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Extremely narrow spectrum of GRB110920A: further evidence for localised, subphotospheric dissipation
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; Erasmus Mundus Joint Doctorate in Relativistic Astrophysics.
2015 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 450, no 2, p. 1651-1663Article in journal (Refereed) Published
##### Abstract [en]

Much evidence points towards that the photosphere in the relativistic outflow in GRBs plays an important role in shaping the observed MeV spectrum. However, it is unclear whether the spectrum is fully produced by the photosphere or whether a substantial part of the spectrum is added by processes far above the photosphere. Here we make a detailed study of the $\gamma$−ray emission from single pulse GRB110920A which has a spectrum that becomes extremely narrow towards the end of the burst. We show that the emission can be interpreted as Comptonisation of thermal photons by cold electrons in an unmagnetised outflow at an optical depth of $\tau \sim 20$. The electrons receive their energy by a local dissipation occurring close to the saturation radius. The main spectral component of GRB110920A and its evolution is thus, in this interpretation, fully explained by the emission from the photosphere including localised dissipation at high optical depths.

##### Place, publisher, year, edition, pages
2015. Vol. 450, no 2, p. 1651-1663
##### Keyword [en]
radiation mechanisms: non-thermal, radiation mechanisms: thermal, gamma-ray burst: general
##### National Category
Astronomy, Astrophysics and Cosmology
##### Research subject
Theoretical Physics
##### Identifiers
ISI: 000356338500039OAI: oai:DiVA.org:su-116247DiVA, id: diva2:805757
Available from: 2015-04-16 Created: 2015-04-16 Last updated: 2017-12-04Bibliographically 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. p. 115
##### 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)
##### 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

#### Open Access in DiVA

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Publisher's full textarxiv.org/abs/1503.05926

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Iyyani, ShabnamAxelsson, Magnus
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Cite
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