LIGO/Virgo S190910d: 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 S190910d (GCN 25695). At the time of the event (2019-09-10 01:26:19 UTC, hereafter T0), INTEGRAL was operating in nominal mode. The peak of the event localization probability was at an angle of 66 deg with respect to the spacecraft pointing axis. This orientation implies strongly suppressed (18% of optimal) response of ISGRI, somewhat suppressed (42% of optimal) response of IBIS/Veto, and near-optimal (91% of optimal) response of SPI-ACS. The background within ±300 seconds around the event was very stable (excess variance 1.2). 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 2.2e-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 ~1.8e-07 (5.4e-08) erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range. For the mean reported distance 606.0 Mpc this corresponds to the limit on the total isotropic equivalent energy in 1 s of 9.5e+48 erg for the short GRB spectrum and for a long GRB spectrum isotropic equivalent luminosity in 1 s (8 s) of 7.1e+48 erg/s (2.4e+48 erg/s) We report for completeness and in order of FAP, all excesses identified in the search region. We find: 5 likely background excesses: scale | T | S/N | luminosity ( x 1e+49 erg/s) | FAP 0.05 | 270 | 12 | 30.7 +/- 3.24 +/- 23.6 | 0.355 0.35 | -30.3 | 3.4 | 2.96 +/- 1.14 +/- 2.28 | 0.72 0.6 | 136 | 3.9 | 25.7 +/- 8.66 +/- 19.8 | 0.819 1.7 | 234 | 3.6 | 13.6 +/- 5.13 +/- 10.5 | 0.87 1.25 | -269 | 3.8 | 17.7 +/- 5.99 +/- 13.6 | 0.934 Note that FAP estimates (especially at timescales above 2s) may be possibly further affected by enhanced non-stationary local background noise. This list excludes any excesses for which FAP is close to unity. We additionally note that the excess at T0+270s despite high S/N of 12 has FAP of only 0.355, due to the large offset from T0 and high rate of excesses at this timescale. 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