The search for fast transients with CZTI

Abstract

The Cadmium Zinc Telluride Imager on AstroSat has proven to be a very effective all-sky monitor in the hard X-ray regime, detecting over three hundred GRBs and putting highly competitive upper limits on X-ray emissions from gravitational wave sources and fast radio bursts. We present the algorithms used for searching for such transient sources in CZTI data, and for calculating upper limits in case of non-detections. We introduce CIFT: the CZTI Interface for Fast Transients, a framework used to streamline these processes. We present details of 88 new GRBs detected by this framework that were previously not detected in CZTI.

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

“…CZTI detected its first GRB, GRB 151006A, on the very first day it was switched on (Bhalerao et al 2015;Rao et al 2016). In the five years since, CZTI has detected more than 400 GRBs, 88 of which are being reported for the first time in Sharma et al (2021). The sensitivity of CZTI is comparable to several other GRB missions (Bhalerao et al 2017a), which has led to many significant results.…”

“…The conversion of effective area to sensitivity depends on the source spectrum, duration, as well as detector noise properties. Sharma et al (2021) reported that the typical minimum detectable count rate for CZTI is 284 counts s À1 for a 1 s burst, and 42 counts s À1 (total 420 counts) for a burst with a 10 s duration. These count rates can be converted into directiondependent sensitivity by assuming 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). Secondly, mass model simulations form a key part of the measurement of polarisation of astrophysical sources (Chattopadhyay et al 2014;Vadawale et al 2015).…”

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

“…The background level is not constant: it varies slowly through the orbit (dominated by proximity to the South Atlantic Anomaly), and also shows variations across orbits (dominated by orbital precession and solar activity). It is known that these background variations for CZTI occur on timescales of several hundreds to thousands of seconds, and is wellapproximated by a quadratic function (Sharma et al 2021;Anumarlapudi et al 2020). Hence we too model background as a quadratic function, b g ðtÞ ¼ aðt À t 0 Þ 2 þbðt À t 0 Þ þ c.…”

“…At energies above $ 100 keV, the instrument and satellite structures become transparent to radiation, giving CZTI sensitivity to sources well outside the primary field of view. CZTI data have been used to detect over 400 Gamma Ray Bursts (GRBs) in the 5 years since launch (Sharma et al 2021), and even study the Crab pulsar at angles from 5 to 70 from the principal axis (Anusree et al 2021). However, CZTI faces the same limitations as other all-sky monitors for studying persistent astrophysical sources.…”