GCN Circular 21611

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 G298389.  The satellite was
pointing at RA=193.46 Dec=-25.644, 43 degrees from far from the peak
of high-probability area of LIGO localization. Depending on the
location within LIGO 90% confidence region the best upper limit is set
by the anti-coincidence shield of the spectrometer on board of
INTEGRAL (SPI/ACS), IBIS/ISGRI, or IBIS/PICsIT. The localization of
G298389 is not optimal for SPI-ACS observation, since part of the
localization it is occulted for SPI-ACS by the coded mask of IBIS. For
harder source spectra, IBIS, and especially IBIS/PICsIT reaches
sensitivity close to optimal in this orientation.

The INTEGRAL IBAS automatically inspects both ISGRI Field
of View and all-sky SPI-ACS light curve. It did not reveal any
significant excess above the background.

We investigated the SPI-ACS, IBIS/PICsIT, and IBIS/ISGRI light curves
between -500 and +500 s from the trigger time (2017-08-19 15:50:46
UTC) on temporal scales from 0.1 to 100 s, and found no evidence for
any significant deviation from the background.  We estimate a median
3-sigma upper limits in 90% LIGO localization region of 1.9e-6 erg/cm2
(75-2000 keV) assuming a duration of 8s and 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 median limiting fluence of 4.0e-7 erg/cm2 (75-2000 keV) at 3
sigma c.l.