TITLE: GCN CIRCULAR NUMBER: 18354 SUBJECT: LIGO/Virgo G184098: INTEGRAL search of temporally coincident prompt hard X-ray emission DATE: 15/09/21 18:52:51 GMT FROM: Carlo Ferrigno at ISDC/INTEGRAL C. Ferrigno (ISDC, University of Geneva, CH), V. Savchenko (APC Paris, France), S. Mereghetti (IASF-Mi, Italy), E. Kuulkers (ESAC/ESA, Madrid, Spain), A. Bazzano (IAPS-Roma, Italy), E. Bozzo, T. J.-L. Courvoisier (ISDC, University of Geneva, CH) The anti-coincidence shield of the spectrometer on board of INTEGRAL (SPI/ACS) is sensitive to gamma-rays above ~50 keV. We investigates the light curve at -300 +900 s from the trigger time (2015-09-14 09:50:45 UTC) on temporal scales from 0.1 to 100 s and derived a maximum post-trial peak significance of ~1.5 sigma with a temporal scale of 9 s, at 200 s before the GW trigger. This excess is not significant. 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 as a function of the incident angle. We estimate typical 3-sigma upper limits corresponding to fluences of 1.4e-6 erg/cm2 for a 100 s duration, 4e-7 erg/cm2 for 10 s, 1.3e-7 erg/cm2 for 1 s, and 6e-8 erg/cm2 for 0.1 s. This assumes an optimal perpendicular direction of the burst to the INTEGRAL pointing direction, low threshold at 100 keV, and Band model parameters â<88><92>1, â<88><92>2.5 with peak E_0 ~ 500 keV. This orientation is compatible with part of the high-probability sky area of the trigger; for the remaining probability region, the upper limits are roughly a factor two higher. A detailed sensitivity map is under computation. The upper limit provided by SPI-ACS disfavours a cosmic origin of the hard X-ray transient detected by Fermi/GBM at 09:50:45.79 (Blackburn et al 2015). We have also investigated the coded-mask imager (IBIS) data on 2015-09-14 in the time interval 09:28 -10:00 UTC, while the instrument was pointing at the sky coordinates RA=270.79 Dec=-31.18, tens of degrees far from the probability peaks of the trigger. No transient point sources appeared in the image at a significance higher than ~3 sigma (post-trial) at a temporal scale of 5 s. The minimum peak flux of gamma-ray bursts detected with IBIS is 0.1 ph/s/cm2 in the 20-200 keV energy band in the central 9x9 deg of the detector, which has a zero response field of view of 29x29 degrees.