A Theory of Mulicolor Black Body Emission from Relativistically Expanding Plasmas

We consider the emission of photons from the inner parts of a relativistically expanding plasma outflow, characterized by a constant Lorentz factor, Gamma. Photons that are injected in regions of high optical depth are advected with the flow until they escape at the photosphere. Due to multiple scattering below the photosphere, the locally emerging comoving photon distribution is thermal. However, as an observer sees simultaneously photons emitted from different angles, hence with different Doppler boosting, the observed spectrum is a multi-color black-body. We calculate here the properties of the observed spectrum at different observed times. Due to the strong dependence of the photospheric radius on the angle to the line of sight, for parameters characterizing gamma-ray bursts (GRBs) thermal photons are seen up to tens of seconds following the termination of the inner engine. At late times, following the inner engine termination, both the number flux and energy flux of the thermal spectrum decay as F ~ t^{-2}. At these times, the multicolor black body emission results in a power law at low energies (below the thermal peak), with power law index F_\nu ~ \nu^{0}. This result is remarkably similar to the average value of the low energy spectral slope index (``\alpha'') seen in fitting the spectra of large GRB sample.Comment: 8 pages, 2 figures; submitted for publication in Ap.

Dynamical Model of an Expanding Shell

Expanding blast waves are ubiquitous in many astronomical sources, such as supernovae remnants (SNRs), X-ray emitting binaries (XRBs) and gamma-ray bursts (GRBs). I consider here the dynamics of such an expanding blast wave, both in the adiabatic and the radiative regimes. As the blast wave collects material from the surrounding, it decelerates. A full description of the temporal evolution of the blast wave requires consideration of both the energy density and the pressure of the shocked material. The obtained equation is different than earlier works in which only the energy was considered. The solution converges to the familiar results in both the ultra-relativistic and the sub-relativistic (Newtonian) regimes.Comment: Minor revision. Some points clarified, references added. Accepted for publication in Ap.J. (Lett.

Constraining Magnetization of Gamma-Ray Bursts Outflows using Prompt Emission Fluence

I consider here acceleration and heating of relativistic outflow by local magnetic energy dissipation process in Poynting flux dominated outflow. Adopting the standard assumption that the reconnection rate scales with the Alfven speed, I show here that the fraction of energy dissipated as thermal photons cannot exceed (13 $\hat \gamma -14)^{-1} = 30$% (for adiabatic index $\hat \gamma = 4/3$) of the kinetic energy at the photosphere. Even in the most radiatively efficient scenario, the energy released as non-thermal photons during the prompt phase is at most equal to the kinetic energy of the outflow. These results imply that calorimetry of the kinetic energy that can be done during the afterglow phase, could be used to constrain the magnetization of gamma-ray bursts (GRB) outflows. I discuss the recent observational status, and its implications on constraining the magnetization in GRB outflows.Comment: (Very) extensive discussions about current observational constraints, implications and limitations. Accepted for publication in the Astrophysical Journa

Radiative Mechanisms in GRB prompt emission

Motivated by the Fermi gamma-ray space telescope results, in recent years immense efforts were given to understanding the mechanism that leads to the prompt emission observed. The failure of the optically thin emission models (synchrotron and synchrotron self Compton) increased interest in alternative models. Optically thick models, while having several advantages, also face difficulty in capturing several key observables. Theoretical efforts are focused in two main directions: (1) mechanisms that act to broaden the Planck spectrum; and (2) combining the optically thin and optically thick models to a hybrid model that could explain the key observables.Comment: 9 pages, 2 figures, 1 table; Invited review, to appear in the proceedings of the Gamma-Ray Burst Symposium 2012- IAA-CSIC - Marbella, editors: Castro-Tirado, A. J., Gorosabel, J. and Park, I.

Plasmas in Gamma-Ray Bursts: particle acceleration, magnetic fields, radiative Processes and environments

Being the most extreme explosions in the universe, gamma-ray bursts (GRBs) provide a unique laboratory to study various plasma physics phenomena. The complex lightcurve and broad-band, non-thermal spectra indicate a very complicated system on the one hand, but on the other hand provide a wealth of information to study it. In this chapter I focus on recent progress in some of the key unsolved physical problems. These include: (1) Particle acceleration and magnetic field generation in shock waves; (2) Possible role of strong magnetic fields in accelerating the plasmas, and accelerating particles via magnetic reconnection process; (3) Various radiative processes that shape the observed lightcurve and spectra, both during the prompt and the afterglow phases, and finally (4) GRB environments and their possible observational signature.Comment: Invited chapter for a special issue of "galaxies", dedicated to "Cosmic Plasmas and Electromagnetic phenomena