[vsnet-grb-info 22053] LIGO/Virgo S190421ar: INTEGRAL prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24152
SUBJECT: LIGO/Virgo S190421ar: INTEGRAL prompt observation
DATE:    19/04/23 05:40:08 GMT
FROM:    Alexander Lutovinov at Space Research Inst.,IKI  <aal at iki.rssi.ru>

D. Götz (CEA, France),
A. 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 combination of INTEGRAL all-sky detectors: SPI/ACS, IBIS/Veto,
and IBIS (Savchenko et al. 2017, A&A 603, A46) we have performed a
search for a prompt gamma-ray counterpart of S190421ar (GCN 24141).

At the time of the event (2019-04-21 21:38:56 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 49 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(19% of optimal) response of ISGRI, strongly suppressed (40% of
optimal) response of IBIS/Veto, and near-optimal (77% of optimal)
response of SPI-ACS. For this first report, we use only 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 (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 1.9e-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 ~1.6e-07 (5.8e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.