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GCN Circular 25323

Subject
LIGO/Virgo S190814bv: No counterpart candidates in INTEGRAL SPI-ACS prompt observation
Date
2019-08-14T22:29:45Z (5 years ago)
From
Sandro Mereghetti at IASF-Milano/INAF <sandro.mereghetti@inaf.it>
Sergey Molkov (IKI, Moscow, Russia), Sandro Mereghetti (INAF IASF-Milano,
Italy),  V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland),
J. Rodi (IAPS-Roma, Italy), A. Coleiro (APC, France)

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 S190814bv.

At the time of the event (2019-08-14 21:10:39 UTC, hereafter T0), INTEGRAL
was operating in nominal mode. The peak of the event
localization probability was at an angle of 117 deg with respect to
the spacecraft pointing axis. This orientation implies strongly suppressed
(3.6% of optimal) response of ISGRI, somewhat suppressed (41% of optimal)
response of IBIS/Veto, and somewhat suppressed (5 1%
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 3.1e-07 erg/cm^2 (within the
50% probability containement 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 ~2.6e-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 236.0 Mpc this corresponds to the limit
on the total isotropic equivalent energy in 1 s of 2.1e+48 erg for the
short GRB spectrum and for a long GRB spectrum isotropic equivalent
luminosity in 1 s (8 s) of 1.7e+48 erg/s (6.1e+47 erg/s)

Overall, in the search region we identify 1 possibly associated
excesses:

scale | T | S/N | luminosity ( x 1e49 erg/s) | FAP
3.1 | 40.9 | 4 | 0.183 +/- 0.0432 +/- 0.0621 | 0.0413

5 likely background excesses:

scale | T | S/N | luminosity ( x 1e49 erg/s) | FAP
0.45 | -10.7 | 3.5 | 0.398 +/- 0.114 +/- 0.135 | 0.163
2.5 | 63.9 | 3.1 | 0.155 +/- 0.0481 +/- 0.0525 | 0.443
1.25 | -34.1 | 3.1 | 0.221 +/- 0.0681 +/- 0.0749 | 0.504
0.3 | -42 | 3.8 | 0.523 +/- 0.14 +/- 0.178 | 0.591
0.25 | -53.4 | 4.1 | 0.616 +/- 0.153 +/- 0.209 | 0.616

Note that FAP estimates (especially at timescales above 2s) may be
further affected possibly enhanced non-stationary local backgro und
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


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