{
  "bibcode": "2017GCN.20755....1S",
  "body": "V. Savchenko (ISDC, University of Geneva, CH)\non behalf of the INTEGRAL group:\nS. Mereghetti (IASF-Milano, Italy),\nC. Ferrigno ((ISDC, University of Geneva, CH),\nE. Kuulkers (ESTEC/ESA, The Netherlands),\nA. Bazzano (IAPS-Roma, Italy), E. Bozzo,\nT. J.-L. Courvoisier (ISDC, University of Geneva, CH)\nS. Brandt (DTU - Denmark) R. Diehl (MPE-Garching, Germany)\nL. Hanlon (UCD, Ireland) P. Laurent (APC, Saclay/CEA, France)\nA. Lutovinov (IKI, Russia) J.P. Roques (CESR, France)\nR. Sunyaev (IKI, Russia) P. Ubertini (IAPS-Roma, Italy)\n\nWe investigated serendipitous INTEGRAL observations carried out at the\ntime of the LIGO/Virgo burst candidate G275404.  The satellite was\npointing at RA=17:44:43 Dec=-25:57:27 (in the direction of the\nGalactic Center region), close to the low-probability area of LIGO\nlocalization. About 2% of the probability was contained in the field\nof view of INTEGRAL IBIS and SPI. Depending on the location within the\nLIGO 90% localization region, as well as the assumed counterpart\nspectrum and duration, the best upper limit is set by the\nanti-coincidence shield of the spectrometer on board of INTEGRAL\n(SPI/ACS), the anti-coincidence shield of the IBIS instrument\n(IBIS/Veto), or by the imaging coded mask instruments (IBIS and SPI).\nThe combination of these instruments covered the full LIGO 90%\nconfidence region and provided stringent constraints on the flux of a\npossible electromagnetic counterpart in the energy range covered by\nthe INTEGRAL instruments.\n\nThe INTEGRAL Burst Alert System (IBAS) did not identify any unusual\ntransients in coincidence with the LIGO/Virgo trigger. The IBAS\ninspects both ISGRI Field of View and all-sky SPI-ACS light curve.\n\nThe IBAS trigger closest in time to the GW candidate (\"Weak\" ISGRI\ntrigger 7706) happened at UTC 2017-02-25 11:32:15.56 and it's location\n(RA=270.48, Dec=-25.08, 3.73 arcmin radius 68% containment,\nstatistical uncertainty only) was outside the 90% probability\ncontainment of the LIGO localization region.\n\nThe nearest excess over the background rate identified by SPI-ACS\noffline pipeline happened 453 seconds after the LIGO/Virgo trigger. It\nhas an SNR of 5.5, duration of 100 ms, and is compatible with cosmic\nray origin.\n\nWe investigated the SPI-ACS, IBIS/Veto, and IBIS/ISGRI light curves\nbetween -500 and +500 s from the trigger time (2017-02-25 18:30:21.3\nUTC) on temporal scales from 0.1 to 100 s, and found no evidence for\nany deviation from the background.  We estimate combined typical\n3-sigma upper limits of 4.3e-7 erg/cm2 (75-2000 keV) for 8s duration\nassuming Band model parameters alpha=-1, beta=-2.5, and E_ peak = 300\nkeV.  To derive a limit for a typical short burst with 1 s duration,\nwe use a harder cutoff power law spectrum with a slope of -0.5 and an\nEpeak = 500 keV: we find a limiting fluence of 1.5e-7 erg/cm2 (75-2000\nkeV) at 3 sigma c.l. These limits assume a perpendicular direction of\nthe burst to the INTEGRAL pointing direction, optimal for SPI-ACS\nsensitivity. However the extent of the region with optimal response\ndepends on the possible source spectrum: we perform a detailed\ncalculation only for a cutoff powerlaw spectrum with a slope of -0.5\nand an Epeak = 500 keV: we estimate that 30% of the LIGO localization\nprobability region is covered with a range of sensitivity from optimal\nfor SPI-ACS (mentioned above) to 50% worse. About 2% of the LIGO\nlocalization in the field of view of IBIS and SPI is covered with at\nleast factor 2 better sensitivity.\n\nThe SPI/ACS light curves, binned at 50 ms, are derived from 91\nindependent detectors with different lower energy thresholds (mainly\nbetween 50 keV and 150 keV) and an upper threshold at about 100\nMeV. The ACS response varies substantially as a function of the source\nincident angle with an optimal effective area of about 6000 cm2 at 1\nMeV.",
  "circularId": 20755,
  "createdOn": 1488129553000,
  "email": "savchenk@apc.in2p3.fr",
  "subject": "LIGO/Virgo G275404: INTEGRAL search for a prompt gamma-ray counterpart",
  "submitter": "Volodymyr Savchenko at APC,Paris  <savchenk@apc.in2p3.fr>",
  "eventId": "LIGO/Virgo G275404"
}