[vsnet-grb-info 22530] LIGO/Virgo S190521r: No counterpart candidates in INTEGRAL SPI/ACS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24629
SUBJECT: LIGO/Virgo S190521r: No counterpart candidates in INTEGRAL SPI/ACS prompt observation
DATE:    19/05/21 07:59:42 GMT
FROM:    Maeve Doyle at U College Dublin, Ireland  <maeve.doyle.1 at ucdconnect.ie>

Maeve Doyle (UCD, Ireland), Enrico Bozzo,
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 <https://www.astro.unige.ch/cdci/integral-multimessenger-collaboration>

Using INTEGRAL/SPI-ACS realtime data (following Savchenko et al. 2017,
A&A 603, A46) we have performed a search for a prompt gamma-ray
counterpart of S190521r.

At the time of the event (2019-05-21 07:43:59 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 125 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (3.3% of optimal) response of ISGRI, strongly suppressed
(36% of optimal) response of IBIS/Veto, and somewhat suppressed (53%
of optimal) response o f SPI-ACS.

The background within +/-300 seconds around the event was stable
(excess variance 1.4).

We have performed a search for any impulsive events in INTEGRAL SPI-
ACS (as described in Savchenko et al. 2012, A&A 541A, 122S) data.

We do not detect any significant counterparts and estimate a 3-sigma
upper limit on the 75-2000 keV fluence of 4e-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 ~3.2e-07 (1.8e-07)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.