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

Abstract

The Cadmium-Zinc-Telluride Imager (CZTI) onboard AstroSat is designed for hard X-ray imaging and spectroscopy in the energy range of 20-100 keV. The CZT detectors are of 5-mm thickness and hence have good efficiency for Compton interactions beyond 100 keV. The polarisation analysis using CZTI relies on such Compton events and have been verified experimentally. The same Compton events can also be used to extend the spectroscopy up to 380 keV. Further, it has been observed that about 20% pixels of the CZTI detector plane have low gain, and they are excluded from the primary spectroscopy. If these pixels are included, then the spectroscopic capability of CZTI can be extended up to 500 keV and further up to 700 keV with a better gain calibration in the future. Here we explore the possibility of using the Compton events as well as the low gain pixels to extend the spectroscopic energy range of CZTI for ON-axis bright X-ray sources. We demonstrate this technique using Crab observations and explore its sensitivity.

High Hard X-Ray Polarization in Cygnus X-1 Confined to the Intermediate Hard State: Evidence for a Variable Jet Component

“…This results from the inclined orbit of AstroSat where some of the orbits pass through the outskirts of the South Atlantic Anomaly giving an increase in count rate when the spacecraft is in these regions of the orbit. Because of the rotation of the Earth, this phase of high count rate reappears every ∼24 hr and this has been seen in other AstroSat instruments also (Antia et al 2022) apart from CZTI (Kumar et al 2021(Kumar et al , 2022. To correct for this effect, we try to match the phases of orbital variation of the count rate during background and Cygnus X-1 observations using a cross-correlation method (Kumar et al 2021) and identify the common or phase-matched regions.…”

“…Because of the rotation of the Earth, this phase of high count rate reappears every ∼24 hr and this has been seen in other AstroSat instruments also (Antia et al 2022) apart from CZTI (Kumar et al 2021(Kumar et al , 2022. To correct for this effect, we try to match the phases of orbital variation of the count rate during background and Cygnus X-1 observations using a cross-correlation method (Kumar et al 2021) and identify the common or phase-matched regions. These phase-matched regions are used to correct for the longterm variation in data before background subtraction.…”

Extending the energy range of AstroSat-CZTI up to 380 keV with compton spectroscopy

“…An alternate method (the 'phase match method') developed for background subtraction is to match the phase of the background and Crab light curves. The phase match method has been discussed and validated in detail in Kumar et al (2021).…”

“…In Kumar et al (2021), we attempted sub-MeV spectroscopy (up to 700 keV) for Crab pulsar and nebula (hereafter referred to as 'Crab') with the use of 'low gain pixels' in CZTI. These are 20% of the total pixels with onboard post-launch gain found to be lower than the other CZTI pixels and hence are sensitive to photons up to 700 keV energy.…”