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Observational evidence of dissipative photospheres in gamma-ray bursts
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.ORCID iD: 0000-0002-9769-8016
Harvard-Smithsonian Center for Astrophysics, United States.
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Sweden.ORCID iD: 0000-0003-4378-8785
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2011 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 415, no 4, 3693-3705 p.Article in journal (Refereed) Published
Abstract [en]

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, a, 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.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2011. Vol. 415, no 4, 3693-3705 p.
Keyword [en]
radiation mechanisms: thermal, gamma-ray burst: general
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-41303DOI: 10.1111/j.1365-2966.2011.18985.xISI: 000294204900064Scopus ID: 2-s2.0-80051807773OAI: oai:DiVA.org:kth-41303DiVA: diva2:444012
Funder
Swedish Research Council, 2009-691
Note

QC 20110927

Available from: 2011-09-27 Created: 2011-09-26 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Photospheric emission in gamma-ray bursts
Open this publication in new window or tab >>Photospheric emission in gamma-ray bursts
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis considers emission from gamma-ray bursts (GRBs), the most powerful explosions known in the Universe. Most GRBs are likely associated with the final stages of stellar evolution, where the core of a massive star collapses, and gives birth to a highly compact object such as a neutron star or black hole. The wide energy range of the Fermi Gamma-ray Space Telescope allows for unprecedented studies of GRBs. Fermi data is used to study the emission released at the photosphere of the relativistic outow ejected from the central compact object. The thesis present studies of two of the strongest GRBs ever detected; GRB 090902B (Papers I, II) and GRB 110721A (Paper III). Photospheric emission is identied and its properties are studied for both GRBs. For the first time, observational evidence is found for spectral broadening of photospheric emission. Motivated by these results, possible mechanisms to make the emission from the photosphere appear broader than the Planck spectrum are examined. Two separate theoretical explanations are presented. Apart from the possibility of energy dissipation below the photosphere (Paper II), geometrical effects in outflows with angle dependent properties is shown to significantly broaden the photospheric spectrum (Paper IV). Most importantly, the observed spectrum below the peak energy may become significantly softer inthe latter case. This thesis thus concludes that photospheric emission in GRBs may be more common than previously thought. This is because the emission spectrum from the jet photosphere does not necessarily need to be a Planck function. On the contrary it is shown that broader and/or multicomponent spectra naturally arise, consistent with what is generally observed. In particular, the thesis presents a new mechanism for spectral broadening due to geometrical effects, which must be taken into consideration in the study of GRB emission.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. xii, 49 p.
Series
Trita-FYS, ISSN 0280-316X ; 2012:72
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-101913 (URN)
Presentation
2012-09-24, Sal FB54, Albanova, RB21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20120907

Available from: 2012-09-07 Created: 2012-09-05 Last updated: 2012-09-07Bibliographically approved

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Ryde, FelixAxelsson, MagnusLundman, Christoffer

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