TITLE: GCN CIRCULAR NUMBER: 33890 SUBJECT: LIGO/Virgo/KAGRA S230529ay: No counterpart candidates in INTEGRAL SPI-ACS prompt observation DATE: 23/05/29 19:14:52 GMT FROM: Volodymyr Savchenko at ISDC, University of Geneva; LASTRO, EPFL volodymyr.savchenko@unige.ch
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 S230529ay (GCN 33889).
At the time of the event (2023-05-29 18:15:00 UTC, hereafter T0), INTEGRAL was operating in nominal mode. The peak of the event localization probability was at an angle of 122 deg with respect to the spacecraft pointing axis. This orientation implies strongly suppressed (2.8% of optimal) response of ISGRI, somewhat suppressed (57% of optimal) response of IBIS/Veto, and somewhat suppressed (47% of optimal) response of SPI-ACS.
The background within +/-300 seconds around the event was somewhat unstable (excess variance 1.8). In addition, we note that excess noise at near Hz frequency has been noted in SPI-ACS data in recent days, related to satellite environment conditions.
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 ~3.2e-07 (8e-08) erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.
For the mean reported distance 217.0 Mpc this corresponds to the limit on the total isotropic equivalent energy in 1 s of 1.9e+48 erg for the short GRB spectrum and for a long GRB spectrum isotropic equivalent luminosity in 1 s (8 s) of 9.4e+47 erg/s (4.5e+47 erg/s)
We report for completeness and in order of FAP, all excesses identified in the search region. We find: 4 likely background excesses:
T-T0 | scale | S/N | luminosity ( x 1e+48 erg/s) | FAP 262 | 1.7 | 4.7 | 33.8 +/- 8.3 +/- 30.4 | 0.276 -12.1 | 0.45 | 3.1 | 3.87 +/- 1.62 +/- 3.48 | 0.498 -6.59 | 0.1 | 3.3 | 8.42 +/- 3.46 +/- 7.58 | 0.728 -91.3 | 0.95 | 3.5 | 3.07 +/- 1.11 +/- 2.76 | 0.756
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.
All results quoted are preliminary.
This circular is an official product of the INTEGRAL Multi-Messenger team.
Note that we send GCNs Circulars only when one of the following conditions is met: merger contains at least one neutron star, a significant counterpart is reported.
[1] Savchenko et al. 2017, A&A 603, A46 [2] Savchenko et al. 2012, A&A 541A, 122S