TITLE: GCN CIRCULAR NUMBER: 25190 SUBJECT: LIGO/Virgo S190728q: No counterpart candidates in INTEGRAL SPI-ACS prompt observation DATE: 19/07/28 09:33:07 GMT FROM: Maeve Doyle at U College Dublin, Ireland Maeve Doyle (UCD, Ireland), Alexander Lutovinov (IKI, Moscow) V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland) J. Rodi (IAPS-Roma, Italy) A. Coleiro (APC, France) S. Mereghetti (INAF IASF-Milano, Italy) on behalf of the INTEGRAL multi-messenger collaboration: https://www.astro.unige.ch/cdci/integral-multimessenger-collaboration Using INTEGRAL/SPI-ACS realtime data (following [1]) we have performed a search for a prompt gamma-ray counterpart of S190728q (GCN 25187). At the time of the event (2019-07-28 06:45:10 UTC, hereafter T0), INTEGRAL was operating in nominal mode. The peak of the event localization probability was at an angle of 91 deg with respect to the spacecraft pointing axis. This orientation implies strongly suppressed (5.3% of optimal) response of ISGRI, strongly suppressed (13% of optimal) response of IBIS/Veto, and somewhat suppressed (58% of optimal) response of SPI-ACS. The background within +/-300 seconds around the event was rather stable (excess variance 1.2). We have performed a search for any impulsive events in INTEGRAL SPI- ACS (as described in [2]) data. We detect a marginal event (S/N 5.22, local FAP 3.27 equivalent sigma) at 0.05s time scale at T0+201.01. Peak count rate of the signal in SPI-ACS is 9629 cts/s, which corresponds to 2.03e-06 to 2.39e-06 erg/cm2/s depending on the location within the 50% LIGO/Virgo Event localization region and assuming a typical short GRB spectrum. This estimate does not explore uncertainty related to the unknown event spectrum, systematic uncertainty on the response of 20%, or any dead-time correction. For the mean distance to the source of 795.0 Mpc this corresponds to the isotropic-equivalent luminosity between 1.53e+50 to 1.8e+50 erg/s. We derive a preliminary estimate of the associated FAP higher than 0.5 (less than 0.67 sigma). This tentatively indicates a random coincidence. Further analysis, taking into account accurate FAR measured on the basis of the study of the background during days surrounding the event will be reported in forthcoming circulars. Otherwise, we estimate a 3-sigma upper limit on the 75-2000 keV fluence of 2.6e-07 erg/cm^2 (within the 50% probability containment region of the source localization) for a burst lasting less than 1 s with a characteristic short GRB spectrum (an exponentially cut off power law with alpha=-0.5 and Ep=600 keV) occurring at any time in the interval within 300 s around T0. For a typical long GRB spectrum (Band function with alpha=-1, beta=-2.5, and Ep=300 keV), the derived peak flux upper limit is ~2.1e-07 (7.9e-08) erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range. In this case, For the mean reported distance 795.0 Mpc this corresponds to the limit on the total isotropic equivalent energy in 1 s of 1.9e+49 erg for the short GRB spectrum and for a long GRB spectrum the isotropic equivalent luminosity can be estimated in 1 s (8 s) as 1.6e+49 erg/s (6e+48 erg/s). All results quoted are preliminary. This circular is an official product of the INTEGRAL Multi-Messenger team. [1] Savchenko et al. 2017, A&A 603, A46 [2] Savchenko et al. 2012, A&A 541A, 122S