{
  "bibcode": "2017GCN.20648....1M",
  "body": "Sujay Mate (IUCAA), Varun Bhalerao (IUCAA), Dipankar Bhattacharya (IUCAA),\nSukanta Bose (IUCAA), Gulab Chand Dewangan (IUCAA), Ranjeev Misra\n(IUCAA), Sanjit Mitra (IUCAA), A R Rao (TIFR), Tarun Souradeep (IUCAA),\nSantosh Vadawale (PRL), on behalf of the Astrosat CZTI team report:\n\nWe carried out offline analysis of data from AstroSat CZTI in a 100\nsecond window centred on the G270580 trigger time, UT 2017-01-20\n12:31:00.35, to look for any coincident hard X-ray flash. CZTI is a\ncoded aperture mask instrument that has considerable effective area for\nabout 29% of the entire sky. Based on the pointing direction of AstroSat\nat the time of the GW event and the LIB skymap provided by LVC\n(LIB_skymap.fits.gz,0), the sky visible to CZTI has 18.6% probability of\ncontaining the EM counterpart.\n\nCZTI data were de-trended to remove orbit-wise background variation. We\nthen searched data from the four independent, identical quadrants to\nlook for coincident spikes in the count rates. Searches were undertaken\nby binning the data in 0.1s, 1s and 10s respectively. Statistical\nfluctuations in count rates were estimated by using data from 10\nneighbouring orbits. We selected confidence levels such that the\nprobability of a false trigger in this 100s window is 10^-4. We do not\nfind any evidence for any hard X-ray transient in this window. We model\nthe source with a band function as used in the INTEGRAL search\n(Savchenko et al, GCN Circular 20496), with alpha = 1, beta = -2.5 and\nE_peak = 300 keV. The sensitivity of CZTI varies with direction. We\nweight the sensitivity by the LIB probability density map to calculate\nupper limits on any coincident emission from the source. In the 30-200\nkeV, the upper limits for source fluence in are 2.05e-07 ergs/cm^2,\n4.75e-07 ergs/cm^2 and 1.36e-06 ergs/cm^2 for search timescales of 0.1,\n1, and 10 seconds respectively. The corresponding flux upper limits for\nthe three timescales are 2.05e-06, 4.75e-07, and 1.36e-07 ergs/cm^2/sec\nrespectively.\n\nPlots showing CZTI sensitivity as a function of direction for this event can be found at\nhttps://gracedb.ligo.org/apiweb/events/G270580/files/G270580_CZTI_limits.pdf,0\n\nCZTI is built by a TIFR-led consortium of institutes across India,\nincluding VSSC, ISAC, IUCAA, SAC and PRL. The Indian Space Research\nOrganisation funded, managed and facilitated the project.",
  "circularId": 20648,
  "createdOn": 1486660189000,
  "email": "varunb@iucaa.in",
  "subject": "LIGO/Virgo G270580: AstroSat CZTI upper limits",
  "submitter": "Varun Bhalerao at IUCAA  <varunb@iucaa.in>",
  "eventId": "LIGO/Virgo G270580"
}