[vsnet-grb-info 22143] LIGO/Virgo S190426c: INTEGRAL prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24242
SUBJECT: LIGO/Virgo S190426c: INTEGRAL prompt observation
DATE:    19/04/26 17:40:08 GMT
FROM:    Antonio Martin-Carrillo at UCD,Space Science Group  <antonio.martin-carrillo at ucd.ie>

Antonio Martin-Carrillo (UCD, Dublin)
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 combination of INTEGRAL all-sky detectors (following Savchenko
et al. 2017, A&A 603, A46): SPI/ACS, IBIS/Veto, and IBIS we have
performed a search for a prompt gamma-ray counterpart of S190426c (GCN
24237).

At the time of the event (2019-04-26 15:21:55 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 59 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(14% of optimal) response of ISGRI, strongly suppressed (29% of
optimal) response of IBIS/Veto, and somewhat suppressed (60% 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 Savchenko et al. 2012, A&A 541A, 122S), 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 1.7e-07 erg/cm^2 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 ~1.4e-07 (4.9e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.