LIGO/Virgo S191204r: No counterpart candidates in INTEGRAL SPI-ACS prompt observation

Using INTEGRAL/SPI-ACS realtime data (following [1]) we have performed a search for a prompt gamma-ray counterpart of S191204r (GCN 26332). At the time of the event (2019-12-04 17:15:26 UTC, hereafter T0), INTEGRAL was operating in nominal mode. The peak of the event localization probability was at an angle of 118 deg with respect to the spacecraft pointing axis. This orientation implies strongly suppressed (2.2% of optimal) response of ISGRI, somewhat suppressed (41% of optimal) response of IBIS/Veto, and somewhat suppressed (5! 0% of optimal) response of SPI-ACS. The background within +/-300 seconds around the event was very stable (excess variance 1.1). We have performed a search for any impulsive events in INTEGRAL SPI- ACS (as described in [2]) data. We do not detect any significant counterparts and estimate a 3-sigma upper limit on the 75-2000 keV fluence of 3.4e-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 ~2.9e-07 (9.1e-08) erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range. For the mean reported distance 678.0 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 isotropic equivalent luminosity in 1 s (8 s) of 1.4e+49 erg/s (5e+48 erg/s) We report for completeness and in order of FAP, all excesses identified in the search region. We find: 1 possibly associated excess: scale | T | S/N | luminosity ( x 1e+49 erg/s) | FAP 1.7 | 19.1 | 3.8 | 17.9 +/- 4.56 +/- 5.02 | 0.0482 5 likely background excesses: scale | T | S/N | luminosity ( x 1e+49 erg/s) | FAP 0.5 | -30.1 | 3.5 | 30.6 +/- 8.43 +/- 8.56 | 0.44 1.25 | 253 | 4 | 21.6 +/- 5.32 +/- 6.05 | 0.661 0.2 | -8.5 | 3.1 | 4.25 +/- 1.34 +/- 1.19 | 0.753 1.4 | 144 | 3.4 | 18.3 +/- 5.02 +/- 5.12 | 0.814 2 | -152 | 3.3 | 14.9 +/- 4.2 +/- 4.16 | 0.864 Note that FAP estimates (especially at timescales above 2s) may be possibly ! further a ffected by enhanced non-stationary local background noise. This list excludes any excesses for which FAP is close to unity. 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 [GCN OPS NOTE(04dec19): Per author's request, the Circular referenced in the first paragraph was changed from "GCN 99999" to "GCN 26332".]