Publications by Year: 2017

Christie I, Petropoulou M, Sironi L, Giannios D. Blazar Variability from Plasmoids in Relativistic Reconnection. In: ; 2017. pp. 40. Website
Petropoulou M. Anatomy of a gamma-ray burst. [Internet]. 2017;1:567 - 568. WebsiteAbstract
Linearly polarized optical emission from a gamma-ray burst reveals the presence of a large-scale distorted magnetic field in the heart of this powerful cosmic explosion.
Petropoulou M, Coenders S, Vasilopoulos G, Kamble A, Sironi L. Point-source and diffuse high-energy neutrino emission from Type IIn supernovae. [Internet]. 2017;470:1881 - 1893. WebsiteAbstract
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.
Nalewajko K, Hayashida M, Madejski G, Petropoulou M. The challenge of rapid gamma-ray variability of blazar 3C 279. In: Vol. 16. ; 2017. pp. 106.23. WebsiteAbstract
Detections of gamma-ray variability of active galaxies on time scales of a few minutes revealed the most extreme regimes of dissipation and particle acceleration in relativistic plasmas. Observations of blazar 3C 279 by the Fermi Large Area Telescope during a successful Target-of-Opportunity pointing campaign in June 2015 detected very clearly and for the first time variability in the GeV band on time scales 5 minutes and possibly shorter. This result presents a unique challenge for the theory of relativistic jets, since 3C 279 is also a quasar with dense radiative environment that can readily absorb gamma rays produced at sub-pc distance scales. The parameters required to explain such variability are extreme, regardless of the assumption of the radiation mechanism (inverse Compton, synchrotron, lepto-hadronic). Very high bulk Lorentz factors, Gamma ~ 100, and kinetic beaming effect of relativistic magnetic reconnection are proposed as ingredients of a complete solution to this problem that remains elusive.
Christie IM, Petropoulou M, Mimica P, Giannios D. Radio emission from Sgr A*: pulsar transits through the accretion disc. [Internet]. 2017;468:L26 - L30. WebsiteAbstract
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.
Petropoulou M, Nalewajko K, Hayashida M, Mastichiadis A. A hadronic minute-scale GeV flare from quasar 3C 279?. [Internet]. 2017;467:L16 - L20. WebsiteAbstract
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.
Petropoulou M, Vasilopoulos G, Giannios D. The TeV emission of Ap Librae: a hadronic interpretation and prospects for CTA. [Internet]. 2017;464:2213 - 2222. WebsiteAbstract
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.
Petropoulou M, Duran RB, Giannios D. Collapsar γ-ray bursts: how the luminosity function dictates the duration distribution. [Internet]. 2017;472:2722 - 2727. WebsiteAbstract
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.