Gregory Vereshchagin

A THEORY OF PHOTOSPHERIC EMISSION FROM RELATIVISTIC OUTFLOWS

We derive the optical depth and photospheric radius of relativistic outflows using a model of a relativistic wind with a finite duration. We also discuss the role of radiative diffusion in such an outflow. We numerically solve the radiative transfer equation and obtain light curves and observed spectra of the photospheric emission. The spectra we obtain are nonthermal and in some cases have Band shapes.

MONTE CARLO SIMULATIONS OF THE PHOTOSPHERIC EMISSION IN GAMMA-RAY BURSTS

We studied the decoupling of photons from ultra-relativistic spherically symmetric outflows expanding with constant velocity by means of Monte Carlo simulations. For outflows with finite widths we confirm the existence of two regimes: photon-thick and photon-thin, introduced recently by Ruffini et al. (RSV). The probability density function of the last scattering of photons is shown to be very different in these two cases. We also obtained spectra as well as light curves. In the photon-thick case, the time-integrated spectrum is much broader than the Planck function and its shape is well described by the fuzzy photosphere approximation introduced by RSV. In the photon-thin case, we confirm the crucial role of photon diffusion, hence the probability density of decoupling has a maximum near the diffusion radius well below the photosphere. The time-integrated spectrum of the photon-thin case has a Band shape that is produced when the outflow is optically thick and its peak is formed at the diffusion radius.

Analysis of GRB 080319B and GRB 050904 within the Fireshell Model: Evidence for a Broader Spectral Energy Distribution

The observation of GRB?080319B, with an isotropic energy E iso = 1.32 ? 1054 erg, and GRB?050904, with E iso = 1.04 ? 1054 erg, offers the possibility of studying the spectral properties of the prompt radiation of two of the most energetic gamma-ray bursts (GRBs). This allows us to probe the validity of the fireshell model for GRBs beyond 1054 erg, well outside the energy range where it has been successfully tested up to now (1049-1053 erg). We find that in the low-energy region, the prompt emission spectra observed by Swift Burst Alert Telescope (BAT) reveals more power than theoretically predicted. The opportunities offered by these observations to improve the fireshell model are outlined in this paper. One of the distinguishing features of the fireshell model is that it relates the observed GRB spectra to the spectrum in the comoving frame of the fireshell. Originally, a fully radiative condition and a comoving thermal spectrum were adopted. An additional power law in the comoving thermal spectrum is required due to the discrepancy of the theoretical and observed light curves and spectra in the fireshell model for GRBs 080319B and 050904. A new phenomenological parameter ? is correspondingly introduced in the model. We perform numerical simulations of the prompt emission in the Swift BAT bandpass by assuming different values of ? within the fireshell model. We compare them with the GRB?080319B and GRB?050904 observed time-resolved spectra, as well as with their time-integrated spectra and light curves. Although GRB?080319B and GRB?050904 are at very different redshifts (z = 0.937 and z = 6.29, respectively), a value of ? = ?1.8 for both of them leads to a good agreement between the numerical simulations and the observed BAT light curves, time-resolved and time-integrated spectra. Such a modified spectrum is also consistent with the observations of previously analyzed less energetic GRBs and reasons for this additional agreement are given. Perspectives for future low-energy missions are outlined.

Joint constraints on the lepton asymmetry of the Universe and neutrino mass from the Wilkinson Microwave Anisotropy Probe

We use the Wilkinson Microwave Anisotropy Probe (WMAP) data on the spectrum of cosmic microwave background anisotropies to put constraints on the present amount of lepton asymmetry

Vacuum polarization and plasma oscillations

L

Thermalization of Electron‐Positron‐Photon Plasmas with an application to GRB

, parametrized by the dimensionless chemical potential (also called degeneracy parameter)

Cosmic absorption of ultra high energy particles

\ensuremath{\xi}

Phase space evolution of pairs created in strong electric fields

and on the effective number of relativistic particle species. We assume a flat cosmological model with three thermally distributed neutrino species having all the same mass and chemical potential, plus an additional amount of effectively massless exotic particle species. The extra energy density associated to these species is parametrized through an effective number of additional species

Kinetics of the mildly relativistic plasma and GRBs

\ensuremath{\Delta}{N}_{\mathrm{others}}^{\mathrm{eff}}

On the frequency of oscillations in the pair plasma generated by a strong electric field

. We find that