Mass modeling and search for transients with AstroSat-CZTI

Abstract

Astrophysical transients are short timescale events, usually lasting from a few seconds to a few hundred seconds, and are sometimes associated with cataclysmic events like death of a star or merger of compact objects. This thesis focuses on the search for and study of astrophysical transients with the Cadmium Zinc Telluride Imager (CZTI), on-board AstroSat. At energies higher than 100 keV, CZTI functions as an open detector and is sensitive to transients from all over the sky. This makes it a very sensitive detector for various classes of astrophysical transients: Gamma Ray Bursts (GRBs), and even X-ray counterparts to gravitational wave events. As an all-sky detector, CZTI can detect transients shining through the satellite body. The analysis, localisation, and interpretation of this data requires a complete understanding of the absorption, scattering, and reprocessing of the incident photons by various elements of the satellite. Such interactions are impossible to compute analytically in a photon starved regime, since many of them are non-deterministic. This warrants the need for a simulation. A simulated ``mass model'' of the satellite has been developed using the GEANT4 simulation package. The scope of this thesis is to enhance the mass model and use it for studying various transients detected by CZTI, and to place strong upper limits on others.