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Statistical Investigations ofthe Emission Processes in Gamma-ray Bursts
KTH, School of Engineering Sciences (SCI), Physics.ORCID iD: 0000-0002-4604-280X
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Physical emission mechanisms responsible for gamma-ray bursts (GRBs) remain elusive to this day, 50 years after their discovery. Although there are well studied physical models, their power to explain the observed data is a matter of debate. In this thesis, the main focus is the statistical studies of the dierent physical models given the available data from the Fermi Gamma-Ray Space Observatory to make better comparisons between these models as well as ascertaining how well they can explain the available observations so far. To this end, theoretically predicted thermal and non-thermal GRB spectra are investigated. This investigation entails both ending groupings in the catalog data (clustering) and then simulating the expected physical emission processes to test how they would look like in the current data acquiry, processing and tting procedures. Finally, a Bayesian model comparison is performed in a sub-sample of these bursts to quantify the preference of different models by the data. In conclusion, it is found that around one third of all bursts include intervals where the emission is from a photosphere which is non-dissipative.

This means that during these intervals, the emission is either emitted close to the saturation radius or in a flow which is laminar. The results further indicate that dissipation below the photosphere is responsible for the spectral shape in a majority of GRB spectra. It is consequently argued that the dominant emission mechanism during the prompt emission phase in GRBs is thermal emission from the jet photosphere at distance of around 1012 cm from the central engine. A small percentage of the bursts are better explained with a non-thermal generating process such as the synchrotron emission.

Abstract [sv]

Stralningsprocessen som ligger till grund for gamma-blixtarnas initiala hogenergetiskautbrott av gamma-stralning ar fortfarande okand trots att de har observeratsi over 50ar. Aven om det nns valformulerade fysikaliska modeller sa debatterasdet huruvida de olika modellerna kan forklara observationerna. Huvudinriktningen iforeliggande avhandling ar statistiska studier av olika fysikaliska stralningsmodellersom testas mot data fran Fermi Gamma-Ray Space Observatory. Syftet ar attuppna forbattrade jamforelser mellan modellanpassningarna och att utrona hur valmodellerna kan beskriva de tillgangliga observationerna. De modeller som studerasar teoretiska beskrivningar av en termisk och en icke-termisk stralningsprocess. Destatistiska studierna innefattar saval klassicering genom klusteranalys av datakatalogersom dataanalys av simulerad data som ar baserade pa de fysikalisk modellerna.Slutligen gors aven en Bayesiansk modelljamforelseanalys av en delmangd av allaobserverade gammablixtar. Syftet ar att undersoka vilken av modellerna som foredrasav datan. Slutsatsen jag drar av mina undersokningar ar att ungefar en tredjedelav gammablixtarna har perioder da den observerade stralningen kommer franfotosfaren i ett utode dar ingen energidissipering sker. Detta innebar att underdessa perioder kommer stralningen antingen fran omraden nara satureringsradieneller fran ett laminart utode. Mina resultat visar vidare att fotonspektrumen franen en majoritet av blixtarna formas av just energidissipering under fotosfaren. Jagargumenterar darfor for att den dominerande stralningsprocessen under den initiala(eng. prompt) fasen hos gammablixtarna ar termisk stralning fran fotosfareni det relativistiska utodet. Enbart en liten del kan ges en battre forklaring med enicke-termisk model som t.ex. synkrotronstralning.v

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2019. , p. 66
Series
TRITA-SCI-FOU ; 2019:61
Keywords [en]
gamma-ray bursts
National Category
Physical Sciences
Research subject
Physics, Atomic, Subatomic and Astrophysics
Identifiers
URN: urn:nbn:se:kth:diva-265527ISBN: 978-91-7873-409-2 (print)OAI: oai:DiVA.org:kth-265527DiVA, id: diva2:1377735
Public defence
2020-01-28, FB42, Roslagstullsbacken 21, Stockholm, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 2020-01-15

Available from: 2020-01-15 Created: 2019-12-12 Last updated: 2020-01-22Bibliographically approved
List of papers
1. Clustering of gamma-ray burst types in the Fermi GBM catalogue: indications of photosphere and synchrotron emissions during the prompt phaseShow affiliations
Open this publication in new window or tab >>Clustering of gamma-ray burst types in the Fermi GBM catalogue: indications of photosphere and synchrotron emissions during the prompt phaseShow affiliations
2018 (English)In: Monthly Notices of the Royal Astronomical Society, Vol. 475, no 2, p. 1708-1724, article id stx3106Article in journal (Refereed) Published
Abstract [en]

Many different physical processes have been suggested to explain the prompt gamma-ray emission in gamma-ray bursts (GRBs). Although there are examples of both bursts with photospheric and synchrotron emission origins, these distinct spectral appearances have not been generalized to large samples of GRBs. Here, we search for signatures of the different emission mechanisms in the full Fermi Gamma-ray Space Telescope/GBM (Gamma-ray Burst Monitor) catalogue. We use Gaussian Mixture Models to cluster bursts according to their parameters from the Band function (α, β, and Epk) as well as their fluence and T90. We find five distinct clusters. We further argue that these clusters can be divided into bursts of photospheric origin (2/3 of all bursts, divided into three clusters) and bursts of synchrotron origin (1/3 of all bursts, divided into two clusters). For instance, the cluster that contains predominantly short bursts is consistent of photospheric emission origin. We discuss several reasons that can determine which cluster a burst belongs to: jet dissipation pattern and/or the jet content, or viewing angle.

National Category
Natural Sciences
Research subject
Physics, Atomic, Subatomic and Astrophysics
Identifiers
urn:nbn:se:kth:diva-265525 (URN)10.1093/mnras/stx3106 (DOI)000427345900019 ()2-s2.0-85045976276 (Scopus ID)
Note

QC 20191212

Available from: 2019-12-12 Created: 2019-12-12 Last updated: 2020-02-19Bibliographically approved
2. Non-dissipative photospheres in GRBs: spectral appearance in the Fermi/GBM catalogue
Open this publication in new window or tab >>Non-dissipative photospheres in GRBs: spectral appearance in the Fermi/GBM catalogue
2019 (English)In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 487, no 4, p. 5508-5519Article in journal (Refereed) Published
Abstract [en]

A large fraction of gamma-ray burst (GRB) spectra are very hard below the peak. Indeed, the observed distribution of sub-peak power-law indices, alpha, has been used as an argument for a photospheric origin of GRB spectra. Here, we investigate what fraction of GRBs have spectra that are consistent with emission from a photopshere in a non-dissipative outflow. This is the simplest possible photospheric emission scenario. We create synthetic spectra, with a range of peak energies, by folding the theoretical predictions through the detector response of the FERMI/GBM detector. These simulated spectral data are fitted with typically employed empirical models. We find that the low-energy photon indices obtain values ranging -0.4 < alpha < 0.0, peaking at around -0.1, thus covering a non-negligible fraction of observed values. These values are significantly softer than the asymptotic value of the theoretical spectrum of alpha similar to 0.4. The reason for the alpha values to be much softer than expected, is the limitation of the empirical functions to capture the true curvature of the theoretical spectrum. We conclude that more than a quarter of the bursts in the GBM catalogue have at least one time-resolved spectrum, whose alpha values are consistent with spectra from a non-dissipative outflow, releasing its thermal energy at the photosphere. The fraction of spectra consistent with emission from the photosphere will increase even more if dissipation of kinetic energy in the flow occurs below the photosphere.

Place, publisher, year, edition, pages
Oxford University Press, 2019
Keywords
radiation mechanisms: thermal, methods: data analysis, methods: numerical, gamma-ray burst: general
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-255552 (URN)10.1093/mnras/stz1356 (DOI)000475888500077 ()2-s2.0-85070069004 (Scopus ID)
Note

QC 20190805

Available from: 2019-08-05 Created: 2019-08-05 Last updated: 2020-03-09Bibliographically approved
3. Photosphere and Synchrotron Emission During the Prompt Phase in GRBs Observed with Fermi/GBM: \\%Comparison of Bayesian Evidences
Open this publication in new window or tab >>Photosphere and Synchrotron Emission During the Prompt Phase in GRBs Observed with Fermi/GBM: \\%Comparison of Bayesian Evidences
(English)Manuscript (preprint) (Other academic)
Abstract [en]

There is no complete description of the emission physics during the prompt phase in gamma-raybursts. Spectral analyses, however, indicate that many spectra are narrower than what is expectedfor non-thermal emission models. Here, we reanalyse the sample of 37 bursts in Yu et al. (2019), byfitting the narrowest time-resolved spectrum in each burst. We perform model comparison between aphotospheric and a synchrotron emission model based on Bayesian evidences. We choose to compare theshape of the narrowest expected spectra: emission from the photosphere in a non-dissipative flow andslow-cooled synchrotron emission from a narrow electron distribution. We find that the photosphericspectral shape is preferred by 54% of the spectra (20/37), while 38% of the spectra (14/37) prefer thesynchrotron spectral shape; three spectra are inconclusive. We hence conclude that GRB spectra areindeed very narrow and that more than half of the bursts have a photospheric emission episode. Wealso find that a third of all spectra, not only prefer, but are also compatible with a non-dissipativephotosphere, confirming previous similar findings. Furthermore, we notice that the spectra, that preferthe photospheric model, all have a low-energy power-law indicesα&−0.5. This means that theαisa good estimator of which model is preferred by the data. Finally, we argue that the spectra whichstatistically prefer the synchrotron model, could equally well be caused by subphotospheric dissipation.If that is the case, photospheric emission during the early, prompt phase would be even more dominant.

National Category
Astronomy, Astrophysics and Cosmology
Research subject
Physics, Atomic, Subatomic and Astrophysics
Identifiers
urn:nbn:se:kth:diva-266553 (URN)
Note

QC 20200115

Available from: 2020-01-15 Created: 2020-01-15 Last updated: 2020-01-15Bibliographically approved

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