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Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). Royal Institute of Technology, Sweden.
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2012 (English)In: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 757, no 2, L31Article 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 similar to 24.5 s (in the GBM) and had a peak flux of (5.7 +/- 0.2) x 10(-5) erg s(-1) 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 +/- 2 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 +/- 0.10. The temperature of the blackbody component also decreased, starting from similar to 80 keV, and the decay showed a significant break after similar to 2 s. 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
Keyword [en]
gamma-ray burst: general, gamma-ray burst: individual (GRB110721A), radiation mechanisms: thermal
National Category
Physical Sciences
URN: urn:nbn:se:su:diva-81833DOI: 10.1088/2041-8205/757/2/L31ISI: 000308921700012OAI: diva2:565368


Available from: 2012-11-07 Created: 2012-11-01 Last updated: 2015-04-16Bibliographically approved
In thesis
1. Photospheric emission in gamma ray bursts: Analysis and interpretation of observations made by the Fermi gamma ray space telescope
Open this publication in new window or tab >>Photospheric emission in gamma ray bursts: Analysis and interpretation of observations made by the Fermi gamma ray space telescope
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The large flashes of radiation that are observed in GRBs are generally believed to arise in a relativistic jetted outflow. This thesis addresses the question of how and where in the jet this radiation is produced. It further explores the jet properties that can be inferred from the observations made by the Fermi GST that regularly observes GRBs in the range 8 keV - 300 GeV.  In my analysis I focus on the observational effects of the emission from the jet photosphere. I show that the photosphere has an important role in shaping the observed radiation spectrum and that its manifestations can significantly vary between bursts. For bursts in which the photospheric  emission component can be identified, the dynamics of the flow can be explored by determining the  jet Lorentz factor and the position of the jet nozzle. I also develop the theory of how to derive the properties of the outflow for general cases. The spectral analysis of the strong burst GRB110721A reveals a two-peaked spectrum, with the peaks evolving differently. I conclude that three main flow quantities can describe the observed spectral behaviour in bursts:  the luminosity, the Lorentz factor, and the nozzle radius. While the photosphere can appear like a pure blackbody it can also be substantially broadened, due to dissipation of the jet energy below the photosphere. I show that Comptonisation of the blackbody can shape the observed spectra and describe its evolution. In particular this model can very well explain GRB110920A which has two prominent breaks in its spectra.  Alternative models including synchrotron emission leads to severe physical constraints, such as the need for very high electron Lorentz factors, which are not expected in internal shocks. Even though different manifestations of the photospheric emission can explain the data, and lead to ambiguous interpretations, I argue that dissipation below the photosphere is the most important process in shaping the observed spectral shapes and evolutions.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2015. 115 p.
gamma ray bursts, photosphere, radiation mechanism
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
urn:nbn:se:su:diva-116244 (URN)978-91-7649-185-0 (ISBN)
Public defence
2015-05-22, Lecture hall FB42, Albanova University Center, Roslagstullsbacken 21, Stockholm, 13:00 (English)

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: In press. Paper 5: Submitted.

Available from: 2015-04-29 Created: 2015-04-16 Last updated: 2015-06-24Bibliographically approved

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Axelsson, MagnusConrad, JanIyyani, ShabnamLarsson, Stefan
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