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Subphotospheric dissipation in gamma-ray bursts observed by the Fermi Gamma-ray Space Telescope
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.ORCID iD: 0000-0003-4000-8341
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Gamma-ray bursts (GRBs) are the brightest events in the Universe, for a short time outshining the rest of the Universe combined, as they explode with isotropic equivalent luminosities up to $10^{54}$ erg s$^{-1}$. These events are believed to be connected to supernovae and to binary compact object mergers, such as binary neutron stars or neutron star -- black hole systems. The origin of the so-called prompt emission in GRBs remains an unsolved problem, although some progress is being made. Spectral analysis of prompt emission has traditionally been performed with the Band function, an empirical model with no physical interpretation, and it is just recently that physical models have started to be fitted to data. This thesis presents spectral analysis of GRB data from the Fermi Gamma-ray Space Telescope using a physical model for subphotospheric dissipation. The model is developed using a numerical code and implemented as a table model in {\scriptsize XSPEC}. Paper \rom{1} presents the model and provides a proof-of-concept of fitting GRB data with such a model. Specifically, two GRBs are fitted and compared with the corresponding Band function fits. In paper \rom{2}, a sample of 37 bursts are fitted with an extended version of the model and improved analysis tools. Overall, about a third of the fitted spectra can be described by the model. From these fits it is concluded that the scenario of subphotospheric dissipation can describe all spectral shapes present in the sample. The key characteristic of the spectra that are not fitted by the model is that they are very luminous. Within the context of the model, this suggests that the assumption of internal shocks as a dissipation mechanism cannot explain the full population of GRBs. Alternatively, additional emission components may required. The thesis concludes that subphotospheric dissipation is viable as a possible origin of GRB prompt emission. Furthermore, it shows the importance of using physically motivated models when analysing GRBs.

Place, publisher, year, edition, pages
Stockholm: Kungliga Tekniska högskolan, 2016. , p. 46
Series
TRITA-FYK ; 2016:81
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-202323ISBN: 978-91-7729-253-1 (print)OAI: oai:DiVA.org:kth-202323DiVA, id: diva2:1075806
Presentation
2017-02-10, FBFB42, Albanova, Stockholm, 10:15 (English)
Opponent
Supervisors
Note

QC 20170221

Available from: 2017-02-21 Created: 2017-02-21 Last updated: 2017-02-21Bibliographically approved
List of papers
1. Confronting GRB prompt emission with a model for subphotospheric dissipation
Open this publication in new window or tab >>Confronting GRB prompt emission with a model for subphotospheric dissipation
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2015 (English)In: Monthly Notices of the Royal Astronomical Society: Letters, ISSN 1745-3925, Vol. 454, no 1, p. L31-L35Article in journal (Refereed) Published
Abstract [en]

The origin of the prompt emission in gamma-ray bursts (GRBs) is still an unsolved problem and several different mechanisms have been suggested. Here, we fit Fermi GRB data with a photospheric emission model which includes dissipation of the jet kinetic energy below the photosphere. The resulting spectra are dominated by Comptonization and contain no significant contribution from synchrotron radiation. In order to fit to the data, we span a physically motivated part of the model's parameter space and create DREAM (Dissipation with Radiative Emission as A table Model), a table model for XSPEC. We show that this model can describe different kinds of GRB spectra, including GRB 090618, representing a typical Band function spectrum, and GRB 100724B, illustrating a double peaked spectrum, previously fitted with a Band+blackbody model, suggesting they originate from a similar scenario. We suggest that the main difference between these two types of bursts is the optical depth at the dissipation site.

Place, publisher, year, edition, pages
Oxford University Press, 2015
Keyword
Gamma-ray burst: general, Gamma-ray burst: individual: GRB 090618, Gamma-ray burst: individual: GRB 100724B, Radiation mechanisms: thermal
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-181222 (URN)10.1093/mnrasl/slv114 (DOI)000378922200006 ()2-s2.0-84944884952 (Scopus ID)
Note

QC 20160203

Available from: 2016-02-03 Created: 2016-01-29 Last updated: 2017-03-09Bibliographically approved
2. Subphotospheric dissipation in GRBs: fits to Fermi data constrain the dissipation scenario and reveal correlations
Open this publication in new window or tab >>Subphotospheric dissipation in GRBs: fits to Fermi data constrain the dissipation scenario and reveal correlations
(English)Manuscript (preprint) (Other academic)
Abstract [en]

 The emission mechanisms that give rise to the prompt emission in gamma-ray bursts (GRBs) is one of the main open questions in understanding these extreme explosions.

We consider a model for subphotospheric dissipation which produces non-thermal spectra from the photosphere, by letting kinetic energy of the outow dissipate below the photosphere using a dissipation mechanism based on internal shocks. Building on the work of Ahlgren et al. (2015), we expand the model parameter space, compensate for adiabatic cooling, and improve the analysis tools. We create two table models for two dierent scenarios regarding the magnetisation, corresponding to no and moderate synchrotron emission, respectively. We t the models to time-resolved spectra of a sample of 37 Fermi GRBs and nd that  approximately a third of the spectra can be described by the models. From these ts we conclude that the scenario of subphotospheric dissipation can describe all spectral shapes present in the sample. However, we are not able to discriminate between no and moderate synchrotron emission. The key characteristic of spectra which are not tted by the model is that they are very luminous. Within the context of our model, we conclude that this is due to the assumption of internal shocks as a dissipation mechanism. Alternatively, some of these luminous bursts may require additional emission components.

National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-202321 (URN)
Note

QC 20170224

Available from: 2017-02-21 Created: 2017-02-21 Last updated: 2017-02-24Bibliographically approved

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