Gamma-ray flares from blazars have been suggested as ideal periods for the detection of high-energy neutrinos. Indeed, the first ∼3σ high-energy neutrino source association was based on the detection of a single neutrino (IC-170922) coincident with the flaring blazar TXS 0506+056. A follow-up analysis of IceCube archival data revealed a past "neutrino flare" (13 +/- 5 events within ~ 6 months) from the direction of TXS 0506+056 which, however, was not accompanied by any electromagnetic flare. Here, we investigate whether leptohadronic models of blazar emission can explain the 2014-15 neutrino flare without violating existing electromagnetic observations. To do so, we perform a wide scan of the available parameter space and numerically compute the neutrino and electromagnetic emission of the hadronic cascade for ~ 50 parameter sets. We explore both synchrotron-supported and Compton-supported electromagnetic cascades in the linear and non-linear regimes. We compare our model predictions against publicly available data from IceCube and Fermi-LAT and the X-ray upper limits we derived by analyzing archival BAT and MAXI data. We find no model that can simultaneously explain the neutrino flare and satisfy all electromagnetic constraints, thus implying the presence of more than one emitting regions in TXS 0506+056.

Magnetic field dissipation via reconnection is a promising process for explaining the non-thermal signatures from a variety of relativistic astrophysical outflows, such as pulsar wind nebulae (PWNe) and jets of active galactic nuclei (AGN). In most relativistic astrophysical outflows reconnection proceeds in the so-called relativistic regime in which the Alfven velocity of the plasma approaches the speed of light. In contrast to PWNe, where the outflow is composed of relativistic pairs, in AGN jets the composition of the plasma is largely unknown. Our goal is to study the general properties of relativistic reconnection in the unexplored regime of plasmas with mixed particle composition. We focus on pair-proton plasmas, as they bridge the gap between the pair plasma and electron-proton plasma cases that have been extensively studied in the past. We perform a suite of 2D PIC simulations using the realistic proton-to-electron mass ratio (mi/me=1836) while varying three physical parameters, namely the plasma magnetization, the plasma temperature, and the pair multiplicity. We study, for the first time, the energy distributions of accelerated particles, the inflows and outflows of plasma in the reconnection region, and the energy partition between pairs, protons, and magnetic fields, as a function of the pair multiplicity in the regime where protons dominate the rest mass energy of the plasma. We finally discuss our results in the context of non-thermal emission from AGN jets.

MeV blazars are the most luminous persistent sources in the Universe and emit most of their energy in the MeV band. These objects display very large jet powers and accretion luminosities and are known to host black holes with a mass often exceeding $10^9 M_{\odot}$. An MeV survey, performed by a new generation MeV telescope which will bridge the entire energy and sensitivity gap between the current generation of hard X-ray and gamma-ray instruments, will detect $>$1000 MeV blazars up to a redshift of $z=5-6$. Here we show that this would allow us: 1) to probe the formation and growth mechanisms of supermassive black holes at high redshifts, 2) to pinpoint the location of the emission region in powerful blazars, 3) to determine how accretion and black hole spin interplay to power the jet.

We present the Spectroscopic Time-Resolving Observatory for Broadband Energy X-rays (STROBE-X), a probe-class mission concept selected for study by NASA. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility. STROBE-X offers an enormous increase in sensitivity for X-ray spectral timing, extending these techniques to extragalactic targets for the first time. It is also an agile mission capable of rapid response to transient events, making it an essential X-ray partner facility in the era of time-domain, multi-wavelength, and multi-messenger astronomy. Optimized for study of the most extreme conditions found in the Universe, its key science objectives include: (1) Robustly measuring mass and spin and mapping inner accretion flows across the black hole mass spectrum, from compact stars to intermediate-mass objects to active galactic nuclei. (2) Mapping out the full mass-radius relation of neutron stars using an ensemble of nearly two dozen rotation-powered pulsars and accreting neutron stars, and hence measuring the equation of state for ultradense matter over a much wider range of densities than explored by NICER. (3) Identifying and studying X-ray counterparts (in the post-Swift era) for multiwavelength and multi-messenger transients in the dynamic sky through cross-correlation with gravitational wave interferometers, neutrino observatories, and high-cadence time-domain surveys in other electromagnetic bands. (4) Continuously surveying the dynamic X-ray sky with a large duty cycle and high time resolution to characterize the behavior of X-ray sources over an unprecedentedly vast range of time scales. STROBE-X's formidable capabilities will also enable a broad portfolio of additional science.

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.

We present cross correlations of the J-band SMARTS light curves and Fermi gamma-ray light curves for 8 bright blazars that have been monitored extensively on sub-weekly time scales over the past decade. Because of the uneven temporal sampling, we use the Discrete Correlation Function (DCF) and we create an empirical boot-strapping method to assess the significance of the DCF amplitude for each blazar. Our results are perhaps surprising. Early on in the Fermi mission, the flaring blazar 3C454.3 showed zero lag between optical and gamma-ray or infrared and gamma-ray fluxes, which Bonning et al. (2012) suggested was consistent with the gamma rays being produced by inverse Compton scattering of ambient photons by synchrotron-emitting electrons. However, of the 8 blazars we examine, only one - 3C454.3 - shows a significant peak at zero lag. The other seven show no significant peak at zero lag. Some blazars show broad peaks at lags of 10s of days, at or just below 3 sigma significance. In addition, analyses of time periods of a year or two only, for a given blazar, show strong changes from one epoch to the next. These results complicate our understanding of blazar emission mechanisms. Possible physical explanations are discussed.

The Mev band holds the key to answering three astrophysical questions: the sites where cosmic Rays are produced and accelerated, the origins of high- energy neutrinos, and the physical mechanisms producing the high energy gamma- ray emission from blazars. Theoretical and experimental capabilities needed in the next decade are discussed.

We discuss the scientific potentials of gamma-ray polarimetry including the theoretical implications, and observational technology advances being made. We explore the primary scientific opportunities and wealth of information expected from synergy of multi-wavelength polarimetry that will be brought to multi-messenger astronomy.

The emission of our Universe at MeV energies is unknown. New measurements of the intensity and the angular fluctuations of the MeV background will allow us to constrain models of SNIa formation, the evolution of radio-loud and radio-quiet AGN, the growth of the most massive black holes and to constrain the cross-section for dark matter interaction

MeV blazars are the most luminous sources in the Universe and host supermassive black holes. An MeV survey will detect >1000 of them up to z > 5. This would allow us: to probe the formation and growth of massive black holes at high z; to pinpoint the emission region location in blazars; to determine the interplay of accretion and black hole spin.

We advocate for a multi-messenger approach that combines high-energy neutrino and broad multi-wavelength electromagnetic observations to study AGN during the coming decade. The unique capabilities of these joint observations promise to solve several long-standing issues in our understanding of AGN as powerful cosmic accelerators.

This new era of multi-messenger astronomy, which will mature in the next decade, offers us the unprecedented opportunity to combine more than one messenger to solve some long-standing puzzles of AGN jet physics. We advocate the support to future instruments with large effective areas, excellent timing resolution, and wide fields of view.

This is a group white paper of 100 authors (each with explicit permission via email) from 51 institutions on the topic of magnetic reconnection which is relevant to 6 thematic areas. Grand challenges and research opportunities are described in observations, numerical modeling and laboratory experiments in the upcoming decade.

With very large fields of view, high-cadence sampling, high angular, and spectral resolutions, and polarization capabilities, the new generation gamma- ray instruments are ready to address the most pressing science questions of the next decades, and they are essential for the time-domain multi-messenger era.

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.

We report the results of cross correlations of the SMARTS optical/infrared and Fermi-LAT gamma-ray light curves for 8 bright blazars that have been monitored with 1 day resolution over the past decade. For the temporal correlation analysis of unevenly sampled variability data, we use the Discrete Correlation Function (DCF), creating an empirical bootstrapping method to assess the significance of the DCF amplitude for each blazar. Our results are perhaps surprising. Early on in the Fermi mission, the brightest gamma-ray blazar 3C 454.3 showed zero lag between optical/infrared and gamma-ray fluxes as reported by Bonning et al. (2012), which was consistent with the leptonic model that optical/infrared photons are produced by synchrotron radiation of relativistic electrons and gamma rays are produced by inverse Compton scattering of ambient photons by the synchrotron-emitting electrons. However, among the 8 blazars, only one blazar - 3C 454.3 - shows a significant peak at zero lag, and the other 7 blazars show no significant peak at zero lag. Some blazars show broad peaks at tens of days of lags at or just below 3 sigma significance. In addition, for a given blazar, strong changes of the DCFs from one epoch to the next are shown by the analyses of time periods of one or two year. These results make it complicated to understand blazar emission mechanisms. We discuss possible physical explanations.

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.

Motivated by the observation of a > 290 TeV muon neutrino by IceCube, coincident with a 6 month-long γ-ray flare of the blazar TXS 0506+056, and an archival search which revealed 13 ± 5 further, lower-energy neutrinos in the direction of the source in 2014-2015 we discuss the likely contribution of blazars to the diffuse high-energy neutrino intensity, the implications for neutrino emission from TXS 0506+056 based on multi-wavelength observations of the source, and a multi-zone model that allows for sufficient neutrino emission so as to reconcile the multi-wavelength cascade constraints with the neutrino emission seen by IceCube in the direction of TXS 0506+056.

The All-sky Medium Energy Gamma-ray Observatory (AMEGO) is a probe class mission concept that will provide essential contributions to multimessenger astrophysics in the late 2020s and beyond. AMEGO combines high sensitivity in the 200 keV to 10 GeV energy range with a wide field of view, good spectral resolution, and polarization sensitivity.

STROBE-X is a probe-class mission concept, selected for study by NASA, for X-ray spectral timing of compact objects across the mass scale. It combines huge collecting area, high throughput, broad energy coverage, and excellent spectral and temporal resolution in a single facility, enabling a broad portfolio of high-priority astrophysics.

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.

IceCube-170922A was an EHE neutrino event (GCN/AMON NOTICE 2016) identified and distributed by the IceCube Observatory via AMON and GCN within δt~43s of its detection at 20:54:30 UT on 2017 September 22 (GCN/AMON NOTICE IceCube-170922A 2017ApJ...843..109G). A refined localization was reported four hours later (Kopper & Blaufuss 2017GCN.21916....1K): RAJ2000=77.43-0.8+1.3°, DEJ2000=+5.72-0.4+0.7° (90% containment ellipse). IceCube-170922A triggered the Neil Gehrels Swift Observatory in automated fashion via AMON cyberinfrastructure, resulting in rapid-response mosaic-type follow-up observations, covering a roughly circular region of sky centered on the prompt localization in a 19-point tiling that began 3.25hr after the neutrino detection. This initial epoch of Swift observations spanned 22.5hr and accumulated ~800s exposure per pointing. The mosaic tiling yielded coverage of a region with radius ~0.8° centered on RAJ2000=05:09:08.784, DEJ2000=+05:45:13.32, amounting to a sky area of 2.1deg2. Source 2 from these observations (marked as X2 on Figure 1), located 4.6' from the center of the neutrino localization, was identified by us as the likely X-ray counterpart to QSO J0509+0541, also known as TXS 0506+056. This was the first report to connect TXS 0506+056 to IceCube-170922A (Keivani+ 2017GCN.21930....1K). Following the Fermi report that TXS 0506+056 was in a rare GeV-flaring state (Tanaka+ 2017ATel10791....1T), we commenced a Swift monitoring campaign on September 27 (Evans+ 2017ATel10792....1E). Swift monitored TXS 0506+056 for 36 epochs by November 30 with 53.7ks total exposure time (Table 1). The Swift-UVOT also participated in the rapid-response follow-up observations of the IceCube-170922A and the subsequent monitoring of the flaring blazar TXS 0506+056. (2 data files).

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.

This white paper is the first of a two-part series highlighting the most well-known high-energy cosmic accelerators and contributions that MeV gamma- ray astronomy will bring to understanding their energetic particle phenomena. This white paper discusses active galactic nuclei and gamma-ray bursts.

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.