GCN Circular 21478

V. Savchenko (ISDC, University of Geneva, CH)
on behalf of the INTEGRAL group:
S. Mereghetti (IASF-Milano, Italy),
C. Ferrigno ((ISDC, University of Geneva, CH),
E. Kuulkers (ESTEC/ESA, The Netherlands),
A. Bazzano (IAPS-Roma, Italy), E. Bozzo,
T. J.-L. Courvoisier (ISDC, University of Geneva, CH)
S. Brandt (DTU - Denmark) R. Diehl (MPE-Garching, Germany)
L. Hanlon (UCD, Ireland) P. Laurent (APC, Saclay/CEA, France)
A. Lutovinov (IKI, Russia) J.P. Roques (CESR, France)
R. Sunyaev (IKI, Russia) P. Ubertini (IAPS-Roma, Italy)

We investigated serendipitous INTEGRAL observations carried out at the
time of the LIGO/Virgo burst candidate G297595.  The satellite was
pointing at RA=240.554 Dec=-55.181, far from the high-probability area
of LIGO localization. For the full LIGO 90% confidence region the best
upper limit is set by the anti-coincidence shield of the spectrometer
on board of INTEGRAL (SPI/ACS). The localization of G297595 is close
to optimal for SPI-ACS observation.

The INTEGRAL Burst Alert System (IBAS) did not identify any unusual
transients in coincidence with the LIGO/Virgo trigger. The IBAS
inspects both ISGRI Field of View and all-sky SPI-ACS light curve.

We investigated the SPI-ACS, IBIS/Veto, and IBIS/ISGRI light curves
between -500 and +500 s from the trigger time (2017-08-14 10:30:43
UTC) on temporal scales from 0.1 to 100 s, and found no evidence for
any significant deviation from the background.  We estimate maximal 
3-sigma upper
limits of 6.6e-7 erg/cm2 (75-2000 keV) for 8s duration assuming Band
model parameters alpha=-1, beta=-2.5, and E_ peak = 300 keV.  To
derive a limit for a typical short burst with 1 s duration, we use a
harder cutoff power law spectrum with a photon index of -0.5 and an
Epeak = 500 keV. We find a limiting fluence of 2.1e-7 erg/cm2 (75-2000
keV) at 3 sigma c.l. Due to high particle background at the current
phase of the Solar Cycle, these upper limits are somewhat higher than
those that can be achieved by SPI-ACS in more favorable conditions.

We do not confirm the report by Pozanenko et al. 2017, GCN 21476
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 . The
fluctuation they report has an S/N marginally exceeding 3 sigma in an
optimized time bin used by the authors. In our systematic search, based
on predefined detection thresholds and time bins, this event is not
detected with a sufficiently high significance to justify a report. We
estimate post-trial significance of a long-timescale fluctuation
shortly following G297595 at 1.9 sigma.

INTEGRAL is scheduled to perform pointed follow-up observations of the
G297595 localization region.