GCN Circular 25825

Antonio Martin-Carrillo (UCD), Francesca Onori, 
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

At the time of the event S190923y (2019-09-23 12:55:59 UTC, GCN 25814
Not found
 
 
) and based on the initial localization map, 8% of the LIGO/Virgo GW localization was within the field of view of IBIS. The background within +/-300 seconds around the event was very stable (excess variance 1.1).

Initial analysis of the pointed observations of IBIS do not reveal any relevant counterparts at this point. 3-sigma upper limits of about 5e-10erg/cm2/s (28-40 keV energy range) in single pointings of about 30 min at ~T0+5.5h have been found in areas at the edge of the 90% LIGO/Virgo contour. 

INTEGRAL will continue to serendipitously observe the localization area
of S190923y over the nex few hours. Any relevant candidates will be reported separately.

We have included this new data to the initial results presented in Onori et al (GCN 25815
Not found
 
 
) to search for any impulsive events in INTEGRAL SPI-
ACS (as described in [1]), IBIS, and IBIS/Veto 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 ~3.4e-07 (3.7e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

For the mean reported distance 438.0 Mpc this corresponds to the limit
on the total isotropic equivalent energy in 1 s of 7.8e+48 erg for
the short GRB spectrum and for a long GRB spectrum isotropic
equivalent luminosity in 1 s (8 s) of 2.6e+48 erg/s (8.6e+47 erg/s)

These upper limits take into account that the peak of the event
localization probability was at an angle of 158 deg with respect to
the spacecraft pointing axis at the time of the trigger. 
This orientation implies strongly
suppressed (3.2% of optimal) response of ISGRI, somewhat suppressed
(62% of optimal) response of IBIS/Veto, and somewhat suppressed (43%
of optimal) response of SPI-ACS.


All results quoted are preliminary.
This circular is an official product of the INTEGRAL Multi-Messenger
team.

[1] Savchenko et al. 2012, A&A 541A, 122S