The AstroSat Mass Model: Imaging and Flux studies of off-axis sources with CZTI

TL;DR

This paper presents a detailed GEANT4-based mass model of AstroSat's CZTI instrument to simulate its response to off-axis X-ray sources, enabling accurate flux and spectrum calculations for sky observations.

Contribution

The paper introduces a comprehensive mass model of CZTI and AstroSat for improved off-axis source imaging and flux estimation, validated through simulations and observations.

Findings

Validated simulation of imaging response matches observations.

Effective response files enable accurate flux and spectrum conversion.

Model supports analysis of off-axis transient sources.

Abstract

The Cadmium Zinc Telluride Imager (CZTI) on AstroSat is a hard X-ray coded-aperture mask instrument with a primary field of view of 4.6 x 4.6 degrees (FWHM). The instrument collimators become increasingly transparent at energies above $\sim$100 keV, making CZTI sensitive to radiation from the entire sky. While this has enabled CZTI to detect a large number of off-axis transient sources, calculating the source flux or spectrum requires knowledge of the direction and energy dependent attenuation of the radiation incident upon the detector. Here, we present a GEANT4-based mass model of CZTI and AstroSat that can be used to simulate the satellite response to the incident radiation, and to calculate an effective "response file" for converting the source counts into fluxes and spectra. We provide details of the geometry and interaction physics, and validate the model by comparing the simulations of imaging and flux studies with observations. Spectroscopic validation of the mass model is discussed in a companion paper, Chattopadhyay 2021.