Master's Thesis

Localising transients in CZTI data using mass model of AstroSat

This thesis focused 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.

Energy redistribution
Photons of different energies incident on the CZTI detector get redistributed to lower energies due to interactions with satellite body.

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 thesis involved devising a method to compare the simulated transients to real ones to obtain an independent localisation to assist multimessenger observations. You can access the thesis document here.