Difference: GCNCIRCULAR20796 ( 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 hard X-ray flash. CZTI is a coded aperture mask instrument tha= t 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 bay= star skymap provided by LVC (baystar.fits.gz), the sky visible to CZTI has = 27.8% probability of containing the EM counterpart.

CZTI data were de-trended to remove orbit-wise background variation. We the= n searched data from three of the four independent, identical quadrants to = look for coincident spikes in the count rates. For this particular event, d= ata from one quadrant (quadrant C) was rejected due to noisy pixels. Search= es were undertaken by binning the data in 0.1s, 1s and 10s respectively. St= atistical fluctuations in count rates were estimated by using data from 12 = neighbouring orbits. We selected confidence levels such that the probabilit= y of a false trigger in this 100s window is 10^-3. We do not find any evide= nce for any hard X-ray transient in this window. We model the source with a= band function using standard parameters, with alpha =3D -1, beta =3D -2.5 = and E_peak =3D 300 keV. The sensitivity of CZTI varies with direction. We = weight the sensitivity by the baystar probability density map to calculate = upper limits on any coincident emission from the source. In the 30-200 keV = band, the upper limits for source fluence are 4.22e-07 ergs/cm^2, 1.03e-06 = ergs/cm^2 and 2.31e-06 ergs/cm^2 for search timescales of 0.1, 1, and 10 se= conds respectively. The corresponding flux upper limits for the three times= cales are 4.2e-06, 1.03e-06, and 2.31e-07 ergs/cm^2/sec respectively.=20

Plots showing CZTI sensitivity as a function of direction for this event ca= n be found at https://gracedb.ligo.org/apiweb/events/G275697/files/G275697_= CZTI_limits.pdf,0

-- ThomasKupfer - 02 Mar 2017