Florencia L. Vieyro

Coronal origin of the polarization of the high-energy emission of Cygnus X-1

Context. Cygnus X-1 is the candidate with the highest probability of containing a black hole among the X-ray binary systems in the Galaxy. It is also by far the most often studied of these objects. Recently, the International Gamma-Ray Astrophysics Laboratory Imager on board the Integral satellite (INTEGRAL/IBIS) detected strong polarization in the high-energy radiation of this source, between 400 keV and 2 MeV. This radiation has been attributed to a jet launched by the black hole. Aims. We consider whether the corona around the black hole might be the site of production of the polarized emission instead of the jet. Methods. We studied self-consistently the injection of nonthermal particles in the hot, magnetized plasma around the black hole. Results. We show that both the high-energy spectrum and polarization of Cygnus X-1 in the low-hard state can originate in the corona, without needing to invoke a jet. We estimate the degree of polarization in the intermediate state, where there is no jet, to provide a tool to test our model. Conclusions. Contrary to the commonly accepted view, the jet might not be the source of the MeV polarized tail in the spectrum of Cygnus X-1.

Particle transport in magnetized media around black holes and associated radiation

Context. Galactic black hole coronae are composed of a hot, magnetized plasma. The spectral energy distribution produced in this component of X-ray binaries can be strongly affected by different interactions between locally injected relativistic particles and the matter, radiation and magnetic fields in the source. Aims. We study the non-thermal processes driven by the injection of relativistic particles into a strongly magnetized corona around an accreting black hole. Methods. We compute in a self-consistent way the effects of relativistic bremsstrahlung, inverse Compton scattering, synchrotron radiation, and the pair-production/annihilation of leptons, as well as hadronic interactions. Our goal is to determine the non-thermal broadband radiative output of the corona. The set of coupled kinetic equations for electrons, positrons, protons, and photons are solved and the resulting particle distributions are computed self-consistently. The spectral energy distributions of transient events in X-ray binaries are calculated, as well as the neutrino production. Results. We show that the application to Cygnus X-1 of our model of non-thermal emission from a magnetized corona yields a good fit to the observational data. Finally, we show that the accumulated signal produced by neutrino bursts in black hole coronae might be detectable for sources within a few kpc on timescales of years. Conclusions. Our work leads to predictions for non-thermal photon and neutrino emission generated around accreting black holes, that can be tested by the new generation of very high energy gamma-ray and neutrino instruments.

Nonthermal processes and neutrino emission from the black hole GRO J0422+32 in a bursting state

Context. GRO J0422+32 is a member of the class of low-mass X-ray binaries (LMXBs). It was discovered during an outburst in 1992. During the entire episode a persistent power-law spectral component extending up to ∼1 MeV was observed, which suggests that nonthermal processes should have occurred in the system. Aims. We study relativistic particle interactions and the neutrino production in the corona of GRO J0422+32, and explain the behavior of GRO J0422+32 during its recorded flaring phase. Methods. We have developed a magnetized corona model to fit the spectrum of GRO J0422+32 during the low-hard state. We also estimate neutrino emission and study the detectability of neutrinos with 1 km 3 detectors, such as IceCube. Results. The short duration of the flares (∼hours) and an energy cutoff around a few TeV in the neutrino spectrum make neutrino detection difficult. There are, however, many factors that can enhance neutrino emission. Conclusions. The northern-sky coverage and full duty cycle of IceCube make it possible to detect neutrino bursts from objects of this kind through time-dependent analysis.

Neutrinos from collapsars

Context. Long gamma-ray bursts (GRBs) are associated with the gravitational collapse of very massive stars. The central engine of a GRB can collimate relativistic jets that propagate inside the stellar envelope. The shock waves produced when the jet disrupts the stellar surface are capable of accelerating particles up to very high energies. Aims. If the jet has hadronic content, neutrinos will be produced via charged pion decays. The main goal of this work is to estimate the neutrino emission produced in the region close to the surface of the star, taking pion and muon cooling into account, along with subtle effects arising from neutrino production in a highly magnetized medium. Methods. We estimate the maximum energies of the different kinds of particles and solve the coupled transport equations for each species. Once the particle distributions are known, we calculate the intensity of neutrinos. We study the different effects on the neutrinos that can change the relative weight of different flavors. In particular, we consider the effects of neutrino oscillations, and of neutrino spin precession caused by strong magnetic fields. Results. The expected neutrino signals from the shocks in the uncorking regions of Population III events is very weak, but the neutrino signal produced by Wolf-Rayet GRBs with z < 0.5 is not far from the level of the atmospheric background. Conclusions. The IceCube experiment does not have the sensitivity to detect neutrinos from the implosion of the earliest stars, but a number of high-energy neutrinos may be detected from nearby long GRBs. The cumulative signal should be detectable over several years (∼10 yr) of integration with the full 86-string configuration.

Modeling the polarization of high-energy radiation from accreting black holes - A case study of XTE J1118+480

Context. The high-energy emission (400 keV−2 MeV) of Cygnus X-1, which is the best-studied Galactic black hole, was recently found to be strongly polarized. The origin of this radiation is still unknown.Aims. We suggest that the emission is the result of non-thermal processes in the hot corona around the accreting compact object and study the polarization of high-energy radiation that is expected for black hole binaries. Methods. Two contributions to the total magnetic field were taken into account in our study: a small-scale random component related to the corona, and an ordered magnetic field associated with the accretion disk. The degree of polarization of gamma-ray emission for this particular geometry was estimated together with the angle of the polarization vector. Results. We obtain that the configuration of corona plus disk can account for the high degree of polarization of gamma-rays that are detected in galactic black holes and does not need to invoke a relativistic jet. We make specific predictions for sources in a low-hard state. In particular, the model is applied to the transient source XTE J1118+480. We show that if a new outburst of XTE J1118+480 is observed, then its gamma-ray polarization should be measurable by future instruments, such as ASTRO-H or the proposed ASTROGAM.

Gamma-ray flares from black hole coronae.

We present results of a study of non-thermal, time-dependent particle injection in a corona around an accreting black hole. We model the spectral energy distribution of high-energy flares in this scenario. We consider particle interactions with magnetic, photon and matter fields in the black hole magnetosphere. Transport equations are solved for all species of particles and the electromagnetic output is predicted. Photon annihilation is taken into account for the case of systems with early-type donor stars.

Primordial black hole evolution in two-fluid cosmology

Several processes in the early universe might lead to the formation of primordial black holes with different masses. These black holes would interact with the cosmic plasma through accretion and emission processes. Such interactions might have affected the dynamics of the universe and generated a considerable amount of entropy. In this paper we investigate the effects of the presence of primordial black holes on the evolution of the early universe. We adopt a two-fluid cosmological model with radiation and a primordial black hole gas. The latter is modelled with different initial mass functions taking into account the available constraints over the initial primordial black hole abundances. We find that certain populations with narrow initial mass functions are capable to produce significant changes in the scale factor and the entropy.

Modeling the polarization of high-energy radiation from accreting black holes

Context. The high-energy emission (400 keV−2 MeV) of Cygnus X-1, which is the best-studied Galactic black hole, was recently found to be strongly polarized. The origin of this radiation is still unknown.Aims. We suggest that the emission is the result of non-thermal processes in the hot corona around the accreting compact object and study the polarization of high-energy radiation that is expected for black hole binaries. Methods. Two contributions to the total magnetic field were taken into account in our study: a small-scale random component related to the corona, and an ordered magnetic field associated with the accretion disk. The degree of polarization of gamma-ray emission for this particular geometry was estimated together with the angle of the polarization vector. Results. We obtain that the configuration of corona plus disk can account for the high degree of polarization of gamma-rays that are detected in galactic black holes and does not need to invoke a relativistic jet. We make specific predictions for sources in a low-hard state. In particular, the model is applied to the transient source XTE J1118+480. We show that if a new outburst of XTE J1118+480 is observed, then its gamma-ray polarization should be measurable by future instruments, such as ASTRO-H or the proposed ASTROGAM.

A model for the polarization of the high-energy radiation from accreting black holes: the case of XTE J1118+480

Context. The high-energy emission (400 keV−2 MeV) of Cygnus X-1, which is the best-studied Galactic black hole, was recently found to be strongly polarized. The origin of this radiation is still unknown.Aims. We suggest that the emission is the result of non-thermal processes in the hot corona around the accreting compact object and study the polarization of high-energy radiation that is expected for black hole binaries. Methods. Two contributions to the total magnetic field were taken into account in our study: a small-scale random component related to the corona, and an ordered magnetic field associated with the accretion disk. The degree of polarization of gamma-ray emission for this particular geometry was estimated together with the angle of the polarization vector. Results. We obtain that the configuration of corona plus disk can account for the high degree of polarization of gamma-rays that are detected in galactic black holes and does not need to invoke a relativistic jet. We make specific predictions for sources in a low-hard state. In particular, the model is applied to the transient source XTE J1118+480. We show that if a new outburst of XTE J1118+480 is observed, then its gamma-ray polarization should be measurable by future instruments, such as ASTRO-H or the proposed ASTROGAM.

Neutron production in black hole coronae and proton loading of jets

We study the production of neutrons in the corona of an accreting black hole through the interaction of locally accelerated protons with matter and radiation. A fraction of these neutrons may escape and penetrate into the base of the jet, later decaying into protons. This is a possible mechanism for loading Poynting-dominated outflows with baryons. We characterize the spatial and energy distribution of neutrons in the corona and that of the protons injected in the jet by neutron decay. We assess the contribution of these protons to the radiative spectrum of the jet. We also investigate the fate of the neutrons that escape the corona into the external medium.