GCN Circular 21699

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 (IRAP, France)
R. Sunyaev (IKI, Russia) P. Ubertini (IAPS-Roma, Italy)

We investigated serendipitous INTEGRAL observations carried out at the
time of the LIGO/Virgo GW event candidate G299232.  The satellite was
pointing at RA=262.993 Dec=-27.052, near the lower-probability arc of
LIGO/Virgo localization. About 10% of LIGO localization probability
was in IBIS and SPI FoV. 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. A fraction of the localization of G299232 is not
optimal for SPI-ACS observation.

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 (2017-08-25 13:13:37
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 1.0e-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 3.6e-7 erg/cm2 (75-2000 keV) at 3
sigma c.l.

For the fraction of LIGO/Virgo localization probability within the Field of
View of INTEGRAL pointing instruments (IBIS, JEM-X, and SPI), the
combination of these instruments allows to set stringent upper
limit in a broad energy range from 3 keV to 8 MeV (Savchenko et al
2017 A&A...603A..46S).  Further analysis of these data is ongoing.