content is dominated by the relativistic proton population, and the inferred jet power is highly super-Eddington. Application of the model to the spectral energy distribution of Mrk 501 on MJD 56857.98 shows that the VHE spectrum of the flare is described well by the sum of a synchrotron self-Compton (SSC) component and a distinct π0 bump centered at 3 TeV. Spectral fitting of simulated SSC+π0 spectra for the Cherenkov Telescope Array (CTA) show that a π0 bump could be detected at a 5σ significance level with a 30-min exposure. Conclusions: A harder VHE γ-ray spectrum than the usual SSC prediction or, more occasionally, a distinct narrow bump at VHE energies during hard X-ray flares, can be suggestive of a relativistic hadronic component in blazar jets that otherwise would remain hidden. The production of narrow features or spectral hardenings due to π0 decay in the VHE spectra of blazars is testable with the advent of CTA.

Context. Jets from supermassive black holes at the centers of active galaxies are the most powerful and persistent sources of electromagnetic radiation in the Universe. To infer the physical conditions in the otherwise out-of-reach regions of extragalactic jets, we usually rely on fitting their spectral energy distributions (SEDs). The calculation of radiative models for the jet's non-thermal emission usually relies on numerical solvers of coupled partial differential equations. Aims: In this work, we use machine learning to tackle the problem of high computational complexity to significantly reduce the SED model evaluation time, which is necessary for SED fittings carried out with Bayesian inference methods. Methods: We computed the SEDs based on the synchrotron self-Compton model for blazar emission using the radiation code ATHEvA. We used them to train neural networks (NNs) to explore whether they can replace the original code, which is computationally expensive. Results: We find that a NN with gated recurrent unit neurons (GRUN) can effectively replace the ATHEvA leptonic code for this application, while it can be efficiently coupled with Markov chain Monte Carlo (MCMC) and nested sampling algorithms for fitting purposes. We demonstrate this approach through an application to simulated data sets, as well as a subsequent application to observational data. Conclusions: We present a proof-of-concept application of NNs to blazar science as the first step in a list of future applications involving hadronic processes and even larger parameter spaces. We offer this tool to the community through a public repository. The results of our work are available in GitHub; https://github.com/tzavellas/blazar_ml. This includes: (a) the NN and accompanied code produced to train them, (b) code for visualization of results in python and jupyter notebooks with instructions, and (c) part of the ATHEvA datasets that can be used for evaluation and plotting examples.

Context. Recent associations of high-energy neutrinos with active galactic nuclei (AGN) have revived the interest in leptohadronic models of radiation from astrophysical sources. The rapid increase in the amount of acquired multi-messenger data will require fast numerical models that may be applied to large source samples. Aims: We develop a time-dependent leptohadronic code, LeHaMoC, that offers several notable benefits compared to other existing codes, such as versatility and speed. Methods: LeHaMoC solves the Fokker-Planck equations of photons and relativistic particles (i.e. electrons, positrons, protons, and neutrinos) produced in a homogeneous magnetized source that may also be expanding. The code utilizes a fully implicit difference scheme that allows fast computation of steady-state and dynamically evolving physical problems. Results: We first present test cases where we compare the numerical results obtained with LeHaMoC against exact analytical solutions and numerical results computed with ATHEvA, a well-tested code of similar philosophy but a different numerical implementation. We find a good agreement (within 10-30%) with the numerical results obtained with ATHEvA without evidence of systematic differences. We then demonstrate the capabilities of the code through illustrative examples. First, we fit the spectral energy distribution from a jetted AGN in the context of a synchrotron-self Compton model and a proton-synchrotron model using Bayesian inference. Second, we compute the high-energy neutrino signal and the electromagnetic cascade induced by hadronic interactions in the corona of NGC 1068. Conclusions: LeHaMoC is easily customized to model a variety of high-energy astrophysical sources and has the potential to become a widely utilized tool in multi-messenger astrophysics. Instructions for downloading the code, accessing online documentation, and reproducing applications presented in this paper can be found at https://github.com/mariapetro/LeHaMoC Github repository.

The recent discovery of astrophysical neutrinos from the Seyfert galaxy NGC 1068 suggests the presence of nonthermal protons within a compact "coronal" region close to the central black hole. The acceleration mechanism of these nonthermal protons remains elusive. We show that a large-scale magnetic reconnection layer, of the order of a few gravitational radii, may provide such a mechanism. In such a scenario, rough energy equipartition between magnetic fields, X-ray photons, and nonthermal protons is established in the reconnection region. Motivated by recent 3D particle-in-cell simulations of relativistic reconnection, we assume that the spectrum of accelerated protons is a broken power law, with the break energy being constrained by energy conservation (i.e., the energy density of accelerated protons is at most comparable to the magnetic energy density). The proton spectrum is ${{dn}}_{p}/{{dE}}_{p}\propto {E}_{p}^{-1}$ below the break and ${{dn}}_{p}/{{dE}}_{p}\propto {E}_{p}^{-s}$ above the break, with IceCube neutrino observations suggesting s ≃ 3. Protons above the break lose most of their energy within the reconnection layer via photohadronic collisions with the coronal X-rays, producing a neutrino signal in good agreement with the recent observations. Gamma rays injected in photohadronic collisions are cascaded to lower energies, sustaining the population of electron-positron pairs that makes the corona moderately Compton thick.

Gamma-ray flares of blazars may be accompanied by high-energy neutrinos due to interactions of high-energy cosmic rays in the jet with photons, as suggested by the detection of the high-energy neutrino IceCube-170922A during a major gamma-ray flare from blazar TXS 0506+056 at the ~3σ significance level. In this work, we present a statistical study of gamma-ray emission from blazars to constrain the contribution of gamma-ray flares to their neutrino output. We construct weekly binned light curves for 145 gamma-ray bright blazars in the Fermi Large Area Telescope Monitored Source List adding TXS 0506+056. We derive the fraction of time spent in the flaring state (flare duty cycle) and the fraction of energy released during each flare from the light curves with a Bayesian blocks algorithm. We find that blazars with lower flare duty cycles and energy fractions are more numerous among our sample. We identify a significant difference in flare duty cycles between blazar subclasses at a significance level of 5%. Then using a general scaling relation for the neutrino and gamma-ray luminosities, ${L}_{\nu }\propto {({L}_{\gamma })}^{\gamma }$ with a weighting exponent of γ = 1.0-2.0, normalized to the quiescent gamma-ray or X-ray flux of each blazar, we evaluate the neutrino energy flux of each gamma-ray flare. The gamma-ray flare distribution indicates that blazar neutrino emission may be dominated by flares for γ ≳ 1.5. The neutrino energy fluxes for 1 week and 10 yr bins are compared with the decl.-dependent IceCube sensitivity to constrain the standard neutrino emission models for gamma-ray flares. Finally, we present the upper-limit contribution of blazar gamma-ray flares to the isotropic diffuse neutrino flux.

For over a century, the identification of high-energy cosmic ray (CR) sources remains an open question. For Galactic CRs with energy up to 1015 eV, supernova remnants (SNRs) have traditionally been thought the main candidate source. However, recent TeV γ-ray observations have questioned the SNR paradigm. Propagating CRs are deflected by the Galactic magnetic field, hence, γ-rays and neutrinos produced via inelastic hadronic interactions are the only means for unveiling the CR sources. In this work, we study the γ-ray and neutrino emission produced by CRs accelerated inside Galactic jets of stellar-mass black holes in X-ray binaries (BHXBs). We calculate the intrinsic neutrino emission of two prototypical BHXBs , Cygnus X-1 and GX 339-4, for which we have high-quality, quasi-simultaneous multiwavelength spectra. Based on these prototypical sources, we discuss the likelihood of the 35 known Galactic BHXBs to be efficient CR accelerators. Moreover, we estimate the potential contribution to the CR spectrum of a viable population of BHXBs that reside in the Galactic plane. When these BHXBs go into outburst, they may accelerate particles up to hundreds of TeV that contribute to the diffuse γ-ray and neutrino spectra while propagating in the Galactic medium. Using HERMES, an open-source code that calculates the hadronic processes along the line of sight, we discuss the contribution of BHXBs to the diffuse γ-ray and neutrino fluxes, and compare these to their intrinsic γ-ray and neutrino emissions. Finally, we discuss the contribution of BHXBs to the observed spectrum of Galactic CRs.

Relativistic hadronic plasmas can become, under certain conditions, supercritical, abruptly and efficiently releasing the energy stored in protons through photon outbursts. Past studies have tried to relate the features of such hadronic supercriticalities (HSCs) to the phenomenology of gamma-ray burst (GRB) prompt emission. In this work we investigate, for the first time, HSC in adiabatically expanding sources. We examine the conditions required to trigger HSC, study the role of expansion velocity, and discuss our results in relation to GRB prompt emission. We find multipulse light curves from slowly expanding regions (≲ 0.01c) that are a manifestation of the natural HSC quasi-periodicity, while single-pulse light curves with a fast rise and slow decay are found for higher velocities. The formation of the photon spectrum is governed by an in-source electromagnetic cascade. The peak photon energy is approximately $1 \cdot \frac{\Gamma }{100} \frac{1+z}{3}$ MeV for maximum proton energies $(1-10) \cdot \frac{\Gamma }{100} \frac{1+z}{3}$ PeV, while the peak γ-ray luminosities are in the range $(10^{49}-10^{52}) \cdot (\frac{\Gamma }{100})^4$ erg s-1. HSC bursts peaking in the MeV energy band are also copious neutrino emitters with peak energies $\sim 10 \cdot \frac{\Gamma }{100} \frac{1+z}{3}$ TeV and an all-flavour neutrino fluence $\sim 10~{{\ \rm per\ cent}}$ of the γ-ray one. The hypothesis that long-duration GRBs are powered by HSCs could be applied therefore only to the most luminous GRBs observed assuming bulk Lorentz factors Γ ≤ 100.

For a subpopulation of energetic gamma-ray bursts (GRBs), a moderate baryonic loading may suffice to power ultra-high-energy cosmic rays (UHECRs). Motivated by this, we study the radiative signatures of cosmic-ray protons in the prompt phase of energetic GRBs. Our framework is the internal shock model with multicollision descriptions of the relativistic ejecta (with different emission regions along the jet), plus time-dependent calculations of photon and neutrino spectra. Our GRB prototypes are motivated by Fermi-Large Area Telescope-detected GRBs (including GRB 221009A) for which further, owing to the large energy flux, neutrino nonobservation of single events may pose a strong limit on the baryonic loading. We study the feedback of protons on electromagnetic spectra in synchrotron- and inverse Compton-dominated scenarios to identify the multiwavelength signatures, to constrain the maximally allowed baryonic loading, and to point out the differences between hadronic and inverse Compton signatures. We find that hadronic signatures appear as correlated flux increases in the optical-UV to soft X-ray and GeV-TeV gamma-ray ranges in the synchrotron scenarios, whereas they are difficult to identify in inverse Compton-dominated scenarios. We demonstrate that baryonic loadings around 10, which satisfy the UHECR energetic requirements, do not distort the predicted photon spectra in the Fermi Gamma-Ray Burst Monitor range and are consistent with constraints from neutrino data if the collision radii are large enough (i.e., the time variability is not too short). It therefore seems plausible that under the condition of large dissipation radii a population of energetic GRBs can be the origin of the UHECRs.

We present the first results for the dust-scattering rings of GRB 221009A, coined as the gamma-ray bursts (GRBs) of the century, as observed by the Neil Gehrels Swift Observatory. We perform analysis of both time resolved observations and stacked data. The former approach enable us to study the expansion of the most prominent rings, associate their origin with the prompt X-ray emission of the GRB and determine the location of the dust layers. The stacked radial profiles increase the signal-to-noise ratio of the data and allows detection of fainter and overlapping peaks in the angular profile. We find a total of 16 dust concentrations (with hints of even more) that span about 15kpc in depth and could be responsible for the highly structured X-ray angular profiles. By comparing the relative scattered fluxes of the five most prominent rings we show that the layer with the largest amount of dust is located at about 0.44 kpc away from us. We finally compare the location of the dust layers with results from experiments that study the 3D structure of our Galaxy via extinction or CO radio observations, and highlight the complementarity of dust X-ray tomography to these approaches.

In this study, we present a method to estimate posterior distributions for standard accretion torque model parameters and binary orbital parameters for X-ray binaries using a nested sampling algorithm for Bayesian parameter estimation. We study the spin evolution of two Be X-ray binary systems in the Magellanic Clouds, RX J0520.5-6932 and RX J0209-7427, during major outbursts, in which they surpassed the Eddington limit. Moreover, we apply our method to the recently discovered Swift J0243.6+6124, the only known Galactic pulsating ultra-luminous X-ray source. This is an excellent candidate for studying the disc evolution at super-Eddington accretion rates, because its luminosity spans several orders of magnitude during its outburst, with a maximum LX that exceeded the Eddington limit by a factor of ~10. Our method, when applied to RX J0520.5-6932 and RX J0209-7427, is able to identify the more favourable torque model for each system, while yielding meaningful ranges for the NS and orbital parameters. Our analysis for Swift J0243.6+6124 illustrates that, contrary to the standard torque model predictions, the magnetospheric radius (Rm) and the Alfvén radius (RA) are not proportional to each other when surpassing the Eddington limit. Reported distance estimates of this source range between 5 and 7 kpc. Smaller distances require non-typical neutron star properties (i.e. mass and radius) and possibly lower radiative efficiency of the accretion column.

We present a multi-messenger model for the prompt emission from GRB 221009A within the internal shock scenario. We consider the time-dependent evolution of the outflow with its impact on the observed light curve from multiple collisions, as well as the self-consistent generation of the electromagnetic spectrum in synchrotron and inverse Compton-dominated scenarios. Our lepto-hadronic model includes UHE protons potentially accelerated in the outflow, and their feedback on spectral energy distribution and on the neutrino emission. We find that we can roughly reproduce the observed light curves with an engine with varying ejection velocity of ultrarelativistic material, which has an intermediate quiescent period of about 200 s and a variability timescale of ~1 s. We consider baryonic loadings of 3 and 30 that are compatible with the hypothesis that the highest-energetic LHAASO photons might come from UHECR interactions with the extragalactic background light, and the paradigm that energetic GRBs may power the UHECR flux. For these values and the high dissipation radii considered, we find consistency with the nonobservation of neutrinos and no significant signatures on the electromagnetic spectrum. Inverse Compton-dominated scenarios from the prompt emission are demonstrated to lead to about an order of magnitude higher fluxes in the HE range; this enhancement is testable via its spectral impact in the Fermi-GBM and LAT ranges.

The blazar PKS 0735+178 is possibly associated with multiple neutrino events observed by the IceCube, Baikal, Baksan, and KM3NeT neutrino telescopes while it was flaring in the γ-ray, X-ray, ultraviolet, and optical bands. We present a detailed study of this peculiar blazar to investigate the temporal and spectral changes in the multiwavelength emission when the neutrino events were observed. The analysis of Swift-XRT snapshots reveal a flux variability of more than a factor 2 in about 5 × 103 s during the observation on 2021 December 17. In the γ-ray band, the source was in its historical highest flux level at the time of the arrival of the neutrinos. The observational comparison between PKS 0735+178 and other neutrino source candidates, such as TXS 0506+056, PKS 1424+240, and GB6 J1542+6129, shows that all these sources share similar spectral energy distributions, very high radio and γ-ray powers, and parsec scale jet properties. Moreover, we present strong supporting evidence for PKS 0735+178 to be, like all the others, a masquerading BL Lac. We perform comprehensive modelling of the multiwavelength emission from PKS 0735+178 within one-zone lepto-hadronic models considering both internal and external photon fields and estimate the expected accompanying neutrino flux. The most optimistic scenario invokes a jet with luminosity close to the Eddington value and the interactions of ~ PeV protons with an external UV photon field. This scenario predicts ~0.067 muon and anti-muon neutrinos over the observed 3-week flare. Our results are consistent with the detection of one very high-energy neutrino like IceCube-211208A.

Blazars are a rare class of active galactic nuclei (AGNs) with relativistic jets pointing towards the observer. Jets are thought to be launched as Poynting-flux dominated outflows that accelerate to relativistic speeds at the expense of the available magnetic energy. In this work, we consider electron-proton jets and assume that particles are energized via magnetic reconnection in parts of the jet where the magnetization is still high (σ ≥ 1). The magnetization and bulk Lorentz factor Γ are related to the available jet energy per baryon as μ = Γ(1 + σ). We adopt an observationally motivated relation between Γ and the mass accretion rate into the black hole $\dot{m}$, which also controls the luminosity of external radiation fields. We numerically compute the photon and neutrino jet emission as a function of μ and σ. We find that the blazar SED is produced by synchrotron and inverse Compton radiation of accelerated electrons, while the emission of hadronic-related processes is subdominant except for the highest magnetization considered. We show that low-luminosity blazars (Lγ ≲ 1045 erg s-1) are associated with less powerful, slower jets with higher magnetizations in the jet dissipation region. Their broad-band photon spectra resemble those of BL Lac objects, and the expected neutrino luminosity is $L_{\nu +\bar{\nu }}\sim (0.3-1)\, L_{\gamma }$. High-luminosity blazars (Lγ ≫ 1045 erg s-1) are associated with more powerful, faster jets with lower magnetizations. Their broad-band photon spectra resemble those of flat spectrum radio quasars, and they are expected to be dim neutrino sources with $L_{\nu +\bar{\nu }}\ll L_{\gamma }$.

Centaurus A is one of the closest radio galaxies to Earth. Its proximity allowed us to extensively study its active galactic nucleus but the core emission mechanism remains elusive because of local strong dust and gas obscuration. The capability of polarimetry to shave-off contaminating emission has been exploited without success in the near-infrared by previous studies but the very recent measurement of the 2-8 keV polarization by the Imaging X-ray Polarimetry Explorer (IXPE) brought the question back to the fore. To determine what is the prevalent photon generation mechanism to the multiwavelength emission from the core of Centaurus A, we retrieved from the archives the panchromatic polarization measurements of the central compact component. We built the total and polarized flux spectral energy distributions of the core and demonstrated that synchrotron self-Compton models nicely fit the polarized flux from the radio to the X-ray band. The linear polarization of the synchrotron continuum is perpendicular to the jet radio axis from the optical to the radio band, and parallel to it at higher energies. The observed smooth rotation of the polarization angle in the ultraviolet band is attributed to synchrotron emission from regions that are getting closer to the particle acceleration site, where the orientation of the jet's magnetic fields become perpendicular to the jet axis. This phenomenon support the shock acceleration mechanism for particle acceleration in Centaurus A, in line with IXPE observations of several high-synchrotron peak blazars.

Magnetic reconnection is often invoked as a source of high-energy particles, and in relativistic astrophysical systems it is regarded as a prime candidate for powering fast and bright flares. We present a novel analytical model-supported and benchmarked with large-scale three-dimensional kinetic particle-in-cell simulations in electron-positron plasmas-that elucidates the physics governing the generation of power-law energy spectra in relativistic reconnection. Particles with Lorentz factor γ ≳ 3σ (here, σ is the magnetization) gain most of their energy in the inflow region, while meandering between the two sides of the reconnection layer. Their acceleration time is ${t}_{\mathrm{acc}}\sim \gamma \,{\eta }_{\mathrm{rec}}^{-1}{\omega }_{{\rm{c}}}^{-1}\simeq 20\,\gamma \,{\omega }_{{\rm{c}}}^{-1}$ , where η rec ≃ 0.06 is the inflow speed in units of the speed of light and ω c = eB 0/mc is the gyrofrequency in the upstream magnetic field. They leave the region of active energization after t esc, when they get captured by one of the outflowing flux ropes of reconnected plasma. We directly measure t esc in our simulations and find that t esc ~ t acc for σ ≳ few. This leads to a universal (i.e., σ-independent) power-law spectrum ${{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{-1}$ for the particles undergoing active acceleration, and ${dN}/d\gamma \propto {\gamma }^{-2}$ for the overall particle population. Our results help to shed light on the ubiquitous presence of power-law particle and photon spectra in astrophysical nonthermal sources.

The origin of extremely fast variability is one of the long-standing questions in the gamma-ray astronomy of blazars. While many models explain the slower, lower energy variability, they cannot easily account for such fast flares reaching hour-to-minute timescales. Magnetic reconnection, a process where magnetic energy is converted to the acceleration of relativistic particles in the reconnection layer, is a candidate solution to this problem. In this work, we employ state-of-the-art particle-in-cell simulations in a statistical comparison with observations of a flaring episode of a well-known blazar, Mrk 421, at a very high energy (VHE, E > 100 GeV). We tested the predictions of our model by generating simulated VHE light curves that we compared quantitatively with methods that we have developed for a precise evaluation of theoretical and observed data. With our analysis, we can constrain the parameter space of the model, such as the magnetic field strength of the unreconnected plasma, viewing angle and the reconnection layer orientation in the blazar jet. Our analysis favours parameter spaces with magnetic field strength 0.1 G, rather large viewing angles (6 − 8°), and misaligned layer angles, offering a strong candidate explanation for the Doppler crisis often observed in the jets of high synchrotron peaking blazars. Full Tables B.1-B.10 are available at the CDS via anonymous ftp to cdsarc.cds.unistra.fr (ftp://130.79.128.5) or via https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/678/A140

Context. Black-hole X-ray binaries (BHXRBs) in the hard and hard-intermediate spectral (and temporal) states exhibit in their power spectra characteristic frequencies called type-C quasi-periodic oscillations (QPOs). Various models that can explain them with various degrees of success have been proposed, but a definitive answer is still missing. Aims: The hot Comptonizing corona interacting with the cold accretion disk, both of which are central in understanding BHXRBs, is essentially a dynamical system. Our aim is to investigate if the radiative coupling between the two components can produce QPOs. Methods: We write and solve the time-dependent equations that describe energy conservation in the system corona - accretion disk. We examine both constant and variable mass accretion rates. By necessity, in this first investigation we use a simple model, but it contains all the essential ingredients. Results: For a constant mass accretion rate and certain justifiable conditions, the dynamic corona - disk system exhibits oscillations, which die out after a few cycles. The characteristic frequencies of these oscillations are similar to the ones observed in the power spectra of BHXRBs. For most parameters, the natural frequencies persist even in the case of variable accretion rates. Conclusions: We argue that type-C QPOs in BHXRBs could, in principle, arise from the interaction of the hot Comptonizing corona with the much colder accretion disk. If this picture is correct, it has immediate implications for other systems that contain the above constituents, such as active galactic nuclei.

Blazar flares have been suggested as ideal candidates for enhanced neutrino production. While the neutrino signal of γ-ray flares has been widely discussed, the neutrino yield of X-ray flares has received less attention. Here, we compute the predicted neutrino signal from X-ray flares detected in 66 blazars observed more than 50 times with the X-ray Telescope (XRT) on board the Neil Gehrels Swift Observatory. We consider a scenario where X-ray flares are powered by synchrotron radiation of relativistic protons, and neutrinos are produced through photomeson interactions between protons with their own synchrotron X-ray photons. Using the 1 keV X-ray light curves for flare identification, the 0.5-10 keV fluence of each flare as a proxy for the all-flavour neutrino fluence, and the IceCube point-source effective area for different detector configurations, we calculate the number of muon and antimuon neutrinos above 100 TeV expected for IceCube from each flaring source. The bulk of the neutrino events from the sample originates from flares with durations ~1-10 d. Accounting for the X-ray flare duty cycle of the sources in the sample, which ranges between ~2 and 24 per cent, we compute an average yearly neutrino rate for each source. The median of the distribution (in logarithm) is ~0.03 yr-1, with Mkn 421 having the highest predicted rate 1.2 ± 0.3 yr-1, followed by 3C 273 (0.33 ± 0.03 yr-1) and PG 1553+113 (0.25 ± 0.02 yr-1). Next-generation neutrino detectors together with regular X-ray monitoring of blazars could constrain the duty cycle of hadronic X-ray flares.

Eight years after the first detection of high-energy astrophysical neutrinos by IceCube, we are still almost clueless as regards to their origin, although the case for blazars being neutrino sources is getting stronger. After the first significant association at the $3\!-\!3.5\, \sigma$ level in time and space with IceCube neutrinos, i.e. the blazar TXS 0506+056 at z = 0.3365, some of us have in fact selected a unique sample of 47 blazars, out of which ~16 could be associated with individual neutrino track events detected by IceCube. Building upon our recent spectroscopy work on these objects, here we characterize them to determine their real nature and check if they are different from the rest of the blazar population. For the first time we also present a systematic study of the frequency of masquerading BL Lacs, i.e. flat-spectrum radio quasars with their broad lines swamped by non-thermal jet emission, in a γ-ray- and IceCube-selected sample, finding a fraction >24 per cent and possibly as high as 80 per cent. In terms of their broad-band properties, our sources appear to be indistinguishable from the rest of the blazar population. We also discuss two theoretical scenarios for neutrino emission, one in which neutrinos are produced in interactions of protons with jet photons and one in which the target photons are from the broad-line region. Both scenarios can equally account for the neutrino-blazar correlation observed by some of us. Future observations with neutrino telescopes and X-ray satellites will test them out.

The broad high-energy spectral component in blazars is usually attributed to various inverse Compton scattering processes in the relativistic jet, but has not been clearly identified in most cases due to degeneracies in physical models. AP Librae, a low-synchrotron-peaking BL Lac object (LBL) detected in 2015 by H.E.S.S. at very high energies (VHE; >0.5 TeV), has an extremely broad high-energy spectrum, covering ~9 decades in energy. Standard synchrotron self-Compton models generally fail to reproduce the VHE emission, which has led to the suggestion that it might arise not from the blazar core, but on kiloparsec scales from inverse Compton (IC) scattering of cosmic microwave background (CMB) photons by a still-relativistic jet (IC/CMB). IC/CMB models for the TeV emission of AP Librae in prior works have implied a high level of infrared emission from the kiloparsec-scale jet. With newly obtained Hubble Space Telescope (HST) imaging, we obtain a deep upper limit on the kiloparsec-scale jet emission at 1.6 μm, well below the expected level. High-resolution Atacama Large Millimeter/submillimeter Array imaging in bands 3-9 reveals a residual dust-disk signature after core subtraction, with a clearly thermal spectrum, and an extent (~500 pc) that matches with a nonjet residual emission seen after point-spread function subtraction in our 1.6 μm HST imaging. We find that the unusually broad GeV and VHE emission in AP Librae can be reproduced through the combined IC scattering of photons from the CMB and the dust disk, respectively, by electrons in both the blazar core and subkiloparsec jet.

We present a toy model for radio emission in high-mass X-ray binaries (HMXBs) with strongly magnetized neutron stars (NSs) where a wind-collision region is formed by the NS outflow and the stellar wind of the massive companion. Radio emission is expected from the synchrotron radiation of shock-accelerated electrons and the free-free emission of the stellar wind. We found that the predicted relation between the GHz luminosity (LR) and the accretion X-ray luminosity (LX) can be written as $L_\mathrm{ R} \propto L_\mathrm{ X}^b$ for most parameters. No correlation with X-rays is expected (b = 0) when the thermal emission of the stellar wind dominates in radio. We typically find a steep correlation (b = 12/7) for sub-Eddington X-ray luminosities and a more shallow one [b = 2(p - 1)/7] for super-Eddington X-ray luminosities, where p is the power-law index of accelerated electrons. The maximum predicted radio luminosity is independent of the NS properties, while it depends on the stellar wind momentum, binary separation distance, and the minimum electron Lorentz factor. Using a Bayesian approach, we modelled the radio observations of Swift J0243.6+6124 that cover a wide range of mass accretion rates. Our results support a shock origin for the radio detections at sub-Eddington X-ray luminosities. However, no physically meaningful parameters could be found for the super-Eddington phase of the outburst, suggesting a different origin. Future observations with more sensitive instruments might reveal a large number of HMXBs with strongly magnetized NSs in radio, allowing the determination of the slope in the LR-LX relation, and putting the wind-collision scenario into test.

Blazars are a class of active galactic nuclei that host relativistic jets oriented close to the observer's line of sight. Blazars have very complex variability properties. Flares, namely flux variations around the mean value with a well-defined shape and duration, are one of the identifying properties of the blazar phenomenon. Blazars are known to exhibit multiwavelength flares, but also "orphan" flares, namely flux changes that appear only in a specific energy range. Various models, sometimes at odds with each other, have been proposed to explain specific flares even for a single source, and cannot be synthesized into a coherent picture. In this paper, we propose a unified model for explaining orphan and multiwavelength flares from blazars in a common framework. We assume that the blazar emission during a flare consists of two components: (i) a quasistable component that arises from the superposition of numerous but comparatively weak dissipation zones along the jet, forming the background (low-state) emission of the blazar, and (ii) a transient component, which is responsible for the sudden enhancement of the blazar flux, forming at a random distance along the jet by a strong energy dissipation event. Whether a multiwavelength or orphan flare is emitted depends on the distance from the base of the jet where the dissipation occurs. Generally speaking, if the dissipation occurs at a small/large distance from the supermassive black hole, the inverse Compton/synchrotron radiation dominates and an orphan γ -ray/optical flare tends to appear. On the other hand, we may expect a multiwavelength flare if the dissipation occurs at an intermediate distance. We show that the model can successfully describe the spectral energy distribution of different flares from the flat spectrum radio quasar 3C 279 and the BL Lacertae object PKS 2155-304.

The origin of Petaelectronvolt (PeV) astrophysical neutrinos is fundamental to our understanding of the high-energy Universe. Apart from the technical challenges of operating detectors deep below ice, oceans, and lakes, the phenomenological challenges are even greater than those of gravitational waves; the sources are unknown, hard to predict, and we lack clear signatures. Neutrino astronomy therefore represents the greatest challenge faced by the astronomy and physics communities thus far. The possible neutrino sources range from accretion disks and tidal disruption events, to relativistic jets and galaxy clusters with blazar TXS 0506+056 the most compelling association thus far. Since that association, immense effort has been put into proving or disproving that jets are indeed neutrino emitters, but to no avail. By generating simulated neutrino counterpart samples, we explore the potential of detecting a significant correlation of neutrinos with jets from active galactic nuclei. We find that, given the existing challenges, even our best experiments could not have produced a > 3σ result. Larger programs over the next few years will be able to detect a significant correlation only if the brightest radio sources, rather than all jetted active galactic nuclei, are neutrino emitters. We discuss the necessary strategies required to steer future efforts into successful experiments.

The exact mechanism for the production of fast γ-ray variability in blazars remains debated. Magnetic reconnection, in which plasmoids filled with relativistic particles and magnetic fields are formed, is a viable candidate to explain the broadband electromagnetic spectrum and variability of these objects. Using state-of-the-art magnetic reconnection simulations, we generate realistic γ-ray light curves that would be observed with the Fermi Large Area Telescope. A comparison with observed γ-ray flares from flat spectrum radio quasars (FSRQs) reveals that magnetic reconnection events lead to comparable flux levels and variability patterns, in particular, when the reconnection layer is slightly misaligned with the line of sight. Emission from fast plasmoids moving close to the line of sight could explain the fast variability on the timescales of minutes for which evidence has been found in observations of FSRQs. Our results motivate improvements in existing radiative transfer simulations as well as dedicated searches for fast variability as evidence for magnetic reconnection events.

Plasmoids—magnetized quasi-circular structures formed self-consistently in reconnecting current sheets—were previously considered to be the graveyards of energetic particles. In this paper, we demonstrate the important role of plasmoids in shaping the particle energy spectrum in relativistic reconnection (i.e., with upstream magnetization σup ≫ 1). Using 2D particle-in-cell simulations in pair plasmas with σup = 10 and 100, we study a secondary particle energization process that takes place inside compressing plasmoids. We demonstrate that plasmoids grow in time, while their interiors compress, amplifying the internal magnetic field. The magnetic field felt by particles injected in an isolated plasmoid increases linearly with time, which leads to particle energization as a result of magnetic moment conservation. For particles injected with a power-law distribution function, this energization process acts in such a way that the shape of the injected power law is conserved, while producing an additional nonthermal tail f(E) ∝ E-3 at higher energies, followed by an exponential cutoff. The cutoff energy, which increases with time as ${E}_{\mathrm{cut}}\propto \sqrt{t}$ , can greatly exceed σupmec2. We analytically predict the secondary acceleration timescale and the shape of the emerging particle energy spectrum, which can be of major importance in certain astrophysical systems, such as blazar jets.

The detection of a high-energy neutrino from the flaring blazar TXS 0506+056 and the subsequent discovery of a neutrino excess from the same direction have strengthened the hypothesis that blazars are cosmic neutrino sources. The lack, however, of γ-ray flaring activity during the latter period challenges the standard scenario of correlated γ-ray and high-energy neutrino emission in blazars. We propose instead that TeV-PeV neutrinos are produced in coincidence with X-ray flares that are powered by proton synchrotron radiation. In this case, neutrinos are produced by photomeson interactions of protons with their own synchrotron radiation, while MeV to GeV γ-rays are the result of synchrotron-dominated electromagnetic cascades developed in the source. Using a time-dependent approach, we find that this "pure hadronic flaring" hypothesis has several interesting consequences. The X-ray flux is a good proxy for the all-flavor neutrino flux, while certain neutrino-rich X-ray flares may be dark in GeV-TeV γ-rays. Lastly, hadronic X-ray flares are accompanied by an equally bright MeV component that is detectable by proposed missions like e-ASTROGAM and AMEGO. We applied this scenario to the extreme blazar 3HSP J095507.9+355101 which has been associated with IceCube-200107A while undergoing an X-ray flare. We showed that the number of muon and anti-muon neutrinos above 100 TeV during hadronic flares can be up to ∼3-10 times higher than the expected number in standard leptohadronic models. Still, frequent hadronic flaring activity is necessary for explaining the detected neutrino event IceCube-200107A.

On July 30th, 2019 IceCube detected a high-energy astrophysical muon neutrino candidate, IC-190730A with a 67% probability of astrophysical origin. The flat spectrum radio quasar (FSRQ) PKS 1502 +106 is in the error circle of the neutrino. Motivated by this observation, we study PKS 1502+106 as a possible source of IC-190730A. PKS 1502+106 was in a quiet state in terms of UV/optical/X-ray/γ-ray flux at the time of the neutrino alert, we therefore model the expected neutrino emission from the source during its average long-term state, and investigate whether the emission of IC-190730A as a result of the quiet long-term emission of PKS 1502+106 is plausible. We analyse UV/optical and X-ray data and collect additional observations from the literature to construct the multi-wavelength spectral energy distribution of PKS 1502+106. We perform leptohadronic modelling of the multi-wavelength emission of the source and determine the most plausible emission scenarios and the maximum expected accompanying neutrino flux. A model in which the multi-wavelength emission of PKS 1502+106 originates beyond the broad-line region and inside the dust torus is most consistent with the observations. In this scenario, PKS 1502+106 can have produced up to of order one muon neutrino with energy exceeding 100 TeV in the lifetime of IceCube. An appealing feature of this model is that the required proton luminosity is consistent with the average required proton luminosity if blazars power the observed ultra-high-energy-cosmic-ray flux and well below the source's Eddington luminosity. If such a model is ubiquitous among FSRQs, additional neutrinos can be expected from other bright sources with energy ≳ 10 PeV.

Decade-long monitoring of blazars at optical and infrared (OIR) wavelengths with the Small and Moderate Aperture Research Telescope System (SMARTS) in Chile and in γ-rays with the Fermi -Large Area Telescope (LAT) has enabled the systematic study of their multiwavelength long-term variability. In this work, we investigate, from a theoretical perspective, the long-term variability properties of blazar emission by introducing an observationally motivated time-dependence to four main parameters of the one-zone leptonic model: injection luminosity of relativistic electrons, strength of magnetic field, Doppler factor, and external photon field luminosity. For the first time, we use both the probability density function and the power spectral density of the 10-yr-long Fermi-LAT light curves to create variation patterns for the model parameters. Using as test beds two bright blazars from the SMARTS sample (PKS 2155-304 and 3C 273), we compute 10-yr-long OIR, X-ray, and γ-ray model light curves for different varying parameters. We compare the findings of our theoretical investigation with multiwavelength observations using various measures of variability. While no single-varying parameter simulation can explain all multiwavelength variability properties, changes in the electron luminosity and external radiation field in PKS 2155-304 and 3C 273, respectively, can account for most of them. Our results motivate future time-dependent studies with coupling between two or more physical parameters to describe the multiwavelength long-term blazar variability.

A small fraction of gamma-ray bursts (GRBs) with available data down to soft X-rays (~0.5 keV) has been shown to feature a spectral break in the low-energy part (~1-10 keV) of their prompt emission spectrum. The overall spectral shape is consistent with optically thin synchrotron emission from a population of particles that have cooled on a time-scale comparable to the dynamic time to energies that are still much higher than their rest-mass energy (marginally fast cooling regime). We consider a hadronic scenario and investigate if the prompt emission of these GRBs can originate from relativistic protons that radiate synchrotron in the marginally fast cooling regime. Using semi-analytical methods, we derive the source parameters, such as magnetic field strength and proton luminosity, and calculate the high-energy neutrino emission expected in this scenario. We also investigate how the emission of secondary pairs produced by photopion interactions and γγ pair production affect the broad-band photon spectrum. We support our findings with detailed numerical calculations. Strong modification of the photon spectrum below the break energy due to the synchrotron emission of secondary pairs is found, unless the bulk Lorentz factor is very large (Γ ≳ 103). Moreover, this scenario predicts unreasonably high Poynting luminosities because of the strong magnetic fields (106-107 G) that are necessary for the incomplete proton cooling. Our results strongly disfavour marginally fast cooling protons as an explanation of the low-energy spectral break in the prompt GRB spectra.

Long duration gamma-ray bursts (GRBs) are among the least understood astrophysical transients powering the high-energy universe. To date, various mechanisms have been proposed to explain the observed electromagnetic GRB emission. In this work, we show that, although different jet models may be equally successful in fitting the observed electromagnetic spectral energy distributions, the neutrino production strongly depends on the adopted emission and dissipation model. To this purpose, we compute the neutrino production for a benchmark high-luminosity GRB in the internal shock model, including a dissipative photosphere as well as three emission components, in the jet model invoking internal-collision-induced magnetic reconnection and turbulence (ICMART), in the case of a magnetic jet with gradual dissipation, and in a jet with dominant proton synchrotron radiation. We find that the expected neutrino fluence can vary up to three orders of magnitude in amplitude and peak at energies ranging from 104 to 108 GeV. For our benchmark input parameters, none of the explored GRB models is excluded by the targeted searches carried out by the IceCube and ANTARES Collaborations. However, our work highlights the potential of high-energy neutrinos of pinpointing the underlying GRB emission mechanism and the importance of relying on different jet models for unbiased stacking searches.

The IceCube report of a ∼ 3.5σ excess of 13 ± 5 neutrino events in the direction of the blazar TXS 0506+056 in 2014-2015 and the 2017 detection of a high-energy neutrino event, IceCube-170922A, during a gamma-ray flare from the same blazar, have revived the interest in scenarios for neutrino production in blazars. We perform comprehensive analyses on the long-term electromagnetic emission of TXS 0506+056 using optical, X-ray, and gamma-ray data from the All-Sky Automated Survey for Supernovae, the Neil Gehrels Swift Observatory, Monitor of All-sky X-ray Image, and the Fermi Large Area Telescope. We also perform numerical modeling of the spectral energy distributions (SEDs) in four epochs prior to 2017 with contemporaneous gamma-ray and lower-energy (optical and/or X-ray) data. We find that the multi-epoch SEDs are consistent with a hybrid leptonic scenario, where the gamma-rays are produced in the blazar zone via external inverse Compton scattering of accelerated electrons, and high-energy neutrinos are produced via the photomeson production process of co-accelerated protons. The multi-epoch SEDs can be satisfactorily explained with the same jet parameters and variable external photon density and electron luminosity. Using the maximal neutrino flux derived for each epoch, we put an upper limit of ∼0.4-2 on the muon neutrino number in 10 years of IceCube observations. Our results are consistent with the IceCube-170922A detection, which can be explained as an upper fluctuation from the average neutrino rate expected from the source, but in strong tension with the 2014-2015 neutrino flare.

Blazar emission models based on magnetic reconnection succeed in reproducing many observed spectral and temporal features, including the short-duration luminous flaring events. Plasmoids, a self-consistent by-product of the tearing instability in the reconnection layer, can be the main source of blazar emission. Kinetic simulations of relativistic reconnection have demonstrated that plasmoids are characterized by rough energy equipartition between their radiating particles and magnetic fields. This is the main reason behind the apparent shortcoming of plasmoid-dominated emission models to explain the observed Compton ratios of BL Lac objects. Here, we demonstrate that the radiative interactions among plasmoids, which have been neglected so far, can assist in alleviating this contradiction. We show that photons emitted by large, slow-moving plasmoids can be a potentially important source of soft photons to be then upscattered, via inverse Compton, by small fast-moving, neighbouring plasmoids. This interplasmoid Compton scattering process can naturally occur throughout the reconnection layer, imprinting itself as an increase in the observed Compton ratios from those short and luminous plasmoid-powered flares within BL Lac sources, while maintaining energy equipartition between radiating particles and magnetic fields.

The IceCube collaboration reported an ∼3.5σ excess of 13 ± 5 neutrino events in the direction of the blazar TXS 0506+056 during an ∼6 month period in 2014-2015, as well as the (∼3σ) detection of a high-energy muon neutrino during an electromagnetic flare in 2017. We explore the possibility that the 2014-2015 neutrino excess and the 2017 multimessenger flare are both explained in a common physical framework that relies on the emergence of a relativistic neutral beam in the blazar jet due to interactions of accelerated cosmic rays (CRs) with photons. We demonstrate that the neutral beam model provides an explanation for the 2014-2015 neutrino excess without violating X-ray and γ-ray constraints and yields results consistent with the detection of one high-energy neutrino during the 2017 flare. If both neutrino associations with TXS 05065+056 are real, our model requires that (I) the composition of accelerated CRs is light, with a ratio of helium nuclei to protons ≳5; (II) a luminous external photon field (∼1046 erg s-1) variable (on yearlong timescales) is present; and (III) the CR injection luminosity, as well as the properties of the dissipation region (I.e., Lorentz factor, magnetic field, and size), vary on yearlong timescales.

Tidal disruption events (TDE) have been considered as cosmic-ray and neutrino sources for a decade. We suggest two classes of new scenarios for high-energy multi-messenger emission from TDEs that do not have to harbor powerful jets. First, we investigate high-energy neutrino and gamma-ray production in the core region of a supermassive black hole. In particular, we show that ∼1-100 TeV neutrinos and MeV gamma rays can efficiently be produced in hot coronae around an accretion disk. We also study the consequences of particle acceleration in radiatively inefficient accretion flows (RIAFs). Second, we consider possible cosmic-ray acceleration by sub-relativistic disk-driven winds or interactions between tidal streams, and show that subsequent hadronuclear and photohadronic interactions inside the TDE debris lead to GeV-PeV neutrinos and sub-GeV cascade gamma rays. We demonstrate that these models should be accompanied by soft gamma rays or hard X-rays as well as optical/UV emission, which can be used for future observational tests. Although this work aims to present models of non-jetted high-energy emission, we discuss the implications of the TDE AT2019dsg that might coincide with the high-energy neutrino IceCube-191001A, by considering the corona, RIAF, hidden sub-relativistic wind, and hidden jet models. It is not yet possible to be conclusive about their physical association and the expected number of neutrinos is typically much less than unity. We find that the most optimistic cases of the corona and hidden wind models could be consistent with the observation of IceCube-191001A, whereas jet models are unlikely to explain the multi-messenger observations.

3HSP J095507.9+355101 is an extreme blazar that has been possibly associated with a high-energy neutrino (IceCube-200107A) detected 1 day before the blazar was found to undergo a hard X-ray flare. We perform a comprehensive study of the predicted multimessenger emission from 3HSP J095507.9+355101 during its recent X-ray flare, but also in the long term. We focus on one-zone leptohadronic models, but we also explore alternative scenarios: (i) a blazar-core model, which considers neutrino production in the inner jet, close to the supermassive black hole; (ii) a hidden external-photon model, which considers neutrino production in the jet through interactions with photons from a weak broad line region; (iii) a proton-synchrotron model, where high-energy protons in the jet produce γ-rays via synchrotron; and (iv) an intergalactic cascade scenario, where neutrinos are produced in the intergalactic medium by interactions of a high-energy cosmic-ray beam escaping the jet. The Poisson probability to detect a single muon neutrino in 10 years from 3HSP J095507.9+355101 with the real-time IceCube alert analysis is ∼1% (3%) for the most optimistic one-zone leptohadronic model (the multi-zone blazar-core model). Meanwhile, detection of a single neutrino during the 44-day-long high X-ray flux-state period following the neutrino detection is 0.06%, according to our most optimistic leptohadronic model. The most promising scenarios for neutrino production also predict strong intrasource γ-ray attenuation above ∼100 GeV. If the association is real, then IceCube-Gen2 and other future detectors should be able to provide additional evidence for neutrino production in 3HSP J095507.9+355101 and other extreme blazars.

The central engine in long gamma-ray bursts (GRBs) is thought to be a compact object produced by the core collapse of massive stars, but its exact nature (black hole or millisecond magnetar) is still debatable. Although the central engine of GRB collapsars is hidden to direct observation, its properties may be imprinted on the accompanying electromagnetic signals. We aim to decipher the generic properties of central engines that are consistent with prompt observations of long GRBs detected by the Burst Alert Telescope (BAT) on board the Neil Gehrels Swift Observatory. Adopting a generic model for the central engine, in which the engine power and activity time-scale are independent of each other, we perform Monte Carlo simulations of long GRBs produced by jets that successfully breakout from the star. Our simulations consider the dependence of the jet breakout time-scale on the engine luminosity and the effects of the detector's flux threshold. The two-dimensional (2D) distribution of simulated detectable bursts in the gamma-ray luminosity versus gamma-ray duration plane is consistent with the observed one for a range of parameter values describing the central engine. The intrinsic 2D distribution of simulated collapsar GRBs peaks at lower gamma-ray luminosities and longer durations than the observed one, a prediction that can be tested in the future with more sensitive detectors. Black hole accretors, whose power and activity time are set by the large-scale magnetic flux through the progenitor star and stellar structure, respectively, are compatible with the properties of the central engine inferred by our model.

The afterglow emission from gamma-ray bursts (GRBs) is believed to originate from a relativistic blast wave driven into the circumburst medium. Although the afterglow emission from radio up to X-ray frequencies is thought to originate from synchrotron radiation emitted by relativistic, non-thermal electrons accelerated by the blast wave, the origin of the emission at high energies (HE; ≳GeV) remains uncertain. The recent detection of sub-TeV emission from GRB 190114C by the Major Atmospheric Gamma Imaging Cherenkov Telescopes (MAGIC) raises further debate on what powers the very high energy (VHE; ≳300 GeV) emission. Here, we explore the inverse Compton scenario as a candidate for the HE and VHE emissions, considering two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton or SSC) and isotropic photon fields external to the blast wave (external Compton). For each case, we compute the multiwavelength afterglow spectra and light curves. We find that SSC will dominate particle cooling and the GeV emission, unless a dense ambient infrared photon field, typical of star-forming regions, is present. Additionally, considering the extragalactic background light attenuation, we discuss the detectability of VHE afterglows by existing and future gamma-ray instruments for a wide range of model parameters. Studying GRB 190114C, we find that its afterglow emission in the Fermi-Large Area Telescope (LAT) band is synchrotron dominated. The late-time Fermi-LAT measurement (I.e. t ∼ 104 s), and the MAGIC observation also set an upper limit on the energy density of a putative external infrared photon field (I.e. ${\lesssim} 3\times 10^{-9}\, {\rm erg\, cm^{-3}}$ ), making the inverse Compton dominant in the sub-TeV energies.

The joint detection of GW170817/GRB 170817 confirmed the long-standing theory that binary neutron star mergers produce short gamma-ray burst (sGRB) jets that can successfully break out of the surrounding ejecta. At the same time, the association with a kilonova provided unprecedented information regarding the physical properties (such as masses and velocities) of the different ejecta constituents. Combining this knowledge with the observed luminosities and durations of cosmological sGRBs detected by the Burst Alert Telescope onboard the Neil Gehrels Swift Observatory, we revisit the breakout conditions of sGRB jets. Assuming self-collimation of sGRB jets does not play a critical role, we find that the time interval between the binary merger and the launch of a typical sGRB jet is $\lesssim 0.1\,{\rm{s}}$ . We also show that for a fraction of at least $\sim 30 \% $ of sGRBs, the usually adopted assumption of static ejecta is inconsistent with observations, even if the polar ejecta mass is an order of magnitude smaller than that in GRB 170817. Our results disfavor magnetar central engines for powering cosmological sGRBs, limit the amount of energy deposited in the cocoon prior to breakout, and suggest that the observed delay of ∼1.7 s in GW170817/GRB 170817 between the gravitational wave and gamma-ray signals is likely dominated by the propagation time of the jet to the gamma-ray production site.

Hadronic supercriticalities are radiative instabilities that appear when large amounts of energy are stored in relativistic protons. When the proton energy density exceeds some critical value, a runaway process is initiated resulting in the explosive transfer of the proton energy into electron-positron pairs and radiation. The runaway also leads to an increase of the radiative efficiency, namely the ratio of the photon luminosity to the injected proton luminosity. We perform a comprehensive study of the parameter space by investigating the onset of hadronic supercriticalities for a wide range of source parameters (I.e. magnetic field strengths of 1 G-100 kG and radii of 1011-1016 cm) and maximum proton Lorentz factors (103-109). We show that supercriticalities are possible for the whole range of source parameters related to compact astrophysical sources, like gamma-ray bursts and cores and jets of active galactic nuclei. We also provide an in-depth look at the physical mechanisms of hadronic supercriticalities and show that magnetized relativistic plasmas are excellent examples of non-linear dynamical systems in high-energy astrophysics.

The origin of high-energy emission in blazars jets (I.e., leptonic versus hadronic) has been a longstanding matter of debate. Here, we focus on one variant of hadronic models where proton synchrotron radiation accounts for the observed steady γ-ray blazar emission. Using analytical methods, we derive the minimum jet power ( ${P}_{j,\min }$ ) for the largest blazar sample analyzed to date (145 sources), taking into account uncertainties of observables and jet's physical parameters. We compare ${P}_{j,\min }$ against three characteristic energy estimators for accreting systems, I.e., the Eddington luminosity, the accretion disk luminosity, and the power of the Blandford-Znajek process, and find that ${P}_{j,\min }$ is about 2 orders of magnitude higher than all energetic estimators for the majority of our sample. The derived magnetic field strengths in the emission region require either large amplification of the jet's magnetic field (factor of 30) or place the γ-ray production site at sub-pc scales. The expected neutrino emission peaks at ∼0.1-10 EeV, with typical peak neutrino fluxes ∼10-4 times lower than the peak γ-ray fluxes. We conclude that if relativistic hadrons are present in blazar jets, they can only produce a radiatively subdominant component of the overall spectral energy distribution of the blazar's steady emission.

A high-energy muon neutrino event, IceCube-170922A, was recently discovered in both spatial and temporal coincidence with a gamma-ray flare of the blazar TXS 0506+056. It has been shown with standard one-zone models that neutrinos can be produced in the blazar jet via hadronic interactions, but with a flux that is mostly limited by the X-ray data. In this work, we explore the neutrino production from TXS 0506+056 by invoking two physically distinct emission zones in the jet, with an inner blob inside of or close to the broad-line region (BLR) and an outer one well beyond the BLR. Using the Doppler-boosted radiation of the BLR as the target photon field, the inner zone accounts for the neutrino and gamma-ray emission via pγ interactions and inverse Compton scattering, respectively, while the outer zone produces the optical and X-ray emission via synchrotron and synchrotron self-Compton processes. The different conditions of the two zones allow us to suppress the X-ray emission from the electromagnetic cascade, and set a much higher upper limit on the muon neutrino flux (i.e., ∼10-11 erg cm-2 s-1) than in one-zone models. We compare our scenario in detail with one-zone models discussed in the literature, and argue that differentiating between such scenarios will become possible with next-generation neutrino telescopes, such as IceCube-Gen2.

NGC 300 ULX1 is an ultraluminous X-ray pulsar, showing an unprecedented spin evolution, from about 126 s to less than 20 s in only 4 yr, consistent with steady mass accretion rate. Following its discovery we have been monitoring the system with Swift and NICER to further study its properties. We found that even though the observed flux of the system dropped by a factor of ≳20, the spin-up rate remained almost constant. A possible explanation is that the decrease in the observed flux is a result of increased absorption of obscuring material due to outflows or a precessing accretion disc.

Inverse Compton-pair cascades are initiated when gamma-rays are absorbed on an ambient soft photon field to produce relativistic pairs, which in turn up-scatter the same soft photons to produce more gamma-rays. If the Compton scatterings take place in the deep Klein-Nishina regime, then triplet pair production (e{γ }b\to {{ee}}+{e}-) becomes relevant and may even regulate the development of the cascade. We investigate the properties of pair-Compton cascades with triplet pair production in accelerating gaps, i.e., regions with an unscreened electric field. Using the method of transport equations for the particle evolution, we compute the growth rate of the pair cascade as a function of the accelerating electric field in the presence of blackbody and power-law ambient photon fields. Informed by the numerical results, we derive simple analytical expressions for the peak growth rate and the corresponding electric field. We show that for certain parameters, which can be realized in the vicinity of accreting supermassive black holes at the centers of active galactic nuclei, the pair cascade may well be regulated by inverse Compton scattering in the deep Klein-Nishina regime and triplet pair production. We present indicative examples of the escaping gamma-ray radiation from the gap, and discuss our results in application to the TeV observations of radio galaxy M87.

Magnetic reconnection is often invoked to explain the nonthermal radiation of relativistic outflows, including jets of active galactic nuclei (AGNs). Motivated by the largely unknown plasma composition of AGN jets, we study reconnection in the unexplored regime of electron-positron-proton (pair-proton) plasmas with large-scale two-dimensional particle-in-cell simulations. We cover a wide range of pair multiplicities (lepton-to-proton number ratio κ = 1-199) for different values of the all-species plasma magnetization (σ = 1, 3, and 10) and electron temperature ({{{\Theta }}}e\equiv {{kT}}e/{m}e{c}2=0.1{--}100). We focus on the dependence of the post-reconnection energy partition and lepton energy spectra on the hot pair plasma magnetization {σ }e,h (i.e., the ratio of magnetic to pair enthalpy densities). We find that the post-reconnection energy is shared roughly equally between magnetic fields, pairs, and protons for {σ }e,h ≳ 3. We empirically find that the mean lepton Lorentz factor in the post-reconnection region depends on σ, Θ e , and {σ }e,h as < {γ }e-1> ≈ \sqrt{σ }(1+4{{{\Theta }}}e)≤ft(1+{σ }e,h/30\right), for σ ≥ 1. The high-energy part of the post-reconnection lepton energy distributions can be described by a power law, whose slope is mainly controlled by {σ }e,h for κ ≳ 3-6, with harder power laws obtained for higher magnetizations. We finally show that reconnection in pair-proton plasmas with multiplicities κ ∼ 1-20, magnetizations σ ∼ 1-10, and temperatures Θ e ∼ 1-10 results in particle power-law slopes and average electron Lorentz factors that are consistent with those inferred in leptonic models of AGN jet emission.

We present evidence that TXS 0506+056, the first plausible non-stellar neutrino source, despite appearances, is not a blazar of the BL Lac type but is instead a masquerading BL Lac, i.e. intrinsically a flat-spectrum radio quasar with hidden broad lines and a standard accretion disc. This reclassification is based on: (1) its radio and O II luminosities; (2) its emission line ratios; (3) its Eddington ratio. We also point out that the synchrotron peak frequency of TXS 0506+056 is more than two orders of magnitude larger than expected by the so-called `blazar sequence', a scenario which has been assumed by some theoretical models predicting neutrino (and cosmic ray) emission from blazars. Finally, we comment on the theoretical implications this reclassification has on the location of the γ-ray emitting region and our understanding of neutrino emission in blazars.

The joint detection of gravitational waves (GWs) and γ-rays from a binary neutron star (NS) merger provided a unique view of off-axis gamma-ray bursts (GRBs) and an independent measurement of the NS merger rate. Comparing the observations of GRB170817 with those of the regular population of short GRBs (sGRBs), we show that an order unity fraction of NS mergers result in sGRB jets that breakout of the surrounding ejecta. We argue that the luminosity function of sGRBs, peaking at {≈ } 2× 10^{52} erg s^{-1}, is likely an intrinsic property of the sGRB central engine and that sGRB jets are typically narrow with opening angles θ0 ≈ 0.1. We perform Monte Carlo simulations to examine models for the structure and efficiency of the prompt emission in off-axis sGRBs. We find that only a small fraction (∼0.01-0.1) of NS mergers detectable by LIGO/VIRGO in GWs is expected to be also detected in prompt γ-rays and that GW170817-like events are very rare. For an NS merger rate of ∼1500 Gpc-3 yr-1, as inferred from GW170817, we expect within the next decade up to ∼12 joint detections with off-axis GRBs for structured-jet models and just ∼1 for quasi-spherical cocoon models where γ-rays are the result of shock breakout. Given several joint detections and the rates of their discoveries, the different structure models can be distinguished. In addition the existence of a cocoon with a reservoir of thermal energy may be observed directly in the ultraviolet, given a sufficiently rapid localization of the GW source.

The multiwavelength spectral and temporal variability observed in blazars set tight constraints on current theoretical emission models. Here, we investigate the relativistic magnetic reconnection process as a source of blazar emission in which quasi-spherical plasmoids, containing relativistic particles and magnetic fields, are associated with the emission sites in blazar jets. By coupling recent two-dimensional particle-in-cell simulations of relativistic reconnection with a time-dependent radiative transfer code, we compute the non-thermal emission from a chain of plasmoids formed during a reconnection event. The derived photon spectra display characteristic features observed in both BL Lac sources and flat spectrum radio quasars, with the distinction made by varying the strength of the external photon fields, the jet magnetization, and the number of pairs per proton contained within. Light curves produced from reconnection events are composed of many fast and powerful flares that appear on excess of a slower evolving envelope produced by the cumulative emission of medium-sized plasmoids. The observed variability is highly dependent upon the orientation of the reconnection layer with respect to the blazar jet axis and to the observer. Our model provides a physically motivated framework for explaining the multitime-scale blazar variability across the entire electromagnetic spectrum.

Blazars are a sub-category of radio-loud active galactic nuclei with relativistic jets pointing towards to the observer. They are well-known for their non-thermal variable emission, which practically extends over the whole electromagnetic spectrum. Despite the plethora of multi-wavelength observations, the issue about the origin of the γ-ray and radio emission in blazar jets remains unsettled. Here, we construct a parametric leptonic model for studying the connection between the γ-ray and radio emission in both steady-state and flaring states of blazars. Assuming that relativistic electrons are injected continuously at a fixed distance from the black hole, we numerically study the evolution of their population as it propagates to larger distances while losing energy due to expansion and radiative cooling. In this framework, γ-ray photons are naturally produced at small distances (e.g., 10‑3 pc) when the electrons are still very energetic, whereas the radio emission is produced at larger distances (e.g., 1 pc), after the electrons have cooled and the emitting region has become optically thin to synchrotron self-absorption due to expansion. We present preliminary results of our numerical investigation for the steady-state jet emission and the predicted time lags between γ-rays and radio during flares.

Frequency-dependent brightness fluctuations of radio sources, the so-called extreme scattering events (ESEs), have been observed over the last three decades. They are caused by Galactic plasma structures whose geometry and origin are still poorly understood. In this paper, we construct axisymmentric two-dimensional (2D) column density profiles for the plasma lens and explore the resulting ESEs for both point-like and extended sources. A quantity that becomes relevant is the impact parameter b, namely the distance of the observer's path from the lens' symmetry axis. We demonstrate its effects on the shape of ESE light curves and use it for a phenomenological classification of ESEs into four main types. Three of them are unique outcomes of the 2D model and do not show a characteristic U-shaped dip in the light curve, which has been traditionally used as an identification means of ESEs. We apply our model to five well-studied ESEs and show that elongated plasma tubes or quasi-spherical clouds are favoured over plasma sheets for four of them, while the remaining one is compatible with both lens geometries.

Magnetic reconnection is invoked as an efficient particle accelerator in a variety of astrophysical sources of non-thermal high-energy radiation. With large-scale two-dimensional particle-in-cell simulations of relativistic reconnection (i.e. with magnetization σ ≫ 1) in pair plasmas, we study the long-term evolution of the power-law slope and high-energy cut-off of the spectrum of accelerated particles. We find that the high-energy spectral cut-off does not saturate at γcut ∼ 4σ, as claimed by earlier studies, but it steadily grows with time as long as the reconnection process stays active. At late times, the cut-off scales approximately as γ _cut∝ √{t}, regardless of the flow magnetization and initial temperature. We show that the particles dominating the high-energy spectral cut-off reside in plasmoids, and in particular in a strongly magnetized ring around the plasmoid core. The growth of their energy is driven by the increase in the local field strength, coupled with the conservation of the first adiabatic invariant. We also find that the power-law slope of the spectrum (p = -d log N/d log γ) evolves with time. For σ ≳ 10, the spectrum is hard at early times (p ≲ 2), but it tends to asymptote to p ∼ 2; the steepening of the power-law slope allows the spectral cut-off to extend to higher and higher energies, without violating the fixed energy budget of the system. Our results demonstrate that relativistic reconnection is a viable candidate for accelerating the high-energy particles emitting in relativistic astrophysical sources.

We consider implications of high-energy neutrino emission from blazar flares, including the recent event IceCube-170922A and the 2014-2015 neutrino flare that could originate from TXS 0506+056. First, we discuss their contribution to the diffuse neutrino intensity taking into account various observational constraints. Blazars are likely to be subdominant in the diffuse neutrino intensity at sub-PeV energies, and we show that blazar flares like those of TXS 0506+056 could make ≲1%-10% of the total neutrino intensity. We also argue that the neutrino output of blazars can be dominated by the flares in the standard leptonic scenario for their γ-ray emission, and energetic flares may still be detected with a rate of ≲ 1 {yr}}-1. Second, we consider multi-messenger constraints on the source modeling. We show that luminous neutrino flares should be accompanied by luminous broadband cascade emission, emerging also in X-rays and γ-rays. This implies that not only γ-ray telescopes like Fermi but also X-ray sky monitors such as Swift and MAXI are critical to test the canonical picture based on the single-zone modeling. We also suggest a two-zone model that can naturally satisfy the X-ray constraints while explaining the flaring neutrinos via either photomeson or hadronuclear processes.

Detection of the IceCube-170922A neutrino coincident with the flaring blazar TXS 0506+056, the first and only ∼3σ high-energy neutrino source association to date, offers a potential breakthrough in our understanding of high-energy cosmic particles and blazar physics. We present a comprehensive analysis of TXS 0506+056 during its flaring state, using newly collected Swift, NuSTAR, and X-shooter data with Fermi observations and numerical models to constrain the blazar’s particle acceleration processes and multimessenger (electromagnetic (EM) and high-energy neutrino) emissions. Accounting properly for EM cascades in the emission region, we find a physically consistent picture only within a hybrid leptonic scenario, with γ-rays produced by external inverse-Compton processes and high-energy neutrinos via a radiatively subdominant hadronic component. We derive robust constraints on the blazar’s neutrino and cosmic-ray emissions and demonstrate that, because of cascade effects, the 0.1-100 keV emissions of TXS 0506+056 serve as a better probe of its hadronic acceleration and high-energy neutrino production processes than its GeV-TeV emissions. If the IceCube neutrino association holds, physical conditions in the TXS 0506+056 jet must be close to optimal for high-energy neutrino production, and are not favorable for ultrahigh-energy cosmic-ray acceleration. Alternatively, the challenges we identify in generating a significant rate of IceCube neutrino detections from TXS 0506+056 may disfavor single-zone models, in which γ-rays and high-energy neutrinos are produced in a single emission region. In concert with continued operations of the high-energy neutrino observatories, we advocate regular X-ray monitoring of TXS 0506+056 and other blazars in order to test single-zone blazar emission models, clarify the nature and extent of their hadronic acceleration processes, and carry out the most sensitive possible search for additional multimessenger sources.

A unique and often overlooked property of a source loaded with relativistic protons is that it can become supercritical, i.e. it can undergo an abrupt transition from a radiatively inefficient to a radiatively efficient state once its proton energy density exceeds a certain threshold. In this paper, we investigate the temporal variability of hadronic systems in this hardly explored regime. We show that there exists a range of proton densities that prevent the system from reaching a steady state, but drive it instead in a quasi-periodic mode. The escaping radiation then exhibits limit cycles, even if all physical parameters are held constant in time. We extend our analysis to cases where the proton injection rate varies with time and explore the variability patterns of escaping radiation as the system moves in and out from the supercritical regime. We examine the relevance of our results to the variability of the prompt gamma-ray burst emission and show that, at least on a phenomenological level, some interesting analogies exist.

We report an optical/UV jet and counterjet in M84, previously unreported in archival Hubble Space Telescope imaging. With archival VLA, ALMA, and Chandra imaging, we examine the first well-sampled spectral energy distribution of the inner jet of M84, where we find that multiple co-spatial spectral components are required. In particular, the ALMA data reveal that the radio spectrum of all four knots in the jet turns over at approximately 100 GHz, which requires a second component for the bright optical/UV emission. Further, the optical/UV has a soft spectrum and is inconsistent with the relatively flat X-ray spectrum, which indicates a third component at higher energies. Using archival VLA imaging, we have measured the proper motion of the innermost knots at 0.9 ± 0.6 and 1.1 ± 0.4c, which when combined with the low jet-to-counterjet flux ratio yields an orientation angle for the system of {74}-18+9°. In the radio, we find high fractional polarization of the inner jet of up to 30% while in the optical no polarization is detected (<8%). We investigate different scenarios for explaining the particular multicomponent spectral energy distribution (SED) of the knots. Inverse Compton models are ruled out due to the extreme departure from equipartition and the unrealistically high total jet power required. The multicomponent SED can be naturally explained within a leptohadronic scenario, but at the cost of very high power in relativistic protons. A two-component synchrotron model remains a viable explanation, but more theoretical work is needed to explain the origin and properties of the electron populations.

Plasmoids, overdense blobs of plasma containing magnetic fields and high-energy particles, are a self-consistent outcome of the reconnection process in the relativistic regime. Recent two-dimensional particle-in-cell (PIC) simulations have shown that plasmoids can undergo a variety of processes (e.g. mergers, bulk acceleration, growth, and advection) within the reconnection layer. We developed a Monte Carlo code, benchmarked with the recent PIC simulations, to examine the effects of these processes on the steady-state size and momentum distributions of the plasmoid chain. The differential plasmoid size distribution is shown to be a power law, ranging from a few plasma skin depths to ∼0.1 of the reconnection layer's length. The power-law slope is shown to be linearly dependent upon the ratio of the plasmoid acceleration and growth rates, which slightly decreases with increasing plasma magnetization. We perform a detailed comparison of our results with those of recent PIC simulations and briefly discuss the astrophysical implications of our findings through the representative case of flaring events from blazar jets.

We report on the X-ray and optical properties of two high-mass X-ray binary systems located in the Large Magellanic Cloud (LMC). Based on the obtained optical spectra, we classify the massive companion as a supergiant star in both systems. Timing analysis of the X-ray events collected by XMM-Newton revealed the presence of coherent pulsations (spin period ∼2013 s) for XMMU J053108.3-690923 and fast flaring behaviour for XMMU J053320.8-684122. The X-ray spectra of both systems can be modelled sufficiently well by an absorbed power law, yielding hard spectra and high intrinsic absorption from the environment of the systems. Due to their combined X-ray and optical properties, we classify both systems as SgXRBs: the 19th confirmed X-ray pulsar and a probable supergiant fast X-ray transient in the LMC, the second such candidate outside our Galaxy.

PSR J2032+4127 is a young and rapidly rotating pulsar on a highly eccentric orbit around the high-mass Be star MT91 213. X-ray monitoring of the binary system over an ∼4000 d period with Swift has revealed an increase of the X-ray luminosity which we attribute to the synchrotron emission of the shocked pulsar wind. We use Swift X-ray observations to infer a clumpy stellar wind with r-2 density profile and constrain the Lorentz factor of the pulsar wind to 105 < γw < 106. We investigate the effects of an axisymmetric stellar wind with polar gradient on the X-ray emission. Comparison of the X-ray light curve hundreds of days before and after the periastron can be used to explore the polar structure of the wind.

Jets in long-duration γ-ray bursts (GRBs) have to drill through the collapsing star in order to break out of it and produce the γ-ray signal while the central engine is still active. If the breakout time is shorter for more powerful engines, then the jet-collapsar interaction acts as a filter of less luminous jets. We show that the observed broken power-law GRB luminosity function is a natural outcome of this process. For a theoretically motivated breakout time that scales with jet luminosity as L-χ with χ ∼ 1/3-1/2, we show that the shape of the γ-ray duration distribution can be uniquely determined by the GRB luminosity function and matches the observed one. This analysis has also interesting implications about the supernova-central engine connection. We show that not only successful jets can deposit sufficient energy in the stellar envelope to power the GRB-associated supernovae, but also failed jets may operate in all Type Ib/c supernovae.

Type IIn supernovae (SNe), a rare subclass of core collapse SNe, explode in dense circumstellar media that have been modified by the SNe progenitors at their last evolutionary stages. The interaction of the freely expanding SN ejecta with the circumstellar medium gives rise to a shock wave propagating in the dense SN environment, which may accelerate protons to multi-PeV energies. Inelastic proton-proton collisions between the shock-accelerated protons and those of the circumstellar medium lead to multimessenger signatures. Here, we evaluate the possible neutrino signal of Type IIn SNe and compare with IceCube observations. We employ a Monte Carlo method for the calculation of the diffuse neutrino emission from the SN IIn class to account for the spread in their properties. The cumulative neutrino emission is found to be ∼10 per cent of the observed IceCube neutrino flux above 60 TeV. Type IIn SNe would be the dominant component of the diffuse astrophysical flux, only if 4 per cent of all core collapse SNe were of this type and 20-30 per cent of the shock energy was channeled to accelerated protons. Lower values of the acceleration efficiency are accessible by the observation of a single Type IIn SN as a neutrino point source with IceCube using up-going muon neutrinos. Such an identification is possible in the first year following the SN shock breakout for sources within 20 Mpc.

Radiatively inefficient accretion flow models have been shown to accurately account for the spectrum and luminosity observed from Sgr A* in the X-ray regime down to mm wavelengths. However, observations at a few GHz cannot be explained by thermal electrons alone but require the presence of an additional non-thermal particle population. Here, we propose a model for the origin of such a population in the accretion flow via means of a pulsar orbiting the supermassive black hole in our Galaxy. Interactions between the relativistic pulsar wind with the disc lead to the formation of a bow shock in the wind. During the pulsar's transit through the accretion disc, relativistic pairs, accelerated at the shock front, are injected into the disc. The radio-emitting particles are long lived and remain within the disc long after the pulsar's transit. Periodic pulsar transits through the disc result in regular injection episodes of non-thermal particles. We show that for a pulsar with spin-down luminosity Lsd ∼ 3 × 1035 erg s-1 and a wind Lorentz factor of γw ∼ 104 a quasi-steady synchrotron emission is established with luminosities in the 1-10 GHz range comparable to the observed one.

The flat spectrum radio quasar 3C 279 is a known γ-ray variable source that has recently exhibited minute-scale variability at energies >100 MeV. One-zone leptonic models for blazar emission are severely constrained by the short time-scale variability that implies a very compact emission region at a distance of hundreds of Schwarzschild radii from the central black hole. Here, we investigate a hadronic scenario where GeV γ-rays are produced via proton synchrotron radiation. We also take into account the effects of the hadronically initiated electromagnetic cascades (EMC). For a γ-ray emitting region in rough equipartition between particles and kG magnetic fields, located within the broad-line region (BLR), the development of EMC redistributes the γ-ray luminosity to softer energy bands and eventually leads to broad-band spectra that differ from the observed ones. Suppression of EMC and energy equipartition are still possible, if the γ-ray emitting region is located beyond the BLR, is fast moving with Doppler factor (>70) and contains strong magnetic fields (>100 G). Yet, these conditions cannot be easily met in parsec-scale jets, thus disfavouring a proton synchrotron origin of the Fermi-LAT flare.

Ap Librae is one out of a handful of low-frequency peaked blazars to be detected at TeV γ-rays and the only one with an identified X-ray jet. Combined observations of Fermi-LAT at high energies (HE) and of H.E.S.S. at very high energies (VHE) revealed a striking spectral property of Ap Librae; the presence of a broad high-energy component that extends more than nine orders of magnitude in energy and is, therefore, hard to be explained by the usual single-zone synchrotron self-Compton model. We show that the superposition of different emission components related to photohadronic interactions can explain the γ-ray emission of Ap Librae without invoking external radiation fields. We present two indicative model fits to the spectral energy distribution of Ap Librae where the VHE emission is assumed to originate from a compact, sub-pc scale region of the jet. A robust prediction of our model is VHE flux variability on time-scales similar to those observed at X-rays and HE γ-rays, which can be further used to distinguish between a sub-pc or kpc scale origin of the TeV emission. We thus calculate the expected variability signatures at X-rays, HE and VHE γ-rays and show that quasi-simultaneous flares are expected, with larger amplitude flares appearing at γ-rays. We assess the detectability of VHE variability from Ap Librae with CTA, next generation of IACT. We show that ∼h time-scale variability at Eγ > 0.1 TeV could be detectable at high significance with shorter exposure times than current Cherenkov telescopes.

Blazars, a subclass of active galactic nuclei, are prime candidate sources for the high energy neutrinos recently detected by IceCube. Being one of the brightest sources in the extragalactic X-ray and γ-ray sky as well as one of the nearest blazars to Earth, Mrk 421 is an excellent source for testing the scenario of the blazar-neutrino connection, especially during flares where time-dependent neutrino searches may have a higher detection probability. Here, we model the spectral energy distribution of Mrk 421 during a 13-day flare in 2010 with unprecedented multi-wavelength coverage, and calculate the respective neutrino flux. We find a correlation between the >1 PeV neutrino and photon fluxes, in all energy bands. Using typical IceCube through-going muon event samples with good angular resolution and high statistics, wederive the mean event rate above 100 TeV (∼0.57 evt/yr) and show that it is comparable to that expected from a four-month quiescent period in 2009. Due to the short duration of the flare, an accumulation of similar flares over several years would be necessary to produce a meaningful signal for IceCube. To better assess this, we apply the correlation between the neutrino and γ-ray fluxes to the 6.9 yr Fermi-LAT light curve of Mrk 421. We find that the mean event count above 1 PeV for the full IceCube detector livetime is 3.59 ± 0.60 (2.73 ± 0.38) νμ +νbarμ with (without) major flares included in our analysis. This estimate exceeds, within the uncertainties, the 95% (90%) threshold value for the detection of one or more muon (anti-)neutrinos. Meanwhile, the most conservative scenario, where no correlation of γ-rays and neutrinos is assumed, predicts 1.60 ± 0.16νμ +νbarμ events. We conclude that a non-detection of high-energy neutrinos by IceCube would probe the neutrino/γ-ray flux correlation during major flares or/and the hadronic contribution to the blazar emission.

We report on the first detection of X-ray dust-scattered rings from the Galactic low-mass X-ray binary V404 Cyg. The observation of the system with Swift/XRT on 2015 June 30 revealed the presence of five concentric ring-like structures centred at the position of V404 Cyg. Follow-up Swift/XRT observations allowed a time-dependent study of the X-ray rings. Assuming that these are the result of small-angle, single X-ray scattering by dust grains along the line of sight, we find that their angular size scales as θ ∝ √{t} in agreement with theoretical predictions. The dust grains are concentrated in five dust layers located at about 2.12, 2.05, 1.63, 1.50 and 1.18 kpc from the observer. These coincide roughly with locations of enhanced extinction as determined by infrared photometry. Assuming that the grain size distribution is described by a generalized Mathis-Rumpl-Nordsieck model, we find that the power-law index of the most distant cloud is q ∼ 4.4, while q ∼ 3.5-3.7 in all other clouds. We constrain at a 3σ level the maximum grain size of the intermediate dust layers in the range 0.16-0.20 μm and set a lower limit of ∼ 0.2 μm in the other clouds. Hints of an exponential cutoff at the angular intensity profile of the outermost X-ray ring suggest that the smallest grains have sizes 0.01 ≤ αmin ≲ 0.03 μm. Based on the relative ratios of dust column densities we find the highest dust concentration at ∼1.6 kpc. Our results indicate a gradient in the dust properties within 1 kpc from V404 Cyg.

Powerful flares from blazars with short (∼min) variability time-scales are challenging for current models of blazar emission. Here, we present a physically motivated ab initio model for blazar flares based on the results of recent particle-in-cell (PIC) simulations of relativistic magnetic reconnection. PIC simulations demonstrate that quasi-spherical plasmoids filled with high-energy particles and magnetic fields are a self-consistent by-product of the reconnection process. By coupling our PIC-based results (I.e. plasmoid growth, acceleration profile, particle and magnetic content) with a kinetic equation for the evolution of the electron distribution function we demonstrate that relativistic reconnection in blazar jets can produce powerful flares whose temporal and spectral properties are consistent with the observations. In particular, our model predicts correlated synchrotron and synchrotron self-Compton flares of duration of several hours-days powered by the largest and slowest moving plasmoids that form in the reconnection layer. Smaller and faster plasmoids produce flares of sub-hour duration with higher peak luminosities than those powered by the largest plasmoids. Yet, the observed fluence in both types of flares is similar. Multiple flares with a range of flux-doubling time-scales (minutes to several hours) observed over a longer period of flaring activity (days or longer) may be used as a probe of the reconnection layer's orientation and the jet's magnetization. Our model shows that blazar flares are naturally expected as a result of magnetic reconnection in a magnetically dominated jet.

Blobs, or quasi-spherical emission regions containing relativistic particles and magnetic fields, are often assumed ad hoc in emission models of relativistic astrophysical jets, yet their physical origin is still not well understood. Here, we employ a suite of large-scale 2D particle-in-cell simulations in electron-positron plasmas to demonstrate that relativistic magnetic reconnection can naturally account for the formation of quasi-spherical plasmoids filled with high-energy particles and magnetic fields. Our simulations extend to unprecedentedly long temporal and spatial scales, so we can capture the asymptotic physics independently of the initial setup. We characterize the properties of the plasmoids, continuously generated as a self-consistent by-product of the reconnection process: they are in rough energy equipartition between particles and magnetic fields; the upper energy cutoff of the plasmoid particle spectrum is proportional to the plasmoid width w, corresponding to a Larmor radius ∼0.2 w; the plasmoids grow in size at ∼0.1 of the speed of light, with most of the growth happening while they are still non-relativistic (`first they grow'); their growth is suppressed once they get accelerated to relativistic speeds by the field line tension, up to the Alfvén speed (`then they go'). The largest plasmoids reach a width wmax ∼ 0.2 L independently of the system length L, they have nearly isotropic particle distributions and contain the highest energy particles, whose Larmor radius is ∼0.03 L. The latter can be regarded as the Hillas criterion for relativistic reconnection. We briefly discuss the implications of our results for the high-energy emission from relativistic jets and pulsar winds.

Sgr A* is an ideal target to study low-luminosity accreting systems. It has been recently proposed that properties of the accretion flow around Sgr A* can be probed through its interactions with the stellar wind of nearby massive stars belonging to the S-cluster. When a star intercepts the accretion disc, the ram and thermal pressures of the disc terminate the stellar wind leading to the formation of a bow shock structure. Here, a semi-analytical model is constructed which describes the geometry of the termination shock formed in the wind. With the employment of numerical hydrodynamic simulations, this model is both verified and extended to a region prone to Kelvin-Helmholtz instabilities. Because the characteristic wind and stellar velocities are in ∼108 cm s-1 range, the shocked wind may produce detectable X-rays via thermal bremsstrahlung emission. The application of this model to the pericentre passage of S2, the brightest member of the S-cluster, shows that the shocked wind produces roughly a month long X-ray flare with a peak luminosity of L ≈ 4 × 1033 erg s-1 for a stellar mass-loss rate, disc number density, and thermal pressure strength of dot{M}_w= 10^{-7} M_{⊙} yr^{-1}, nd = 105 cm-3, and α = 0.1, respectively. This peak luminosity is comparable to the quiescent X-ray emission detected from Sgr A* and is within the detection capabilities of current X-ray observatories. Its detection could constrain the density and thickness of the disc at a distance of ∼3000 gravitational radii from the supermassive black hole.

The absolute power of a relativistic black hole jet includes the power in the magnetic field, the leptons, the hadrons, and the radiated photons. A power analysis of a relativistic radio/γ-ray blazar jet leads to bifurcated leptonic synchrotron-Compton (LSC) and leptohadronic synchrotron (LHS) solutions that minimize the total jet power. Higher Doppler factors with increasing peak synchrotron frequency are implied in the LSC model. Strong magnetic fields {B}\prime ≳ 100 {{G}} are found for the LHS model with variability times ≲ {10}3 {{s}}, in accord with highly magnetized, reconnection-driven jet models. Proton synchrotron models of ≳ 100 {GeV} blazar radiation can have sub-Eddington absolute jet powers, but models of dominant GeV radiation in flat spectrum radio quasars require excessive power.

Several supernovae (SNe) with an unusually dense circumstellar medium (CSM) have been recently observed at radio frequencies. Their radio emission is powered by relativistic electrons that can be either accelerated at the SN shock (primaries) or injected as a by-product (secondaries) of inelastic proton-proton collisions. We investigate the radio signatures from secondary electrons, by detailing a semi-analytical model to calculate the temporal evolution of the distributions of protons, primary and secondary electrons. With our formalism, we track the cooling history of all the particles that have been injected into the emission region up to a given time, and calculate the resulting radio spectra and light curves. For an SN shock propagating through the progenitor wind, we find that secondary electrons control the early radio signatures, but their contribution decays faster than that of primary electrons. This results in a flattening of the light curve at a given radio frequency that depends only upon the radial profiles of the CSM density and of the shock velocity, υ0. The relevant transition time at the peak frequency is {∼ } {190} d K_ep,-3^{-1} A_{w, 16}{/β _{0, -1.5}^2}, where Aw is the wind mass-loading parameter, β0 = υ0/c and Kep are the electron-to-proton ratio of accelerated particles. We explicitly show that late peak times at 5 GHz (I.e. tpk ≳ 300-1000 d) suggest a shock wave propagating in a dense wind (Aw ≳ 1016-1017 gr cm-1), where secondary electrons are likely to power the observed peak emission.

We present a method of constraining the properties of the γ-ray emitting region in flat spectrum radio quasars in the one-zone proton synchrotron model, where the γ-rays are produced by synchrotron radiation of relativistic protons. We show that for low enough values of the Doppler factor δ, the emission from the electromagnetic (EM) cascade which is initiated by the internal absorption of high-energy photons from photohadronic interactions may exceed the observed ∼GeV flux. We use that effect to derive an absolute lower limit of δ; first, an analytical one, in the asymptotic limit where the external radiation from the broad-line region (BLR) is negligible, and then a numerical one in the more general case that includes BLR radiation. As its energy density in the emission region depends on δ and the region's distance from the galactic centre, we use the EM cascade to determine a minimum distance for each value of δ. We complement the EM cascade constraint with one derived from variability arguments and apply our method to the FSRQ 3C 273. We find that δ ≳ 18-20 for B ≲ 30 G and ∼day time-scale variability; the emission region is located outside the BLR, namely at r ≳ 10RBLR ∼ 3 pc; the model requires at pc-scale distances stronger magnetic fields than those inferred from core shift observations; while the jet power exceeds by at least one order of magnitude the accretion power. In short, our results disfavour the proton synchrotron model for the FSRQ 3C 273.

Blazars have been suggested as possible neutrino sources long before the recent IceCube discovery of high-energy neutrinos. We re-examine this possibility within a new framework built upon the blazar simplified view and a self-consistent modelling of neutrino emission from individual sources. The former is a recently proposed paradigm that explains the diverse statistical properties of blazars adopting minimal assumptions on blazars' physical and geometrical properties. This view, tested through detailed Monte Carlo simulations, reproduces the main features of radio, X-ray, and γ-ray blazar surveys and also the extragalactic γ-ray background at energies ≳ 10 GeV. Here, we add a hadronic component for neutrino production and estimate the neutrino emission from BL Lacertae objects as a class, `calibrated' by fitting the spectral energy distributions of a preselected sample of such objects and their (putative) neutrino spectra. Unlike all previous papers on this topic, the neutrino background is then derived by summing up at a given energy the fluxes of each BL Lac in the simulation, all characterized by their own redshift, synchrotron peak energy, γ-ray flux, etc. Our main result is that BL Lacs as a class can explain the neutrino background seen by IceCube above ∼0.5 PeV while they only contribute ∼10 per cent at lower energies, leaving room to some other population(s)/physical mechanism. However, one cannot also exclude the possibility that individual BL Lacs still make a contribution at the ≈20 per cent level to the IceCube low-energy events. Our scenario makes specific predictions, which are testable in the next few years.

The inverse Compton catastrophe is defined as a dramatic rise in the luminosity of inverse Compton scattered photons. It is described by a non-linear loop of radiative processes that sets in for high values of the electron compactness and is responsible for the efficient transfer of energy from electrons to photons, predominantly through inverse Compton scatterings. We search for the conditions that drive a magnetized non-thermal source to the inverse Compton catastrophe regime and study its multiwavelength (MW) photon spectrum. We develop a generic analytical framework and use numerical calculations as a backup to the analytical predictions. We find that the escaping radiation from a source in the Compton catastrophe regime bears some unique features. The MW photon spectrum is a broken power law with a break at ∼mec2 due to the onset of the Klein-Nishina suppression. The spectral index below the break energy depends on the electron and magnetic compactnesses logarithmically, while it is independent of the electron power-law index (s). The maximum radiating power emerges typically in the γ-ray regime, at energies ∼mec2 (∼γmax mec2) for s > 2 (s ≲ 2), where γmax is the maximum Lorentz factor of the injected electron distribution. We apply the principles of the inverse Compton catastrophe to blazars and γ-ray bursts using the analytical framework we developed, and show how these can be used to impose robust constraints on the source parameters.

Observations of gamma-ray-bursts and jets from active galactic nuclei reveal that the jet flow is characterized by a high radiative efficiency and that the dissipative mechanism must be a powerful accelerator of non-thermal particles. Shocks and magnetic reconnection have long been considered as possible candidates for powering the jet emission. Recent progress via fully-kinetic particle-in-cell simulations allows us to revisit this issue on firm physical grounds. We show that shock models are unlikely to account for the jet emission. In fact, when shocks are efficient at dissipating energy, they typically do not accelerate particles far beyond the thermal energy, and vice versa. In contrast, we show that magnetic reconnection can deposit more than 50 per cent of the dissipated energy into non-thermal leptons as long as the energy density of the magnetic field in the bulk flow is larger than the rest-mass energy density. The emitting region, i.e. the reconnection downstream, is characterized by a rough energy equipartition between magnetic fields and radiating particles, which naturally accounts for a commonly observed property of blazar jets.

In 2012 Markarian 421 underwent the largest flare ever observed in this blazar at radio frequencies. In the present study, we start exploring this unique event and compare it to a less extreme event in 2013. We use 15 GHz radio data obtained with the Owens Valley Radio Observatory 40-m telescope, 95 GHz millimetre data from the Combined Array for Research in Millimeter-Wave Astronomy, and GeV γ-ray data from the Fermi Gamma-ray Space Telescope. The radio light curves during the flaring periods in 2012 and 2013 have very different appearances, in both shape and peak flux density. Assuming that the radio and γ-ray flares are physically connected, we attempt to model the most prominent sub-flares of the 2012 and 2013 activity periods by using the simplest possible theoretical framework. We first fit a one-zone synchrotron self-Compton (SSC) model to the less extreme 2013 flare and estimate parameters describing the emission region. We then model the major γ-ray and radio flares of 2012 using the same framework. The 2012 γ-ray flare shows two distinct spikes of similar amplitude, so we examine scenarios associating the radio flare with each spike in turn. In the first scenario, we cannot explain the sharp radio flare with a simple SSC model, but we can accommodate this by adding plausible time variations to the Doppler beaming factor. In the second scenario, a varying Doppler factor is not needed, but the SSC model parameters require fine-tuning. Both alternatives indicate that the sharp radio flare, if physically connected to the preceding γ-ray flares, can be reproduced only for a very specific choice of parameters.

The recent IceCube discovery of 0.1-1 PeV neutrinos of astrophysical origin opens up a new era for high-energy astrophysics. Although there are various astrophysical candidate sources, a firm association of the detected neutrinos with one (or more) of them is still lacking. A recent analysis of plausible astrophysical counterparts within the error circles of IceCube events showed that likely counterparts for nine of the IceCube neutrinos include mostly BL Lacs, among which Mrk 421. Motivated by this result and a previous independent analysis on the neutrino emission from Mrk 421, we test the BL Lac-neutrino connection in the context of a specific theoretical model for BL Lac emission. We model the spectral energy distribution (SED) of the BL Lacs selected as counterparts of the IceCube neutrinos using a one-zone leptohadronic model and mostly nearly simultaneous data. The neutrino flux for each BL Lac is self-consistently calculated, using photon and proton distributions specifically derived for every individual source. We find that the SEDs of the sample, although different in shape and flux, are all well fitted by the model using reasonable parameter values. Moreover, the model-predicted neutrino flux and energy for these sources are of the same order of magnitude as those of the IceCube neutrinos. In two cases, namely Mrk 421 and 1H 1914-194, we find a suggestively good agreement between the model prediction and the detected neutrino flux. Our predictions for all the BL Lacs of the sample are in the range to be confirmed or disputed by IceCube in the next few years of data sampling.

We present the spectral signatures of the Bethe-Heitler pair production (pe) process on the spectral energy distribution (SED) of blazars, in scenarios where the hard γ-ray emission is of photohadronic origin. If relativistic protons interact with the synchrotron blazar photons producing γ rays through photopion processes, we show that, besides the ∼2-20 PeV neutrino emission, the typical blazar SED should have an emission feature due to the synchrotron emission of pe secondaries that bridges the gap between the low- and high-energy humps of the SED, namely in the energy range 40 keV-40 MeV. We first present analytical expressions for the photopion and pe loss rates in terms of observable quantities of blazar emission. For the pe loss rate in particular, we derive a new approximate analytical expression for the case of a power-law photon distribution, which has an excellent accuracy with the numerically calculated exact one, especially at energies much above the threshold for pair production. We show that for typical blazar parameters, the photopair synchrotron emission emerges in the hard X-ray/soft γ-ray energy range with a characteristic spectral shape and non-negligible flux, which may even be comparable to the hard γ-ray flux produced through photopion processes. We argue that the expected `pe bumps' are a natural consequence of leptohadronic models, and as such, they may indicate that blazars with a three-hump SED are possible emitters of high-energy neutrinos.

We explore a one-zone hadronic model that may be able to reproduce γ-ray burst (GRB) prompt emission with a minimum of free parameters. Assuming only that GRBs are efficient high-energy proton accelerators and without the presence of an ab initio photon field, we investigate the conditions under which the system becomes supercritical, i.e. there is a fast, non-linear transfer of energy from protons to secondary particles initiated by the spontaneous quenching of proton-produced γ-rays. We first show analytically that the transition to supercriticality occurs whenever the proton injection compactness exceeds a critical value, which favours high proton injection luminosities and a wide range of bulk Lorentz factors. The properties of supercriticality are then studied with a time-dependent numerical code that solves concurrently the coupled equations of proton, photon, electron, neutron and neutrino distributions. For conditions that drive the system deep into the supercriticality, we find that the photon spectra obtain a Band-like shape due to Comptonization by cooled pairs and that the energy transfer efficiency from protons to γ-rays and neutrinos is high reaching ∼0.3. Although some questions concerning its full adaptability to the GRB prompt emission remain open, supercriticality is found to be a promising process in that regard.

Aims: We apply both leptonic and leptohadronic emission scenarios for modelling the multiwavelength photon spectra and the observed variability in the optical, X-ray, and TeV gamma-ray energy bands of blazar PKS 2155-304 while being in a low state between 25 August and 6 September 2008. Methods: We consider three emission models, namely a one-component synchrotron self-Compton model (1-SSC), a one-zone proton synchrotron model (LHs), and a two-component SSC model (2-SSC). Only in the first scenario can the emission from the optical up to very high-energy (VHE) gamma-rays be attributed to a single particle population from one emission region. In the LHs model, the low-energy and high-energy bumps of the spectral energy distribution (SED) are the result of electron and proton synchrotron radiation, respectively, i.e. two different particle populations are required. In the 2-SSC model, the emission from one component dominates in the optical and gamma-ray energy bands, while the other one contributes only to the X-ray flux. Using a time-dependent numerical code that solves the kinetic equations for each particle species, we derived, in all cases, acceptable fits to the time-averaged SED. By imposing variations to one (or more) model parameters according to observed variability pattern in one (or more) frequencies we calculated the respective lightcurves and compared them with the observations. Results: We show that the 1-SSC model cannot account for the anticorrelation observed between the X-rays and VHE gamma-rays, although it can explain the time-averaged SED. The anticorrelation can be more naturally explained by the two-component emission models. Both of them reproduce satisfactorily the optical, X-ray, and TeV variability but at the cost of additional free parameters, which from four in the 2-SSC model increase to six in the LHs model. Although the results of our time-resolved analysis do not favour one of the aforementioned models, they suggest that a two-component scenario is more adequate for the emission of PKS 2155-304 in the low state of 2008, which agrees with a recent independent analysis. This suggests that the quiescent blazar radiation might result from a superposition of the radiation from different components, while a flare might still be the result of a single component.

We investigate the effects of hadronic cascades on the gamma-ray burst (GRB) prompt emission spectra in scenarios of efficient neutrino production. By assuming a fiducial GRB spectrum and a power-law proton distribution extending to ultrahigh energies, we calculate the proton cooling rate and the neutrino emission produced through photopion processes. For this, we employ a numerical code that follows the formation of the hadronic cascade by taking into account non-linear feedback effects, such as the evolution of the target photon field itself due to the contribution of secondary particles. We show that in cases of efficient proton cooling and subsequently efficient high-energy neutrino production, the emission from the hadronic cascade distorts and may even dominate the GRB spectrum. Taking this into account, we constrain the allowable values of the ratio ηp = Lp/Lγ, where Lp and Lγ are the isotropic equivalent proton and prompt gamma-ray luminosities. For the highest value of ηp that does not lead to the dominance of the cascading emission, we then calculate the maximum neutrino luminosity from a single burst and show that it ranges between (0.01-0.6)Lp and (0.5-1.4)Lγ for various parameter sets. We discuss possible implications of other parameters, such as the magnetic field strength and the shape of the initial gamma-ray spectrum, on our results. Finally, we compare the upper limit on ηp derived here with various studies in the field, and we point out the necessity of a self-consistent treatment of the hadronic emission in order to avoid erroneously high neutrino fluxes from GRB models.

Aims: We investigate the role of the second synchrotron self-Compton (SSC) photon generation to the multiwavelength emission from the compact regions of sources that are characterized as misaligned blazars. For this, we focus on the nearest high-energy emitting radio galaxy Centaurus A and we revisit the one-zone SSC model for its core emission. Methods: We have calculated analytically the peak luminosities of the first and second SSC components by first deriving the steady-state electron distribution in the presence of synchrotron and SSC cooling, and then by using appropriate expressions for the positions of the spectral peaks. We have also tested our analytical results against those derived from a numerical code where the full emissivities and cross-sections were used. Results: We show that the one-zone SSC model cannot account for the core emission of Centaurus A above a few GeV, where the peak of the second SSC component appears. We thus propose an alternative explanation for the origin of the high-energy (≳0.4 GeV) and TeV emission, where these are attributed to the radiation emitted by a relativistic proton component through photohadronic interactions with the photons produced by the primary leptonic component. We show that the required proton luminosities are not extremely high, i.e. ~1043 erg/s, provided that the injection spectra are modelled by a power law with a high value of the lower energy cutoff. Finally, we find that the contribution of the core emitting region of Cen A to the observed neutrino and ultra-high-energy cosmic-ray fluxes is negligible.

We examine the neutrino and cosmic ray spectra resulting from two models of fitting the spectral energy distribution (SED) of the blazar Mrk 421 using a self-consistent leptohadronic code. The γ -ray emission is attributed to either synchrotron radiation of ultra-high energy protons (LHs model) or to synchrotron radiation from electrons that result from photopion interactions of lower energy protons (LH π model). Although both models succeed in fitting satisfactorily the SED, the parameter values that they use result in significantly different neutrino and cosmic-ray spectra. For the LH π model, which requires high proton energy density, we find that the neutrino spectrum peaks at an energy Eν,peak = 3.3 PeV which falls well within the energy range of recent neutrino observations. While at the same time its peak flux is just under the sensitivity limit of IC-40 observations, it cannot produce ultra-high energy cosmic rays. In the LHs model, on the other hand, neutrinos are far from being detectable because of their low flux and peak energy at Eν,peak ≃ 100 PeV. However, the propagation of protons produced by the decay of escaping neutrons results in an ultra-high energy cosmic ray flux close to that observed by Pierre Augere, HiRes and Telescope Array at energies Ep ≃ 30 EeV.

The recent discovery of extragalactic PeV neutrinos opens a new window to the exploration of cosmic ray accelerators. The observed PeV neutrino flux is close to the Waxman-Bahcall upper bound implying that gamma-ray bursts (GRBs) may be the source of ultrahigh energy cosmic rays (UHECRs). Starting with the assumption of the GRB-UHECR connection, we show using both analytical estimates and numerical simulations that the observed neutrinos can originate at the jet as a result of photopion interactions with the following implications: the neutrino spectra are predicted to have a cut-off at energy ≲10 PeV; the dissipation responsible for the GRB emission and cosmic ray acceleration takes place at distances rdiss ≃ 3 × 1011-3 × 1013 cm from the central engine; the Thomson optical depth at the dissipation region is τT ∼ 1; the jet carries a substantial fraction of its energy in the form of Poynting flux at the dissipation region, and has a Lorentz factor Γ ≃ 100-500. The non-detection of PeV neutrinos coincident with GRBs will indicate that GRBs are either poor cosmic accelerators or the dissipation takes place at small optical depths in the jet.

We investigate the origin of high-energy emission in blazars within the context of the leptohadronic one-zone model. We find that γ-ray emission can be attributed to synchrotron radiation either from protons or from secondary leptons produced via photohadronic processes. These possibilities imply differences not only in the spectral energy distribution (SED) but also in the variability signatures, especially in the X- and γ-ray regime. Thus, the temporal behaviour of each leptohadronic scenario can be used to probe the particle population responsible for the high-energy emission as it can give extra information not available by spectral fits. In this work, we apply these ideas to the non-thermal emission of Mrk 421, which is one of the best monitored TeV blazars. We focus on the observations of 2001 March, since during that period Mrk 421 showed multiple flares that have been observed in detail both in X-rays and γ-rays. First, we obtain pre-flaring fits to the SED using the different types of leptohadronic scenarios. Then, we introduce random-walk-type, small-amplitude variations on the injection compactness or on the maximum energy of radiating particles and follow the subsequent response of the radiated photon spectrum. For each leptohadronic scenario, we calculate the X-ray and γ-ray fluxes and investigate their possible correlation. Whenever the `input' variations lead, apart from flux variability, also to spectral variability, we present the resulting relations between the spectral index and the flux, both in X-rays and γ-rays. We find that proton synchrotron models are favoured energetically but require fine tuning between electron and proton parameters to reproduce the observed quadratic behaviour between X-rays and TeV γ-rays. On the other hand, models based on pion decay can reproduce this behaviour in a much more natural way.

Aims: We have studied a mechanism for producing intrinsic broken power-law γ-ray spectra in compact sources. This is based on the principles of automatic photon quenching, according to which γ-rays are being absorbed on spontaneously produced soft photons whenever the injected luminosity in γ-rays lies above a certain critical value. Methods: We derived an analytical expression for the critical γ-ray compactness in the case of power-law injection. For the case where automatic photon quenching is relevant, we calculated analytically the emergent steady-state γ-ray spectra. We also performed numerical calculations in order to back up our analytical results. Results: We show that a spontaneously quenched power-law γ-ray spectrum obtains a photon index 3Γ/2, where Γ is the photon index of the power-law at injection. Thus, large spectral breaks of the γ-ray photon spectrum, e.g. ΔΓ ≳ 1, can be obtained by this mechanism. We also discuss additional features of this mechanism that can be tested observationally. Finally, we fit the multiwavelength spectrum of a newly discovered blazar (PKS 0447-439) by using such parameters to explain the break in the γ-ray spectrum by means of spontaneous photon quenching, under the assumption that its redshift lies in the range 0.1 < z < 0.24.

In this work we propose an innovative estimation method for the minimum Doppler factor and energy content of the γ-ray emitting region of quasar 3C 279, using a standard proton synchrotron blazar model and the principles of automatic photon quenching. The latter becomes relevant for high enough magnetic fields and results in spontaneous annihilation of γ-rays. The absorbed energy is then redistributed into electron-positron pairs and soft radiation. We show that as quenching sets an upper value for the source rest-frame γ-ray luminosity, one has, by necessity, to resort to Doppler factors that lie above a certain value in order to explain the TeV observations. The existence of this lower limit for the Doppler factor also has implications on the energetics of the emitting region. In this aspect, the proposed method can be regarded as an extension of the widely used method for estimating the equipartition magnetic field using radio observations. In our case, the leptonic synchrotron component is replaced by the proton synchrotron emission and the radio by the very high energy γ-ray observations. We show specifically that one can model the TeV observations by using parameter values that minimize both the energy density and the jet power at the cost of high values of the Doppler factor. On the other hand, the modelling can also be done by using the minimum possible Doppler factor; this, however, leads to a particle-dominated region and high jet power for a wide range of magnetic field values. Despite the fact that we have focused on the case of 3C 279, our analysis can be of relevance to all TeV blazars favouring hadronic modelling that have, moreover, simultaneous X-ray observations.

The hadronic model of active galactic nuclei and other compact high-energy astrophysical sources assumes that ultra-relativistic protons, electron-positron pairs and photons interact via various hadronic and electromagnetic processes inside a magnetized volume, producing the multiwavelength spectra observed from these sources. A less studied property of such systems is that they can exhibit a variety of temporal behaviours due to the operation of different feedback mechanisms. We investigate the effects of one possible feedback loop, where γ-rays produced by photopion processes are being quenched whenever their compactness increases above a critical level. This causes a spontaneous creation of soft photons in the system that result in further proton cooling and more production of γ-rays, thus making the loop operate. We perform an analytical study of a simplified set of equations describing the system, in order to investigate the connection of its temporal behaviour with key physical parameters. We also perform numerical integration of the full set of kinetic equations verifying not only our analytical results but also those of previous numerical studies. We find that once the system becomes 'supercritical', it can exhibit either a periodic behaviour or a damped oscillatory one leading to a steady state. We briefly point out possible implications of such a supercriticality on the parameter values used in active galactic nuclei spectral modelling, through an indicative fitting of the VHE emission of blazar 3C 279.

Aims: We investigate photon quenching in compact non-thermal sources. This involves photon-photon annihilation and lepton synchrotron radiation in a network that can become non-linear. As a result the γ-ray luminosity of a source cannot exceed a critical limit that depends only on the radius of the source and on the magnetic field. Methods: We perform analytic and numerical calculations that verify previous results and extend them so that the basic properties of photon quenching are investigated. Results: We apply the above to the 2006 TeV observations of quasar 3C 279 and obtain the parameter space of allowed values for the radius of the emitting source, its magnetic field strength and the Doppler factor of the flow. We argue that the TeV observations favour either a modest Doppler factor and a low magnetic field or a high Doppler factor and a high magnetic field.

Aims: We investigate the behavior of the frequency-centered light curves expected within the standard model of gamma ray bursts, allowing the maximum electron energy (γmax) to be a free parameter that may take low values. Methods: We solve the spatially averaged kinetic equations that describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectra as a function of time. From these we construct the frequency-centered light curves with an emphasis on the X-ray and optical bands. Results: We show that in cases where γmax takes low values, the produced X-ray light curves show a plateau as the synchrotron component gives its place to the synchrotron self-Compton one in the X-ray band.

Aims: Drawing an analogy with active galactic nuclei, we investigate the one-zone synchrotron self-compton (SSC) model of gamma ray bursts (GRB) afterglows in the presence of electron injection and cooling both by synchrotron and SSC losses. Methods: We solve the spatially averaged kinetic equations which describe the simultaneous evolution of particles and photons, obtaining the multi-wavelength spectrum as a function of time. We back up our numerical calculations with analytical solutions of the equations using various profiles of the magnetic field evolution under certain simplifying assumptions. Results: We apply the model to the afterglow evolution of GRBs in a uniform density environment and examine the impact various parameters have on the multiwavelength spectra. We find that in cases where the electron injection and/or the ambient density is high, the losses are dominated by SSC and the solutions depart significantly from the ones derived in the synchrotron standard cases.