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GCN Circular 20834

Subject
LIGO/Virgo G275404 and AGL J1914+1043: INTEGRAL follow-up
Date
2017-03-07T22:38:53Z (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 ((ISDC, University of Geneva, CH),
S. Mereghetti (IASF-Milano, Italy),
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)


INTEGRAL performed a target-of-opportunity (ToO) observation
containing the peak of the LIGO localization probability of G275404
(Sachdev et al 2017, GCN 20738) and its possible gamma-ray counterpart
AGL J1914+1043 (Tavani et al 2017, GCN 20754). In total 56% of the
LIGO probability and the whole probable localization region of AGL
J1914+1043 were observed in the field of view (FoV) of the INTEGRAL
instruments.

The observation containing the peak localization probability of
G275404 peak (RA= 299.1 Dec=49.3), started 81.5 hours after the
G275404 trigger time (2017-02-25 18:30:21.4 UTC , T0 hereafter) from
2017-03-01 04:09:58 UTC to 2017-03-02 05:45:58 UTC, corresponding to
an on-target time of 85.5 ks. We covered in total 40% of LIGO
localization probability with INTEGRAL/IBIS FoV. Every single
hour-long spacecraft pointing contained about 30% of the LIGO
localization probability.

The observation of AGL J1914+1043 started at T0+107.5 hours and
spanned from 2017-03-02 05:57:54 UTC to 2017-03-03 07:03:05 UTC, with
a total on-target time of 86.5 ks and including an extended region
within a distance of about 15 degrees from the target, securely
encompassing the region of probable localization reported by AGILE (4
deg 68% containment + 1 deg systematic error). This observation
includes 16% of G275404 localization in INTEGRAL/IBIS FoV and every
single hour-long instrument pointing contained about 13% of the LIGO
localization probability.

Additionally, by chance, INTEGRAL observed a non-negligible fraction
of the G275404 probability before the start of the dedicated
ToO. Despite limited coverage, these observations are useful, since
they were performed during the LIGO trigger and as well as immediately
before and after. The observation of the Galactic Bulge region from
2017-02-26T12:22:54 to 2017-02-26T16:03:52.396 collected 12.5ks of
data covering 1.5% of LIGO probability, complementary to that of the
ToO observation. The observation of the Norma Region between
2017-02-27T07:13:58.431 and 2017-02-28T01:48:14.473 collected 61ks of
data covering additionally 2% of the LIGO probability.

We investigated the data collected by INTEGRAL IBIS/ISGRI and
JEM-X. We did not find any new significant sources in the 90%
localization region of G275404.

In INTEGRAL/ISGRI observation of AGL J1914+1043 data between 25 and 80
keV we have found a weak excess with an SNR of 4.3 at RA=292.27
Dec=8.14 (radius of 5 arcmin, 90% containment), 4.6 deg from the
center of AGILE localization (RA=288.43 deg, DEC=10.72 deg, and error
radius R = 4 deg at 68% confidence + 1 deg systematic error).  We have
split the observation in two periods of equal duration, and found that
this excess is not detected with SNR more than 4 in any of them, hence
revealing no sign of evolution. If real, the excess would correspond
to a source with an average flux of 5.1+/-1.2 mCrab (5.1+-1.2e-11
erg/cm2/s) in the 25-80 keV band and 3.5+/-1.1 mCrab (2.9+-0.9e-11
erg/cm2/s) in 25-40 keV band. While this possible source is located in
a region frequently observed by INTEGRAL (for a total of 14Ms between
2003 and 2017), no possible detection was previously reported at its
position.

We estimate a probability of this excess happening randomly within the
5 degree radius of the AGILE localization to 3.5% or a significance of
2.1 sigma.

We derived an upper limit on the flux of a new source at the peak of
the LIGO localization probability of 6 mCrab (1.9e-10 erg/cm2/s) in
the 3-35 keV band, 3.6 mCrab (3.6e-11 erg/cm2/s) in the 25-80 keV
band, and 7 mCrab (8.2x10-11 erg/cm2/s) in the 80-200 keV band. The
upper limit depends on the source position in the field of view. About
40% of G275404 is observed with the sensitivity of down to 10 mCrab or
better.

The localization region of another LIGO event, G275697 (Siellez et al
2017, GCN 20763), was in the FoV of the INTEGRAL instruments, with the
30% of the LIGO localization observed by IBIS/ISGRI at any time from
the LIGO trigger time to 3.5 days after. At the moment of the LIGO
trigger as well as from -0.5 days to 0.3 days around it up to 8% of
the probability was in the ISGRI FoV, while at any other moment until
3.5 days after the trigger up to 15% of the probability was in the
FoV.

We note that omni-directional INTEGRAL/SPI-ACS observations around the
G275404 and G275697 triggers are reported in LIGO/Virgo Circulars
20755 and GCN 20768.
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