GCN Circular 20648
Subject
LIGO/Virgo G270580: AstroSat CZTI upper limits
Date
2017-02-09T17:09:49Z (8 years ago)
From
Varun Bhalerao at IUCAA <varunb@iucaa.in>
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.