TITLE: GCN CIRCULAR NUMBER: 20768 SUBJECT: LIGO/Virgo G275697: INTEGRAL search for a prompt gamma-ray counterpart DATE: 17/02/27 23:08:20 GMT FROM: Volodymyr Savchenko at APC,Paris V. Savchenko (ISDC, University of Geneva, CH) on behalf of the INTEGRAL group: S. Mereghetti (IASF-Milano, Italy), C. Ferrigno ((ISDC, University of Geneva, CH), E. Kuulkers (ESTEC/ESA, The Netherlands), A. Bazzano (IAPS-Roma, Italy), E. Bozzo, T. J.-L. Courvoisier (ISDC, University of Geneva, CH) S. Brandt (DTU - Denmark) R. Diehl (MPE-Garching, Germany) L. Hanlon (UCD, Ireland) P. Laurent (APC, Saclay/CEA, France) A. Lutovinov (IKI, Russia) J.P. Roques (CESR, France) R. Sunyaev (IKI, Russia) P. Ubertini (IAPS-Roma, Italy) 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 low-probability 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 anti-coincidence shield of the spectrometer on board of INTEGRAL (SPI/ACS), the anti-coincidence 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 all-sky SPI-ACS light curve. We investigated the SPI-ACS, IBIS/Veto, and IBIS/ISGRI light curves between -500 and +500 s from the trigger time (2017-02-27 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 3-sigma upper limits of 4.3e-7 erg/cm2 (75-2000 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 1.5e-7 erg/cm2 (75-2000 keV) at 3 sigma c.l. These limits assume a perpendicular direction of the burst to the INTEGRAL pointing direction, optimal for SPI-ACS 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 SPI-ACS (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. The SPI/ACS light curves, binned at 50 ms, are derived from 91 independent detectors with different lower energy thresholds (mainly between 50 keV and 150 keV) and an upper threshold at about 100 MeV. The ACS response varies substantially as a function of the source incident angle with an optimal effective area of about 6000 cm2 at 1 MeV.