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AT2018cow - AstroSat CZTI limits on prompt emission

ATel #11809; Y. Sharma (IIT Bombay), V. Bhalerao (IIT Bombay), D. Bhattacharya (IUCAA), A. R. Rao (TIFR), S. Vadawale (PRL)
on 3 Jul 2018; 02:47 UT
Credential Certification: Varun Bhalerao (varunb@iitb.ac.in)

Subjects: X-ray, Gamma Ray, Supernovae, Transient

We report results from offline analysis of data from Astrosat CZTI from last confirmed non-detection 2018-06-13 04:07 UTC (ATel #11738) to detection time 2018-06-16 10:35 UTC (ATel #11727), to look for any coincident hard X-ray flash. CZTI is a coded aperture mask instrument that has considerable effective area for about 29% of the entire sky, but is also sensitive to brighter transients from the entire sky. After removing orbit-wise background variation in data, we binned the data in 0.1s, 1s, and 10s bins to search for coincident spikes in the count rates in the four independent, identical detector quadrants. We selected confidence levels such that the probability of a false trigger in one AstroSat orbit window (~6000s) is 10^-4. We do not find evidence for any hard X-ray transient in this window corresponding to AT2018cow, in the CZTI energy range of 20-200 keV. Using the position (RA, Dec) = (244.00090, 22.26800) as provided in Smartt et al. (ATel #11727), a detailed mass model of the AstroSat satellite to calculate the instrument response in this direction and assuming a power law with alpha = -1.0 for the spectrum, we get the following upper limits for source flux in the 20-200 keV band: 0.1 s: flux = 6.3e-6 ergs cm^-2 s^-1 1.0 s: flux = 9.8e-7 ergs cm^-2 s^-1 10.0 s: flux = 1.5e-7 ergs cm^-2 s^-1 CZTI is built by a TIFR-led consortium of institutes across India, including VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research Organisation funded, managed and facilitated the project.