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Cosmic rays and shock physics in gamma-ray bursts

Time: Fri 2022-06-03 14.00

Location: FB42, Roslagstullsbacken 21, Stockholm

Language: English

Subject area: Physics, Atomic, Subatomic and Astrophysics

Doctoral student: Filip Samuelsson , Partikel- och astropartikelfysik, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden.

Opponent: Professor Robert Mochkovitch, Sorbonne Université

Supervisor: Felix Ryde, Partikel- och astropartikelfysik

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QC 220509


Gamma-ray bursts (GRBs) are the most luminous events in the known universe. Due to their tremendous energy output, they serve as laboratories of physics far beyond anything that we can hope to achieve in terrestrial experiments. However, the insights we can gain from these violent phenomena depend on our understanding of the relevant physical processes at work. In this thesis, I study emission processes in GRBs. Specifically, I focus on GRBs as potential sources of ultra-high-energy cosmic rays (UHECRs) and investigate the cause of the early electromagnetic emission. 

UHECRs are extraterrestrial particles with incredible energies. Despite decades of research, the origin of UHECRs remains unknown. GRBs have long been one of the most promising source candidates. In Papers I and II, we estimate the emission expected from electrons that are co-accelerated with the UHECRs at the source. We show that GRBs would have to be much brighter in the optical band if they harbored substantial UHECR acceleration, disfavoring a UHECR-GRB connection. 

In Papers III, IV, and V, we study the possible cause of the γ-ray emission that has given GRBs their name. In Paper III, we develop a model capable of describing emission from shocks in the optically thick regions of GRBs. Specifically, our model is uniquely capable of performing fits to the observed data. In paper V, we use this model to examine observational characteristics of the γ-ray emission expected from optically thick shocks. We find that many key signatures of GRBs, such as the low-energy slope and the peak energy of the spectrum, are naturally reproduced by the model. In Paper IV, we focus on synchrotron radiation from high-energy protons as the possible cause for the γ-ray emission and limit the parameter space where such models are viable. However, within the allowed parameter range, we find that some very specific spectral features are obtained, which are consistent with a subset of observed GBRs.