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GRB110721A: An extreme peak energy and signatures of the photosphere
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.ORCID iD: 0000-0003-4378-8785
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.ORCID iD: 0000-0002-0642-1055
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
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Number of Authors: 111
2012 (English)In: The Astrophysical Journal. Letters, ISSN 2041-8205, Vol. 757, no 2, L31- p.Article in journal (Refereed) Published
Abstract [en]

GRB110721A was observed by the Fermi Gamma-ray Space Telescope using its two instruments the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM). The burst consisted of one major emission episode which lasted for ~24.5 seconds (in the GBM) and had a peak flux of 5.7\pm0.2 x 10^{-5} erg/s/cm^2. The time-resolved emission spectrum is best modeled with a combination of a Band function and a blackbody spectrum. The peak energy of the Band component was initially 15\pm2 MeV, which is the highest value ever detected in a GRB. This measurement was made possible by combining GBM/BGO data with LAT Low Energy Events to achieve continuous 10--100 MeV coverage. The peak energy later decreased as a power law in time with an index of -1.89\pm0.10. The temperature of the blackbody component also decreased, starting from ~80 keV, and the decay showed a significant break after ~2 seconds. The spectrum provides strong constraints on the standard synchrotron model, indicating that alternative mechanisms may give rise to the emission at these energies.

Place, publisher, year, edition, pages
2012. Vol. 757, no 2, L31- p.
Keyword [en]
gamma-ray burst: general; gamma-ray burst: individual (GRB110721A); radiation mechanisms: thermal
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-102053DOI: 10.1088/2041-8205/757/2/L31ISI: 000308921700012Scopus ID: 2-s2.0-84866432468OAI: oai:DiVA.org:kth-102053DiVA: diva2:550527
Note

QC 20121015. Author count: 111. Updated from manuscript to article in journal.

Available from: 2012-09-07 Created: 2012-09-07 Last updated: 2012-10-25Bibliographically 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)
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Supervisors
Note

QC 20120907

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

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Axelsson, MagnusLundman, Christoffer

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