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GCN Circular 21695

Subject
LIGO/Virgo G297595: INTEGRAL pointed follow-up observations
Date
2017-08-25T14:33:57Z (7 years ago)
From
Volodymyr Savchenko at APC,Paris <savchenk@apc.in2p3.fr>
V. Savchenko (ISDC, University of Geneva, CH)
on behalf of the INTEGRAL group:
C. Ferrigno, E. Bozzo ((ISDC, University of Geneva, CH),
E. Kuulkers (ESTEC/ESA, The Netherlands),
S. Mereghetti (IASF-Milano, Italy),
T. J.-L. Courvoisier (ISDC, University of Geneva, CH)
J. Chenevez, S. Brandt (DTU - Denmark)
R. Diehl (MPE-Garching, Germany)
L. Hanlon (UCD, Ireland)
P. Laurent (APC, Saclay/CEA, France), D. Gotz (DRF/Irfu/DAp Saclay/CEA)
J.P. Roques, E. Jourdain (IRAP, France)
P. Ubertini, A. Bazzano, J. Rodi (IAPS-Roma, Italy)
A. Lutovinov, R. Sunyaev (IKI, Russia)

INTEGRAL is an observatory with multiple instruments: a gamma-ray
spectrometer (20 keV -4 MeV, SPI), an imager (15 keV - 2 MeV, IBIS),
an X-ray monitor (3-25 keV, JEM-X), and an optical monitor (V band,
OMC). Our group requested and obtained pointed follow-up observations
of the LIGO/Virgo candidate BBH merger G297595 (GCN 21474).

The INTEGRAL follow-up observation was centered on RA=41.07
Dec=-45.49, close to the peak of the LIGO/Virgo localization
probability, taking into account observational constrains. This
observation spanned from 2017-08-16 at 10:04 to 2017-08-17 at 18:19
(starting about 47 hours after the LIGO/Virgo event), with a maximum
on-source time of 97.5 ks (depending on the instrument).

IBIS and SPI observed more than 92% of the of LIGO/Virgo localization
in the combined observation mosaic. JEM-X covered about 79%, owing to
its smaller Field of View.  INTEGRAL provides the most stringent
constraint on any possible emission associated with G297595, above 80
keV.  We searched IBIS/ISGRI and JEM-X data for new point sources in
the whole 90% LIGO/Virgo localization region, and did not find any. We
set the following 3-sigma upper limits for the average flux:

JEM-X:

3-10 keV: 2.0 mCrab (3.2e-11  erg/cm2/s )
10-25 keV: 1.0 mCrab (1.1e-11  erg/cm2/s )

ISGRI:

20-80 keV: 3.3 mCrab (4.9e-11 erg/cm2/s)
80-300 keV: 13.1 mCrab (1.5e-10 erg/cm2/s)


In the case of ISGRI (JEM-X), 82% (59%) of the of LIGO/Virgo
localization is covered with sensitivity no more than factor two worse
than the best.

During these observation online INTEGRAL/IBAS system identified
several weak triggers. After offline inspection, we consider these
triggers unrelated noise.

INTEGRAL/IBAS online search in the SPI-ACS identified a bright
GRB170816A on 2017-08-16 at 14:22:58. The location of this GRB is
inconsistent with the LIGO/Virgo 90% localization (GCN 21504, 21517).

Finally, the BNS merger G298048 (GCN 21509, 21513) candidate
counterpart (GRB 170817A, GCN 21506, 21520) was detected in the
INTEGRAL data during this observation, as reported in GCN 21507.

We have also inspected PICsIT spectral-timing data (200 - 2600 keV)
and did not find any evidence for significant variability.

Further analysis is ongoing.

We acknowledge the essential support by the teams at ESAC and ESOC for
the scheduling of this follow-up observation.
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