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Photospheric emission in gamma ray bursts: Analysis and interpretation of observations made by the Fermi gamma ray space telescope
Stockholm University, Faculty of Science, Department of Physics.
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 [en]
gamma ray bursts, photosphere, radiation mechanism
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
URN: urn:nbn:se:su:diva-116244ISBN: 978-91-7649-185-0 (print)OAI: oai:DiVA.org:su-116244DiVA: diva2:805741
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
List of papers
1. GRB110721A: AN EXTREME PEAK ENERGY AND SIGNATURES OF THE PHOTOSPHERE
Open this publication in new window or tab >>GRB110721A: AN EXTREME PEAK ENERGY AND SIGNATURES OF THE PHOTOSPHERE
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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.

Keyword
gamma-ray burst: general, gamma-ray burst: individual (GRB110721A), radiation mechanisms: thermal
National Category
Physical Sciences
Identifiers
urn:nbn:se:su:diva-81833 (URN)10.1088/2041-8205/757/2/L31 (DOI)000308921700012 ()
Note

AuthorCount:111;

Available from: 2012-11-07 Created: 2012-11-01 Last updated: 2015-04-16Bibliographically approved
2. Variable jet properties in GRB 110721A: time resolved observations of the jet photosphere
Open this publication in new window or tab >>Variable jet properties in GRB 110721A: time resolved observations of the jet photosphere
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2013 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 433, no 4, 2739-2748 p.Article in journal (Refereed) Published
Abstract [en]

Fermi Gamma-ray Space Telescope observations of GRB 110721A have revealed two emission components from the relativistic jet: emission from the photosphere, peaking at similar to 100 keV, and a non-thermal component, which peaks at similar to 1000 keV. We use the photospheric component to calculate the properties of the relativistic outflow. We find a strong evolution in the flow properties: the Lorentz factor decreases with time during the bursts from G similar to 1000 to similar to 150 (assuming a redshift z = 2; the values are only weakly dependent on unknown efficiency parameters). Such a decrease is contrary to the expectations from the internal shocks and the isolated magnetar birth models. Moreover, the position of the flow nozzle measured from the central engine, r(0), increases by more than two orders of magnitude. Assuming a moderately magnetized outflow we estimate that r(0) varies from 10(6) to similar to 10(9) cm during the burst. We suggest that the maximal value reflects the size of the progenitor core. Finally, we show that these jet properties naturally explain the observed broken power-law decay of the temperature which has been reported as a characteristic for gamma-ray burst pulses.

Keyword
gamma-ray burst: individual: GRB 110721A
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-93178 (URN)10.1093/mnras/stt863 (DOI)000322405900004 ()
Note

AuthorCount:11;

Available from: 2013-09-05 Created: 2013-09-04 Last updated: 2017-12-06Bibliographically approved
3. TRANSPARENCY PARAMETERS FROM RELATIVISTICALLY EXPANDING OUTFLOWS
Open this publication in new window or tab >>TRANSPARENCY PARAMETERS FROM RELATIVISTICALLY EXPANDING OUTFLOWS
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.

Keyword
gamma-ray burst: general, plasmas, radiation mechanisms: thermal
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-107790 (URN)10.1088/0004-637X/792/1/42 (DOI)000341172100042 ()
Note

AuthorCount:2;

Available from: 2014-10-06 Created: 2014-09-29 Last updated: 2017-12-05Bibliographically approved
4. Extremely narrow spectrum of GRB110920A: further evidence for localised, subphotospheric dissipation
Open this publication in new window or tab >>Extremely narrow spectrum of GRB110920A: further evidence for localised, subphotospheric dissipation
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2015 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 450, no 2, 1651-1663 p.Article 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. 

Keyword
radiation mechanisms: non-thermal, radiation mechanisms: thermal, gamma-ray burst: general
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-116247 (URN)10.1093/mnras/stv636 (DOI)000356338500039 ()
Available from: 2015-04-16 Created: 2015-04-16 Last updated: 2017-12-04Bibliographically approved
5. Synchrotron emission in GRBs observed with Fermi: Its limitations and the role of the photosphere
Open this publication in new window or tab >>Synchrotron emission in GRBs observed with Fermi: Its limitations and the role of the photosphere
(English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966Article in journal (Refereed) Submitted
National Category
Physical Sciences
Research subject
Theoretical Physics
Identifiers
urn:nbn:se:su:diva-116735 (URN)
Available from: 2015-04-26 Created: 2015-04-26 Last updated: 2017-12-04

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