Mathematics – Logic
Scientific paper
Aug 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000phdt.........1r&link_type=abstract
Thesis (PhD). Stockholm University. ISBN 91-7265-106-7.
Mathematics
Logic
Gamma-Ray Bursts, Spectral Evolution, Grb Pulse Spectra, Hardness-Intensity Correlation, Batse Observation
Scientific paper
Ever since their discovery at the end of the 1960s, the occasional, short flashes of gamma-rays, denoted gamma-ray bursts (GRBs), have been some of the most enigmatic phenomena to have been encountered in astrophysics. Large resources are being put into the quest to understand these objects and great progress has been made. In particular, during recent years it has become evident that GRBs lie at large, cosmological distances, which implies, from the measured energies, that they are the most powerful explosions in the Universe since its creation. They are detected approximately once per day and occur in an average galaxy probably once every 10 million years. This thesis discusses various aspects of the spectral and temporal behaviour of the gamma-ray emission in long and bright pulses of prompt GRBs. This is studied both by analytical derivations and through the study of data from the Burst And Transient Source Experiment (BATSE) on board the Compton Gamma-Ray Observatory (CGRO) satellite. A self-consistent formulation of the spectral and temporal evolution during the decay of a GRB pulse is presented and explored. This leads to the finding that the decay of GRB pulses can be described by a particular power-law function and that there is a bimodality in the distribution of the associated power-law index. The importance of studying the temporally resolved spectra during a GRB, and especially during a pulse, is stressed. These spectra have a direct connection with the underlying emission process (possibly affected by relativistic effects due to the outflow emitting the gamma-rays). The time-integrated spectrum, on the other hand, reflects mainly the spectral evolution. Analytical results are given, which connect the properties of the time-integrated spectrum with those of the time-resolved spectra, and are thus useful when studying observed GRB pulse spectra. The correlation between the peak energy of the instantaneous spectrum (as a measure of spectral hardness) and the corresponding intensity during a burst is of great importance and special attention is devoted to it. A new method for studying this relation is introduced, which is advantageous when a large fraction of the bolometric flux lies outside the narrow band over which the spectrum is observed. It is found that this correlation, between the hardness of the spectrum and the intensity, among pulses within a single burst is more similar than that among pulses from different bursts. Also, a characteristic signature in the hardness-intensity correlation is found, which is interpreted as the result of heavily overlapping pulses. Furthermore, there is an indication that this correlation might be the fundamental one, valid for GRB pulses even if their light curves have diverse behaviours. Finally, a new parametric function for fitting spectra is presented. This function is especially useful for broad spectra and it is compared with previously introduced functions. This young and highly active field of astrophysical research will continue to blossom with the help of numerous new satellite missions and will help in unveiling the secrets of GRBs and may eventually provide a way of studying the early Universe.
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