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Photospheric emission in gamma-ray bursts
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.ORCID iD: 0000-0002-0642-1055
2012 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis considers emission from gamma-ray bursts (GRBs), the most powerful explosions known in the Universe. Most GRBs are likely associated with the final stages of stellar evolution, where the core of a massive star collapses, and gives birth to a highly compact object such as a neutron star or black hole. The wide energy range of the Fermi Gamma-ray Space Telescope allows for unprecedented studies of GRBs. Fermi data is used to study the emission released at the photosphere of the relativistic outow ejected from the central compact object. The thesis present studies of two of the strongest GRBs ever detected; GRB 090902B (Papers I, II) and GRB 110721A (Paper III). Photospheric emission is identied and its properties are studied for both GRBs. For the first time, observational evidence is found for spectral broadening of photospheric emission. Motivated by these results, possible mechanisms to make the emission from the photosphere appear broader than the Planck spectrum are examined. Two separate theoretical explanations are presented. Apart from the possibility of energy dissipation below the photosphere (Paper II), geometrical effects in outflows with angle dependent properties is shown to significantly broaden the photospheric spectrum (Paper IV). Most importantly, the observed spectrum below the peak energy may become significantly softer inthe latter case. This thesis thus concludes that photospheric emission in GRBs may be more common than previously thought. This is because the emission spectrum from the jet photosphere does not necessarily need to be a Planck function. On the contrary it is shown that broader and/or multicomponent spectra naturally arise, consistent with what is generally observed. In particular, the thesis presents a new mechanism for spectral broadening due to geometrical effects, which must be taken into consideration in the study of GRB emission.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , xii, 49 p.
Series
Trita-FYS, ISSN 0280-316X ; 2012:72
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:kth:diva-101913OAI: oai:DiVA.org:kth-101913DiVA: diva2:549845
Presentation
2012-09-24, Sal FB54, Albanova, RB21, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20120907

Available from: 2012-09-07 Created: 2012-09-05 Last updated: 2012-09-07Bibliographically approved
List of papers
1. IDENTIFICATION AND PROPERTIES OF THE PHOTOSPHERIC EMISSION IN GRB090902B
Open this publication in new window or tab >>IDENTIFICATION AND PROPERTIES OF THE PHOTOSPHERIC EMISSION IN GRB090902B
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2010 (English)In: ASTROPHYSICAL JOURNAL LETTERS, ISSN 2041-8213, Vol. 709, no 2, L172-L177 p.Article in journal (Refereed) Published
Abstract [en]

The Fermi Gamma-ray Space Telescope observed the bright and long GRB090902B, lying at a redshift of z = 1.822. Together the Large Area Telescope (LAT) and the Gamma-ray Burst Monitor (GBM) cover the spectral range from 8 keV to >300 GeV. Here we show that the prompt burst spectrum is consistent with emission from the jet photosphere combined with nonthermal emission described by a single power law with photon index -1.9. The photosphere gives rise to a strong quasi-blackbody spectrum which is somewhat broader than a single Planck function and has a characteristic temperature of similar to 290 keV. We model the photospheric emission with a multicolor blackbody, and its shape indicates that the photospheric radius increases at higher latitudes. We derive the averaged photospheric radius R-ph = (1.1 +/- 0.3) x 10(12) Y-1/4 cm and the bulk Lorentz factor of the flow, which is found to vary by a factor of 2 and has a maximal value of Gamma = 750 Y-1/4. Here, Y is the ratio between the total fireball energy and the energy emitted in the gamma rays. We find that during the first quarter of the prompt phase the photospheric emission dominates, which explains the delayed onset of the observed flux in the LAT compared to the GBM. We interpret the broadband emission as synchrotron emission at R similar to 4 x 10(15) cm. Our analysis emphasizes the importance of having high temporal resolution when performing spectral analysis on gamma-ray bursts, since there is strong spectral evolution.

Keyword
gamma-ray burst: individual (GRB090902B), gamma rays: general, radiation mechanisms: thermal
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-28856 (URN)10.1088/2041-8205/709/2/L172 (DOI)000274209300017 ()2-s2.0-77749336341 (Scopus ID)
Note

QC 20110207

Available from: 2011-02-07 Created: 2011-01-21 Last updated: 2016-05-04Bibliographically approved
2. Observational evidence of dissipative photospheres in gamma-ray bursts
Open this publication in new window or tab >>Observational evidence of dissipative photospheres in gamma-ray bursts
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2011 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 415, no 4, 3693-3705 p.Article in journal (Refereed) Published
Abstract [en]

The emission from a gamma-ray burst (GRB) photosphere can give rise to a variety of spectral shapes. The spectrum can retain the shape of a Planck function or it can be broadened and have the shape of a Band function. This fact is best illustrated by studying GRB090902B. The main gamma-ray spectral component is initially close to a Planck function, which can only be explained by emission from the jet photosphere. Later, the same component evolves into a broader Band function. This burst thus provides observational evidence that the photosphere can give rise to a non-thermal spectrum. We show that such a broadening is most naturally explained by subphotospheric dissipation in the jet. The broadening mainly depends on the strength and location of the dissipation, the magnetic field strength and the relation between the energy densities of thermal photons and electrons. We suggest that the evolution in spectral shape observed in GRB090902B is due to a decrease in the bulk Lorentz factor of the flow, leading to the main dissipation becoming subphotospheric. Such a change in the flow parameters can also explain the correlation observed between the peak energy of the spectrum and low-energy power-law slope, a, a correlation commonly observed in GRBs. We conclude that photospheric emission could indeed be a ubiquitous feature during the prompt phase in GRBs and play a decisive role in creating the diverse spectral shapes and spectral evolutions that are observed.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2011
Keyword
radiation mechanisms: thermal, gamma-ray burst: general
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-41303 (URN)10.1111/j.1365-2966.2011.18985.x (DOI)000294204900064 ()2-s2.0-80051807773 (Scopus ID)
Funder
Swedish Research Council, 2009-691
Note

QC 20110927

Available from: 2011-09-27 Created: 2011-09-26 Last updated: 2017-12-08Bibliographically approved
3. 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: The Astrophysical Journal. Letters, ISSN 2041-8205, Vol. 757, no 2, L31- p.Article 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 ~24.5 seconds (in the GBM) and had a peak flux of 5.7\pm0.2 x 10^{-5} erg/s/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\pm2 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\pm0.10. The temperature of the blackbody component also decreased, starting from ~80 keV, and the decay showed a significant break after ~2 seconds. 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
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-102053 (URN)10.1088/2041-8205/757/2/L31 (DOI)000308921700012 ()2-s2.0-84866432468 (Scopus ID)
Note

QC 20121015. Author count: 111. Updated from manuscript to article in journal.

Available from: 2012-09-07 Created: 2012-09-07 Last updated: 2012-10-25Bibliographically approved
4. A theory of photospheric emission from collimated outflows
Open this publication in new window or tab >>A theory of photospheric emission from collimated outflows
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Relativistic outflows in the form of jets are common in many astrophysical objects. By their very nature, jets have angle dependent velocity profiles, Gamma = Gamma(r, theta, phi), where Gamma is the outflow Lorentz factor. In this work we consider photospheric emission from non-dissipative jets with various Lorentz factor profiles, of the approximate form Gamma \approx Gamma_0/[(theta/theta_j)^p + 1], were theta_j is the characteristic jet opening angle. In collimated jets, the observed spectrum depends on the viewing angle, theta_v. We show that for narrow jets (theta_j Gamma_0 \lesssim few), the obtained low energy photon index is alpha \approx -1 (dN/dE \propto E^alpha), independent of viewing angle, and weakly dependent on the Lorentz factor gradient (p). A similar result is obtained for wider jets observed at theta_v \approx theta_j. This result is surprisingly similar to the average low energy photon index seen in gamma-ray bursts. For wide jets (theta_j Gamma_0 \gtrsim few) observed at theta_v \ll theta_j, a multicolor blackbody spectrum is obtained. We discuss the consequences of this theory on our understanding of the prompt emission in gamma-ray bursts.

National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:kth:diva-102051 (URN)
Note

QS 2012

Available from: 2012-09-07 Created: 2012-09-07 Last updated: 2012-09-07Bibliographically approved

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