Arvind Balasubramanian (IISER Pune), Sujay Mate (IIT Bombay), Varun Bhalerao (IIT Bombay), Dipankar Bhattacharya (IUCAA), Ajay Vibhute (IUCAA), Sukanta Bose (IUCAA), Gulab Chand Dewangan (IUCAA), Ranjeev Misra (IUCAA), Sanjit Mitra (IUCAA), A R Rao (TIFR), Tarun Souradeep (IUCAA), Santosh Vadawale (PRL), on behalf of the Astrosat CZTI team report:
We carried out offline analysis of data from Astrosat CZTI in a 100 second window centred on the G298048 trigger time, UT 2017-08-17 12:41:04, 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. Based on the pointing direction of Astrosat at the time of the GW event and the Bayestar skymap provided by LVC (bayestar.fits.gz,0), the sky visible to CZTI has 12% probability of containing the EM counterpart.
CZTI data were de-trended to remove orbit-wise background variation. We then searched data from the four independent, identical quadrants to look for coincident spikes in the count rates. Searches were undertaken by binning the data in 0.1s, 1s and 10s respectively. Statistical fluctuations in count rates were estimated by using data from 5 previous orbits. We selected confidence levels such that the probability of a false trigger in this 100s window is 10^-4. We do not find any evidence for any hard X-ray transient in this window. We convert our count rates into fluence and flux limits by assuming that the source spectrum has band parameters alpha = -1.0, beta = -2.5, Epeak = 300. The upper limits for source fluence and flux in a 30-200 keV band at different timescales are:
Calculating fluxes assuming band parameters alpha = -1.0, beta = -2.5, Epeak = 300
0.1 s: Effective fluence limit= 6.93e-7 ergs/cm^2; flux= 6.93e-6 ergs/cm^2/s
1.0 s: Effective fluence limit= 1.65e-6 ergs/cm^2; flux= 1.65e-6 ergs/cm^2/s
10.0s: Effective fluence limit= 2.05e-6 ergs/cm^2; flux= 2.05e-7 ergs/cm^2/s
The corresponding all-sky maps are uploaded at https://gracedb.ligo.org/apiweb/events/G298048/files/G298048_CZTI_limits.pdf,0
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.