TITLE: GCN CIRCULAR NUMBER: 25309 SUBJECT: HAWC-190806A: upper limits from INTEGRAL SPI-ACS and IBIS prompt observation DATE: 19/08/09 22:44:12 GMT FROM: Volodymyr Savchenko at ISDC,U of Geneve 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, and following [1], we have performed a search for prompt gamma-ray counterparts of HAWC-190806A (GCN 25284). At the time of the event (2019-08-06 13:20:48 UTC, hereafter T0), INTEGRAL was operating in nominal mode. The event localization was at an angle of 33 deg with respect to thespacecraft pointing axis. This orientation implies a strongly suppressed(17% of optimal) response of ISGRI, a strongly suppressed (31% ofoptimal) response of IBIS/Veto, and a near-optimal (75% of optimal)response of SPI-ACS. This orientation implies a favorableresponse of IBIS/PICsIT. Thebackground within +/-300 seconds around  the event was ratherstable (excess variance 1.3, slightly higher than usual). 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 3.3) at a 3.5s time scale atT0-1.8s. The peak count rate of the signal in SPI-ACS is 713 cts/s, which corresponds to a flux between 1.2e-07 and 1.7e-07 erg/cm2/s, depending on thelocation within the source localization region, and assuming a typical short GRB spectrum (an exponentially cut offpower law with alpha=-0.5 and Ep=600 keV). This estimate does not take into account the uncertaintyrelated to the unknown event spectrum, the 20% systematic uncertainty on theresponse, or any dead-time correction. We derive preliminary estimate of the association FAP at the level of 0.011 (2.3 sigma). This tentatively indicates a random association. Further analysis, taking into account accurate FAR measured on the basis of the study ofthe background during days surrounding the event might be reported in aforthcoming circulars. In the IBIS/PICsIT analysis, we do not detect any relevant signal, despite an orientation favorable for this instrument. This disfavours a cosmic origin of the observed SPI-ACS excess. Otherwise, we estimate a 3-sigma upper limit on the 75-2000 keV fluence of 2e-07 erg/cm^2 for a burst lasting less than 1 s with a characteristic short GRB spectrum  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 ~1.7e-07 (6.9e-08) erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range. In addition, we identify the following low-S/N excesses, which likely belong to thebackground: scale | T     | S/N | flux ( x 1e-7 erg/cm2/s) | FAP 2.9   | -135  | 3.8 | 1.9 ± 0.5 ± 0.8          | 0.2 2.7   | -238  | 4.2 | 2.1 ± 0.5 ± 0.8          | 0.231 0.1   | -9.26 | 4.2 | 10.6 ± 2.7 ± 4.4         | 0.248 0.8   | 92.7  | 4.1 | 3.6 ± 0.9 ± 1.5          | 0.341 0.5   | -113  | 3.9 | 4.5 ± 1.2 ± 1.8          | 0.786 1.1   | -275  | 3.9 | 3.0 ± 0.8 ± 1.2          | 0.896 Note that FAP estimates (especially at timescales above 2s) may be further affected by possibly 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