GCN Circular 20694

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 G274296. The satellite was pointing at
RA=17:30:10.94 Dec=-25:04:47.6, close to the low-probability area of 
LIGO localization.
About 2% of the probability was contained in the field of view of 
INTEGRAL IBIS and SPI.
Depending on the location within the LIGO 90% localization region, as 
well as the assumed
counterpart spectrum and duration, the best upper limit is set by the 
anti-coincidence shield
of the spectrometer on board of INTEGRAL (SPI/ACS), the anti-coincidence 
shield of
the IBIS instrument (IBIS/Veto), or by the imaging coded mask 
instruments (IBIS and SPI).
The combination of these instruments covered the full LIGO 90% 
confidence region and
provided stringent constraints on the flux of a possible electromagnetic 
counterpart
in the energy range covered by the INTEGRAL instruments.

We investigated the SPI-ACS, IBIS/Veto, and IBIS/ISGRI light curves between
-500 and +500 s from the trigger time (2017-02-17 06:05:53 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  3.8e-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 slope of ���0.5 and an Epeak = 500 keV: we find 
a limiting fluence
of 1.4e-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 slope of ���0.5 and
an Epeak = 500 keV: we estimate that 35% of the LIGO localization 
probability region is covered
with a range of sensitivity from optimal for SPI-ACS (mentioned above) 
to 50% worse. About 2%
of the LIGO localization in the field of view of IBIS and SPI is covered 
with at least
factor 2 better sensitivity.

The SPI/ACS light curves, binned at 50 ms, are derived from 91 
independent detectors with different
lower energy thresholds (mainly between 50 keV and 150 keV) and an upper 
threshold at about 100 MeV.
The ACS response varies substantially as a function of the source 
incident angle with an optimal
effective area of about 6000 cm2 at 1 MeV.