GCN Circular 33815

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 S230518h (GCN 33813
Not found
 
 
).

At the time of the event (2023-05-18 12:59:08 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 136 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (3.7% of optimal) response of ISGRI, somewhat suppressed
(57% of optimal) response of IBIS/Veto, and strongly suppressed (35%
of optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was rather
stable (excess variance 1.3).

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 3.8e-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 ~3.7e-07 (5.8e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

We report for completeness and in order of FAP, all excesses
identified in the search region. We find: 6 likely background
excesses:

T-T0     | scale   | S/N | flux ( x 1e-06 erg/cm2/s)     | FAP    
7.98     | 1.15    | 3.1 |    0.494 +/- 0.211  +/- 0.477  | 0.123
-27.1    | 0.2     | 4.3 |      1.6 +/- 0.512  +/- 1.55   | 0.331
213      | 0.8     | 4.3 |    0.815 +/- 0.254  +/- 0.787  | 0.618
10.1     | 0.1     | 3.4 |     1.76 +/- 0.726  +/- 1.7    | 0.923
128      | 2.1     | 3.2 |    0.386 +/- 0.156  +/- 0.373  | 0.945
-28.9    | 0.3     | 3.3 |        1 +/- 0.415  +/- 0.965  | 0.991


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.

Open SPI-ACS data can be retrieved in here

https://www.astro.unige.ch/mmoda/?T1=2023-05-18T12%3A59%3A00.000&T2=2023-05-18T12%3A59%3A18.000&T_format=isot&data_level=ordinary&instrument=spi_acs&product_type=spi_acs_lc&query_status=new&query_type=Real&time_bin=0.05&time_bin_format=sec

Note that we send GCNs Circulars only when one merger contains at least one neutron star or a
significance counterpart is found by any telescope. This policy may be revised, in which case we'll announce it in a GCN.

[1] Savchenko et al. 2017, A&A 603, A46 
[2] Savchenko et al. 2012, A&A 541A, 122S