LIGO/Virgo G275697: INTEGRAL search for a prompt gammaray counterpart
Abstract
We investigated serendipitous INTEGRAL observations carried out at %the time of the LIGO/Virgo burst candidate G275697. The satellite was pointing at RA=15:29:10 Dec=55:09:03 (in the direction of the Norma Region), close to the lowprobability area of LIGO localization. About 6.5% of the probability was contained in the field of view of INTEGRAL IBIS and SPI. Depending on the location within the LIGO 90% localization region, as well as the assumed counterpart spectrum and duration, the best upper limit is set by the anticoincidence shield of the spectrometer on board of INTEGRAL (SPI/ACS), the anticoincidence shield of the IBIS instrument (IBIS/Veto), or by the imaging coded mask instruments (IBIS and SPI). The combination of these instruments covered the full LIGO 90% confidence region and provided stringent constraints on the flux of a possible electromagnetic counterpart in the energy range covered by the INTEGRAL instruments.
The INTEGRAL Burst Alert System (IBAS) did not identify any unusual transients in coincidence with the LIGO/Virgo trigger. The IBAS inspects both ISGRI Field of View and allsky SPIACS light curve.
We investigated the SPIACS, IBIS/Veto, and IBIS/ISGRI light curves between 500 and +500 s from the trigger time (20170227 18:57:31.375 UTC) on temporal scales from 0.1 to 100 s, and found no evidence for any deviation from the background. We estimate combined typical 3sigma upper limits of 4.3e7 erg/cm2 (752000 keV) for 8s duration assuming Band model parameters alpha=1, beta=2.5, and E_ peak = 300 keV. To derive a limit for a typical short burst with 1 s duration, we use a harder cutoff power law spectrum with a slope of 0.5 and an Epeak = 500 keV: we find a limiting fluence of 1.5e7 erg/cm2 (752000 keV) at 3 sigma c.l. These limits assume a perpendicular direction of the burst to the INTEGRAL pointing direction, optimal for SPIACS sensitivity. However the extent of the region with optimal response depends on the possible source spectrum: we perform a detailed calculation only for a cutoff powerlaw spectrum with a slope of 0.5 and an Epeak = 500 keV: we estimate that 20% of the LIGO localization probability region is covered with a range of sensitivity from optimal for SPIACS (mentioned above) to 50% worse. About 2% of the LIGO localization in the field of view of IBIS and SPI is covered with at least factor 2 better sensitivity.
 Publication:

GRB Coordinates Network
 Pub Date:
 February 2017
 Bibcode:
 2017GCN.20768....1S