GCN Circular 21063

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 G284239.  The satellite was
pointing at RA=294.829 Dec=17.726, 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 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 (2017-05-02 22:26:07
UTC) on temporal scales from 0.1 to 100 s, and found no evidence for
any deviation from the background.  We estimate combined typical
3-sigma upper limits of 3.7e-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 1.5e-7 erg/cm2
(75-2000 keV) at 3 sigma c.l. These limits assume a perpendicular
direction of the burst to the INTEGRAL pointing direction, optimal for
SPI-ACS sensitivity. However the extent of the region with optimal
response depends on the possible source spectrum: we perform a
detailed calculation only for a cutoff powerlaw spectrum with a photon
index of -0.5 and an Epeak = 500 keV. We estimate that 60% of the LIGO
localization probability is covered with a range of sensitivity from
optimal for SPI-ACS (mentioned above) to 50% worse than optimal. The
90% LIGO confidence region includes a small area for which the SPI-ACS
sensitivity is unusually strongly suppressed. The region in which
SPI-ACS sensitivity is reduced more than by factor 3 contains about
10% of LIGO localization probability.