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Clustering of gamma-ray burst types in the Fermi GBM catalogue: indications of photosphere and synchrotron emissions during the prompt phaseShow affiliations
KTH, School of Engineering Sciences (SCI). (KTH Astroparticle Physics)ORCID iD: 0000-0002-4604-280X
KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. (KTH Astroparticle Physics)ORCID iD: 0000-0002-9769-8016
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.

Place, publisher, year, edition, pages
2018. Vol. 475, no 2, p. 1708-1724, article id stx3106
National Category
Natural Sciences
Research subject
Physics, Atomic, Subatomic and Astrophysics
Identifiers
URN: urn:nbn:se:kth:diva-265525DOI: 10.1093/mnras/stx3106OAI: oai:DiVA.org:kth-265525DiVA, id: diva2:1377714
Note

QC 20191212

Available from: 2019-12-12 Created: 2019-12-12 Last updated: 2019-12-12Bibliographically approved
In thesis
1. Statistical Investigations ofthe Emission Processes in Gamma-ray Bursts
Open this publication in new window or tab >>Statistical Investigations ofthe Emission Processes in Gamma-ray Bursts
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

nding 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 dierent

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
gamma-ray bursts
National Category
Natural Sciences
Research subject
Physics, Atomic, Subatomic and Astrophysics
Identifiers
urn:nbn:se:kth:diva-265527 (URN)978-91-7873-409-2 (ISBN)
Public defence
2019-01-28, FB42, Roslagstullsbacken 21, Stockholm, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2020-01-15 Created: 2019-12-12 Last updated: 2020-01-20Bibliographically approved

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