Difference: GCNCIRCULAR20766 ( vs. 1)

Sujay Mate (IUCAA), Varun Bhalerao (IIT Bombay), Dipankar Bhattacharya (IUC= AA), Sukanta Bose (IUCAA), Gulab Chand Dewangan (IUCAA), Ranjeev Misra (IUC= AA), 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 G275404 trigger time, UT 2017-02-25 18:30:21.374, to = look for any coincident hard X-ray flash. CZTI is a coded aperture mask ins= trument that has considerable effective area for about 29% of the entire sk= y. Based on the pointing direction of AstroSat at the time of the GW event = and the baystar skymap provided by LVC (baystar.fits.gz), the sky visible t= o CZTI has 22.1% probability of containing the EM counterpart.

CZTI data were de-trended to remove orbit-wise background variation. We the= n searched data from the four independent, identical quadrants to look for = coincident spikes in the count rates. Searches were undertaken by binning t= he data in 0.1s, 1s and 10s respectively. Statistical fluctuations in count= rates were estimated by using data from 10 neighbouring orbits. We selecte= d confidence levels such that the probability of a false trigger in this 10= 0s window is 10^-4. We do not find any evidence for any hard X-ray transien= t in this window. We model the source with a band function using standard p= arameters, with alpha 3D -1, beta =3D -2.5 and E_peak =3D 300 keV. The sen sitivity of CZTI varies with direction. We weight the sensitivity by the ba= ystar probability density map to calculate upper limits on any coincident e= mission from the source. In the 30-200 keV, the upper limits for source flu= ence are 1.6e-07 ergs/cm^2, 3.9e-07 ergs/cm^2 and 9.0e-07 ergs/cm^2 for sea= rch timescales of 0.1, 1, and 10 seconds respectively. The corresponding fl= ux upper limits for the three timescales are 1.6e-06, 3.9e-07, and 9.0e-08 = ergs/cm^2/sec respectively.

Plots showing CZTI sensitivity as a function of direction for this event ca= n be found at https://gracedb.ligo.org/apiweb/events/G275404/files/G275404_= CZTI_limits.pdf,0

-- ThomasKupfer - 27 Feb 2017