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TRANSPARENCY PARAMETERS FROM RELATIVISTICALLY EXPANDING OUTFLOWS
Stockholm University, Faculty of Science, Department of Physics. Stockholm University, Faculty of Science, The Oskar Klein Centre for Cosmo Particle Physics (OKC). KTH Royal Institute of Technology, Sweden.
2014 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 792, no 1, 42Article in journal (Refereed) Published
Abstract [en]

In many gamma-ray bursts a distinct blackbody spectral component is present, which is attributed to the emission from the photosphere of a relativistically expanding plasma. The properties of this component (temperature and flux) can be linked to the properties of the outflow and have been presented in the case where there is no sub-photospheric dissipation and the photosphere is in coasting phase. First, we present the derivation of the properties of the outflow for finite winds, including when the photosphere is in the accelerating phase. Second, we study the effect of localized sub-photospheric dissipation on the estimation of the parameters. Finally, we apply our results to GRB 090902B. We find that during the first epoch of this burst the photosphere is most likely to be in the accelerating phase, leading to smaller values of the Lorentz factor than the ones previously estimated. For the second epoch, we find that the photosphere is likely to be in the coasting phase.

Place, publisher, year, edition, pages
2014. Vol. 792, no 1, 42
Keyword [en]
gamma-ray burst: general, plasmas, radiation mechanisms: thermal
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-107790DOI: 10.1088/0004-637X/792/1/42ISI: 000341172100042OAI: oai:DiVA.org:su-107790DiVA: diva2:753025
Note

AuthorCount:2;

Available from: 2014-10-06 Created: 2014-09-29 Last updated: 2017-12-05Bibliographically 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.
Keyword
gamma ray bursts, photosphere, radiation mechanism
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
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)
Opponent
Supervisors
Note

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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