Indrek Vurm

GAMMA-RAY BURSTS FROM MAGNETIZED COLLISIONALLY HEATED JETS

Jets producing gamma-ray bursts (GRBs) are likely to carry a neutron component that drifts with respect to the proton component. The neutron-proton collisions strongly heat the jet and generate electron-positron pairs. We investigate radiation produced by this heating using a new numerical code. Our results confirm the recent claim that collisional heating generates the observed Band-type spectrum of GRBs. We extend the model to study the effects of magnetic fields on the emitted spectrum. We find that the spectrum peak remains near 1?MeV for the entire range of the magnetization parameter 0 < ?B < 2 that is explored in our simulations. The low-energy part of the spectrum softens with increasing ?B, and a visible soft excess appears in the keV band. The high-energy part of the spectrum extends well above the GeV range and can contribute to the prompt emission observed by Fermi/LAT. Overall, the radiation spectrum created by the collisional mechanism appears to agree with observations, with no fine tuning of parameters.

ON THERMALIZATION IN GAMMA-RAY BURST JETS AND THE PEAK ENERGIES OF PHOTOSPHERIC SPECTRA

The low energy spectral slopes of the prompt emission of most gamma-ray bursts (GRBs) are difficult to reconcile with radiatively efficient optically thin emission models irrespective of the radiation mechanism. An alternative is to ascribe the radiation around the spectral peak to a thermalization process occurring well inside the Thomson photosphere. This quasi-thermal spectrum can evolve into the observed non-thermal shape by additional energy release at moderate to small Thomson optical depths, which can readily give rise to the hard spectral tail. The position of the spectral peak is determined by the temperature and Lorentz factor of the flow in the termalization zone, where the total number of photons carried by the jet is established. To reach thermalization, dissipation alone is not sufficient and photon generation requires an efficient emission/absorption process in addition to scattering. We perform a systematic study of all relevant photon production mechanisms searching for possible conditions in which thermalization can take place. We find that a significant fraction of the available energy should be dissipated at intermediate radii,

Time-Dependent Modeling of Radiative Processes in Hot Magnetized Plasmas

\sim 10^{10}

ON THE ORIGIN OF SPECTRAL STATES IN ACCRETING BLACK HOLES

-- a few

Hot accretion flow in black hole binaries: a link connecting X -rays to the infrared

\times 10^{11}

ON THE ORIGIN OF GeV EMISSION IN GAMMA-RAY BURSTS

cm and the flow there should be relatively slow: the bulk Lorentz factor could not exceed a few tens for all but the most luminous bursts with the highest

A SYNCHROTRON SELF-COMPTON-DISK REPROCESSING MODEL FOR OPTICAL/X-RAY CORRELATION IN BLACK HOLE X-RAY BINARIES

\Epk

Gamma-ray novae as probes of relativistic particle acceleration at non-relativistic shocks

-s. The least restrictive constraint for successful thermalization,

Shocks in nova outflows – I. Thermal emission

\Gamma\lesssim 20

RADIATIVE TRANSFER MODELS FOR GAMMA-RAY BURSTS

, is obtained if synchrotron emission acts as the photon source. This requires, however, a non-thermal acceleration deep below the Thomson photosphere transferring a significant fraction of the flow energy to relativistic electrons with Lorentz factors between 10 and 100. Other processes require bulk flow Lorentz factors of order of a few for typical bursts. We examine the implications of these results to different GRB photospheric emission models.