TITLE: GCN CIRCULAR NUMBER: 20648 SUBJECT: LIGO/Virgo G270580: AstroSat CZTI upper limits DATE: 17/02/09 17:09:49 GMT FROM: Varun Bhalerao at IUCAA Sujay Mate (IUCAA), Varun Bhalerao (IUCAA), Dipankar Bhattacharya (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 G270580 trigger time, UT 2017-01-20 12:31:00.35, 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 LIB skymap provided by LVC (LIB_skymap.fits.gz,0), the sky visible to CZTI has 18.6% 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 10 neighbouring 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 model the source with a band function as used in the INTEGRAL search (Savchenko et al, GCN Circular 20496), with alpha = 1, beta = -2.5 and E_peak = 300 keV. The sensitivity of CZTI varies with direction. We weight the sensitivity by the LIB probability density map to calculate upper limits on any coincident emission from the source. In the 30-200 keV, the upper limits for source fluence in are 2.05e-07 ergs/cm^2, 4.75e-07 ergs/cm^2 and 1.36e-06 ergs/cm^2 for search timescales of 0.1, 1, and 10 seconds respectively. The corresponding flux upper limits for the three timescales are 2.05e-06, 4.75e-07, and 1.36e-07 ergs/cm^2/sec respectively. Plots showing CZTI sensitivity as a function of direction for this event can be found at https://gracedb.ligo.org/apiweb/events/G270580/files/G270580_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.