GCN Circular 21231

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 G288732.  The satellite was
pointing at RA=180.649, Dec=-60.391, far from the high-probability
area of LIGO localization. Depending on the location within the LIGO
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) or by the
anti-coincidence shield of the IBIS instrument (IBIS/Veto).  The
combination of these instruments covered the full LIGO localization
region and provided constraints on the flux of a possible
electromagnetic counterpart in the energy range covered by the
INTEGRAL instruments, although the localization was somewhat
unfavorable for the INTEGRAL 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 and IBIS/Veto light curves between -500
and +500 s from the trigger time (2017-06-08 02:01:16.5 UTC) on
temporal scales from 0.1 to 100 s, and found no evidence for any
deviation from the background.

We estimate a 3-sigma fluence upper limit enclosing 90% of LIGO
probability of 1.65e-6 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 and we find a limiting fluence of 4.3e-7 erg/cm2
(75-2000 keV) at 3 sigma c.l. This limit applies to 90% of LIGO
localization probability.