Comparative studies of astrophysical gamma-ray events detected with FERMI/GBM

Official URL: https://risc01.sabanciuniv.edu/record=b2579466_(Table of contents)

The aim of this study is to define well-constrained parameters for classification of gamma-ray transient events found in comprehensive untriggered-event searches. To this end, we studied the triggered events detected with Gamma-ray Burst Monitor (GBM) and classified by the Flight Software (FSW) on the Fermi Gamma-ray Space Telescope. The main classifications of gamma-ray events in Fermi/GBM Trigger Catalog are gammaray bursts (GRB), soft gamma repeaters (SGR), terrestrial gamma-ray flashes (TGF), solar flares (SFL), transients (TRANSNT), and charged-particle events. In this thesis, we present a comparative study of these events, based on which we suggest possible classification methods and demonstrate how this study will be used to classify untriggered GBM events. We first performed duration and hardness analysis to identify characteristic properties of event types. The durations of all triggered events were calculated via Bayesian Block method, and the duration distribution of each class was obtained. Then, all burst types were divided into two categories as short (. 2 s ; SGRB, SGR, TGF) and long (& 2 s ; LGRB, SFL, TRANSNT) events based on the results. For further classification, we calculated spectral hardness in the energy range of 10-2000 keV. The least overlap in the hardness distributions of different event classes was obtained when we used 50 keV and 75 keV as breaking (pivot) points between the soft and hard energy bands for short and long lasting events, respectively. We additionally developed an algorithm to differentiate transient bursts from pulsations and Earth occultations of known astrophysical sources, all of which are classified as transients in the Fermi/GBM Trigger Catalog. Moreover, we differentiated particle events with the help of the McIlwain L coordinate of the spacecraft and maximum to minimum ratio of the count rates of the 12 sodium iodide (NaI(Tl)) detectors in the energy range of 50-300 keV at the time of the trigger.