Journal Article

Observational evidence of dissipative photospheres in gamma-ray bursts

Felix Ryde, Asaf Pe’er, Tanja Nymark, Magnus Axelsson, Elena Moretti, Christoffer Lundman, Milan Battelino, Elisabetta Bissaldi, James Chiang, Miranda S. Jackson, Stefan Larsson, Francesco Longo, Sinead McGlynn and Nicola Omodei

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 415, issue 4, pages 3693-3705
Published in print August 2011 | ISSN: 0035-8711
Published online August 2011 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2011.18985.x
Observational evidence of dissipative photospheres in gamma-ray bursts

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The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape of a Band function. This fact is best illustrated by studying GRB090902B. The main gamma-ray spectral component is initially close to a Planck function, which can only be explained by emission from the jet photosphere. Later, the same component evolves into a broader Band function. This burst thus provides observational evidence that the photosphere can give rise to a non-thermal spectrum. We show that such a broadening is most naturally explained by subphotospheric dissipation in the jet. The broadening mainly depends on the strength and location of the dissipation, the magnetic field strength and the relation between the energy densities of thermal photons and electrons. We suggest that the evolution in spectral shape observed in GRB090902B is due to a decrease in the bulk Lorentz factor of the flow, leading to the main dissipation becoming subphotospheric. Such a change in the flow parameters can also explain the correlation observed between the peak energy of the spectrum and low-energy power-law slope, α, a correlation commonly observed in GRBs. We conclude that photospheric emission could indeed be a ubiquitous feature during the prompt phase in GRBs and play a decisive role in creating the diverse spectral shapes and spectral evolutions that are observed.

Keywords: radiation mechanisms: thermal; gamma-ray burst: general

Journal Article.  10923 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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