Sub-MeV spectroscopy with AstroSat-CZT imager for gamma ray bursts

Abstract

Cadmium-Zinc-Telluride Imager (CZTI) onboard AstroSat has been a prolific Gamma-Ray Burst (GRB) monitor. While the 2-pixel Compton scattered events (100-300 keV) are used to extract sensitive spectroscopic information, the inclusion of the low-gain pixels ( $ 20% of the detector plane) after careful calibration extends the energy range of Compton energy spectra to 600 keV. The new feature also allows single-pixel spectroscopy of the GRBs to the sub-MeV range which is otherwise limited to 150 keV. We also introduced a new noise rejection algorithm in the analysis ('Compton noise'). These new additions not only enhances the spectroscopic sensitivity of CZTI, but the sub-MeV spectroscopy will also allow proper characterization of the GRBs not detected by Fermi. This article describes the methodology of single, Compton event and veto spectroscopy in 100-900 keV combined for the GRBs detected in the first year of operation. CZTI in last five years has detected $ 20 bright GRBs. The new methodologies, when applied on the spectral analysis for this large sample of GRBs, has the potential to improve the results significantly and help in better understanding the prompt emission mechanism.

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

“…Singhal et al (2021) used the same older version to measure fluxes of bright sources by the earth occultation technique. (Chattopadhyay et al 2021) demonstrated the validity of the mass model simulations for spectroscopic analyses, using this current mass model version.…”

“…Using an older version of this mass model with minor differences, a satisfactory agreement in spectral parameters between Fermi and CZTI was reported in Chattopadhyay et al (2019) for GRB 160821A. Chattopadhyay et al (2021) used this latest version of the mass model to expand the work to eleven bright GRBs detected in the first year of CZTI operation and showcases capability of CZTI as a sub-MeV spectrometer, and thereby validating this AstroSat-CZTI mass model further.…”

“…In this paper, we give the full details of the mass model and give results on imaging and flux studies. A companion paper (Chattopadhyay et al 2021) discusses details of sub-MeV spectroscopy using the mass model, and compares the results with various other sub-MeV spectroscopic methods.…”

“…First, these simulations are used for mapping the observed count rates and spectra to a source spectrum. We discuss examples with count rates in this paper and spectra are discussed in Chattopadhyay et al (2021). The same technique is used to calculate flux upper limits for non-detections, based on an assumed source spectrum (Sharma et al 2021).…”

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

“…Another challenge is to calibrate these pixels in the absence of any mono energetic lines at energies above 100 keV to estimate their gains. A careful calibration of these pixels have been attempted in a companion paper by Chattopadhyay et al (2021) in this issue, and spectroscopy up to 900 keV is explored for Gamma Ray Bursts (GRBs). Compared to the ON-axis sources, spectroscopy of GRBs is relatively easy because of the availability of background (from the immediate pre-GRB and post-GRB observations) and the significantly higher signal to noise ratio for the GRBs.…”