[vsnet-grb-info 23091] LIGO/Virgo S190728q: No counterpart candidates in INTEGRAL SPI-ACS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25190
SUBJECT: LIGO/Virgo S190728q: No counterpart candidates in INTEGRAL SPI-ACS prompt observation
DATE:    19/07/28 09:33:07 GMT
FROM:    Maeve Doyle at U College Dublin, Ireland  <maeve.doyle.1 at ucdconnect.ie>

Maeve Doyle (UCD, Ireland), Alexander Lutovinov (IKI, Moscow)
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 S190728q (GCN 25187).

At the time of the event (2019-07-28 06:45:10 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 91 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(5.3% of optimal) response of ISGRI, strongly suppressed (13% of
optimal) response of IBIS/Veto, and somewhat suppressed (58% of
optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was rather
stable (excess variance 1.2).

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 5.22, local FAP
3.27 equivalent sigma) at 0.05s time scale at T0+201.01. Peak count
rate of the signal in SPI-ACS is 9629 cts/s, which corresponds to
2.03e-06 to 2.39e-06 erg/cm2/s depending on the location within the
50% LIGO/Virgo Event localization region and assuming a typical short GRB
spectrum. This estimate does not explore uncertainty related to the
unknown event spectrum, systematic uncertainty on the response of 20%,
or any dead-time correction. For the mean distance to the source of
795.0 Mpc this corresponds to the isotropic-equivalent luminosity
between 1.53e+50 to 1.8e+50 erg/s. We derive a preliminary estimate of
the associated FAP higher than 0.5 (less than 0.67 sigma). This
tentatively indicates a random coincidence. Further analysis, taking
into account accurate FAR measured on the basis of the study of the
background during days surrounding the event will be reported in
forthcoming circulars.

Otherwise, we estimate a 3-sigma upper limit on the 75-2000 keV
fluence of 2.6e-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 ~2.1e-07 (7.9e-08) erg/cm^2/s at 1 s (8 s) time
scale in 75-2000 keV energy range.

In this case, For the mean reported distance 795.0 Mpc this corresponds to the limit
on the total isotropic equivalent energy in 1 s of 1.9e+49 erg for the
short GRB spectrum and for a long GRB spectrum the isotropic equivalent
luminosity can be estimated in 1 s (8 s) as 1.6e+49 erg/s (6e+48 erg/s).

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