The AstroSat mass model: Imaging and flux studies of off-axis sources with CZTI

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 Â 4:6 (FWHM). The instrument collimators become increasingly transparent at energies above $ 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 et al.

Revisiting the Earth’s atmospheric scattering of X-ray/γ-rays and its effect on space observation: Implication for GRB spectral analysis

“…The problem may be further exacerbated when accounting for the direction-dependent sensitivity of the detector instrument. For instance, the effective area of AstroSat-CZTI varies by more than two orders of magnitude over the entire sky (Mate et al 2021). If the GRB were to be incident from a lowsensitivity direction while the scattered radiation arrives from a high-sensitivity direction for the satellite, the scattered component can be an order of magnitude larger than the incident componentfurther magnifying the effect of any uncertainties in the scattering model.…”

“…?, initially developed for the simulation of high energy physics and gradually got enhanced, in order to be applied to lower energies also. Currently, it is used extensively in high energy detector and instrument simulation (mass model) of most of the astronomical missions including AstroSat-CZTI (Mate et al 2021). Below we describe the construction of the simulation geometry including the Earth's atmosphere, the generic detector at LEO height, the physics models used in the simulation, and the methods for particle generation, data extraction, and analysis.…”

Using collimated CZTI as all-sky X-ray detector based on Earth occultation technique

“…The sensitivity of CZTI to off-axis sources is a strong function of direction, owing to varying degrees of obscuration by various satellite elements. This has been calculated in detail using the CZTI Mass Model (Mate et al 2021). Here, we use an older version of the mass model with minor differences.…”

Exploring sub-MeV sensitivity of AstroSat–CZTI for ON-axis bright sources

“…The response for EB1 is generated using the standard pipeline of the AstroSat and for both EB2 and the EB3 using the GEANT4 simulation of the AstroSat mass model. Details of the AstroSat mass model and its validation has been discussed in Chattopadhyay et al (2019) and Mate et al (2021). We simulated the mass model for 56 mono-energies ranging from 100 keV to 2 MeV (at every 20 keV up to 1 MeV and 200 keV till 2 MeV) for 10 9 photons.…”