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Asaf Pe'er

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The origin of gamma-ray burst prompt emission

  Gamma-ray bursts are flashes of gamma-rays, that appear at random in the sky. Residing in distant galaxies, these are the most luminous objects in the universe, releasing as much as 10^{52} ergs in few seconds (equivalent to a few % of solar binding energy). We know today that this massive energy release is acompanied by relativistic outflows (jets), in which material is accelerated to velocities very close to the speed of light, having typical Lorentz factors of hundreds. This ejection is most likely accompanying the collapse of material, and the birth of black holes.
  In spite of over two decades of extensive research, the origin of GRB prompt emission is still a mystery. While it was initially thought to be thermal, it was noticed in the early 1990's that the observed spectra appears to have a broken power law shape that is very different than a "Planck" spectrum. Thus, this idea was replaced by a combination of synchrotron emission and inverse Compton scattering - both being well known mechanisms that produce non-thermal radiation. However, in the late 1990's, evidence began to accumulate that these mechanisms, by themselves are insufficient to explain the prompt spectra. First, both mechanism suffer from efficiency problems, making it difficult to explain the release of such a huge amount of energy; and most importantly, the observed spectral shapes are, in many cases, in contradiction to the theoretical expectations (which became known as "synchrotron line of death").
example graphic   A major part of my research in recent years was devoted to study possible solutions to this problem. I worked in two complementing directions.
  First, I studied sub-photospheric energy dissipation. This study was done mainly in collaboration with P. Meszaros and M. Rees. Sub-photospheric energy dissipation occurs when (kinetic) energy is released in regions of intermediate optical depth, and is a natural outcome of GRB "fireball" model. In such regimes, the underlying assumptions of optically-thin emission calculations do not hold, but on the other hand the optical depth is not so high as to force the spectrum to converge to "Planck". Complex spectra therefor eemerges (see examples in the Figure to the right, taken from Pe'er, Meszaros & Rees 2006). Under different conditions, the obtain spectra can resemble observations.
&nbsb An independent line of research was the study of light abberation caused by the relativistic expansion itself. As it turnes out, since photons that reach the observer originate from different radii and different angles to the line of sight, the observed spectrum from unresolved, relativistically expanding plasma is not the naively expected Planck spectrum, but could be very different. This is terminated "geometrical broadening".
Selected Publications
  • Pe'er, A., Meszaros, P., & Rees, M.J. (2005), "Peak Energy Clustering and Efficiency in Compact Objects" Ap.J., 635, 476
  • Pe'er, A., Meszaros, P., & Rees, M.J. (2006), "The Observable Effects of a Photospheric Component on GRB and XRF Prompt Emission Spectrum" Ap.J., 642, 995
  • Pe'er, A. et. al. (2012), "The connection between thermal and non-thermal emission in gamma-ray bursts: general considerations and GRB 090902B as a case study" MNRAS, 420, 468

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