| 1. |
- Acuner, Zeynep, 1990-
(författare)
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Statistical Investigations ofthe Emission Processes in Gamma-ray Bursts
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- 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.
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| 2. |
- Cunningham, Virginia, et al.
(författare)
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GRB 160625B : Evidence for a Gaussian-shaped Jet
- 2020
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Ingår i: Astrophysical Journal. - : American Astronomical Society. - 0004-637X .- 1538-4357. ; 904:2
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Tidskriftsartikel (refereegranskat)abstract
- We present multiwavelength modeling of the afterglow from the long gamma-ray burst (GRB) 160625B using Markov Chain Monte Carlo techniques of the afterglowpy Python package. GRB 160625B is an extremely bright burst with a rich set of observations spanning from radio to gamma-ray frequencies. These observations range from similar to 0.1 days to >1000 days, thus making this event extremely well suited to such modeling. In this work we compare top-hat and Gaussian jet structure types in order to find best-fit values for the GRB jet collimation angle, viewing angle, and other physical parameters. We find that a Gaussian-shaped jet is preferred (2.7 sigma-5.3 sigma) over the traditional top-hat model. Our estimate for the opening angle of the burst ranges from 126 to 390, depending on jet-shape model. We also discuss the implications that assumptions on jet shape, viewing angle, and particularly the participation a fraction of electrons have on the final estimation of GRB intrinsic energy release and the resulting energy budget of the relativistic outflow. Most notably, allowing the participation fraction to vary results in an estimated total relativistic energy of similar to 10(53) erg. This is two orders of magnitude higher than when the total fraction is assumed to be unity; thus, this parameter has strong relevance for placing constraints on long GRB central engines, details of the circumburst media, and host environment.
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