TITLE: GCN CIRCULAR NUMBER: 19401 SUBJECT: LIGO/Virgo G211117: Astrosat CZTI upper limits DATE: 16/05/07 09:25:01 GMT FROM: Varun Bhalerao at IUCAA Varun Bhalerao (IUCAA), 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 G211117 trigger time, UT 2015-12-26 03:38:53.648, 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 LALInference skymap provided by LVC (LALInference_skymap.fits.gz,0), the sky visible to CZTI has 32% 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 20 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 convert our count rates into fluence and flux limits by assuming that the source spectrum is a power-law with a photon index Gamma = 1 and 2 respectively ( N(E) \propto E^-\Gamma ). The upper limits for source fluence and flux in a 30-200 keV band at different timescales are: Calculating fluxes assuming photon power law index Gamma = 1.0 0.1 s: Effective fluence limit= 2.3e-7 ergs/cm^2; flux= 2.3e-6 ergs/cm^2/s 1.0 s: Effective fluence limit= 5.8e-7 ergs/cm^2; flux= 5.8e-7 ergs/cm^2/s 10.0s: Effective fluence limit= 1.5e-6 ergs/cm^2; flux= 1.5e-7 ergs/cm^2/s Calculating fluxes assuming phton perlaw index Gamma = 2.0 0.1 s: Effective fluence limit= 2.9e-7 ergs/cm^2; flux= 2.9e-6 ergs/cm^2/s 1.0 s: Effective fluence limit= 7.5e-7 ergs/cm^2; flux= 7.5e-7 ergs/cm^2/s 10.0s: Effective fluence limit= 1.9e-6 ergs/cm^2; flux= 1.9e-7 ergs/cm^2/s 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.