GCN Circular 24571

M. Doyle (UCD, Dublin), Bozzo Enrico,
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 (following Savchenko
et al. 2017, A&A 603, A46) we have
performed a search for a prompt gamma-ray counterpart of S190517h (GCN
24570
Not found
 
 
).

At the time of the event (2019-05-17 05:51:01 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 90 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(8.7% of optimal) response of ISGRI, strongly suppressed (19% of
optimal) response of IBIS/Veto, and somewhat suppressed (61% of
optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was somewhat
unstable (excess variance 1.9).

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

The real time pipeline has found a collection of weak excesses
in the vicinity of the event, the most prominent being at T0+37s with
a S/N of 5.4 at the timescale of 0.1s. Note that this pipeline excludes
the shortest 0.05s timescale since it is usually strongly polluted by CR
effects.

We note the presence of a short time-scale variance excess in the days before
the event, and advise caution in interpreting this signal as astrophysical.
Further analysis of local noise properties will be reported elsewhere.

We do not detect any secure significant counterparts and estimate a 3-sigma
upper limit on the 75-2000 keV fluence of 2.7e-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 ~2.3e-07 (1.1e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.