GCN Circular 19401

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.