[vsnet-grb-info 22079] LIGO/Virgo S190425z: further analysis of INTEGRAL data

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  24178
SUBJECT: LIGO/Virgo S190425z: further analysis of INTEGRAL data
DATE:    19/04/25 13:06:11 GMT
FROM:    Volodymyr Savchenko at ISDC,U of Geneve  <savchenk at in2p3.fr>

V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland)
A. Martin-Carillo (UCD, Dublin)
J. Rodi (IAPS-Roma, Italy)
A. Coleiro (APC, France)
S. Mereghetti (INAF IASF-Milano, Italy)

on behalf of the INTEGRAL multi-messenger collaboration:
https://www.astro.unige.ch/cdci/integral-multimessenger-collaboration

We detect only one possible low-S/N excess after correcting
for the local background variance at 2019-04-25T08:18:05
(T0)+6s, as reported in Martin-Carillo 2019 (GCN 24169).
Its fluence strongly depends on the true source location,
since large 90% localization region provided by LIGO/Virgo
includes sky directions in which INTEGRAL detectors able to
detect impulsive signal from all-sky (SPI-ACS and IBIS)
featured both low and high sensitivity.

For the excess at T0+6s, we estimate a possible 75-2000 keV
fluence range due to uncertainty of the location
from 2.0e-10 to 2e-9 erg/cm2 (in addition to systematic
uncertainty of response of 20% and statistical uncertainty of 30%),
assuming the duration of 1s and a characteristic short GRB spectrum
(an exponentially cut off power law with alpha=-0.5 and Ep=600 keV).

The sensitivity map is provided:

https://www.astro.unige.ch/cdci/content/int-combined-sensitivity-map-lvc-s190425z

We stress that the FAP of association of this excess to S190425z
is below 3 sigma. Further estimate of the FAP, taking into account detail
noise estimates during the days around the event, will be
provided in the future publications.

INTEGRAL is unable to provide an accurate localization of
the above mentioned low-S/N excess.
The best localization constraint we are currently able to
provide excludes the FoV of INTEGRAL/IBIS at the time of the event
(18:24:43.61, +29:45:25.9, 15 deg radius).
An event comparable to the excess at T0+6s occurring in the FoV
would have been detected by INTEGRAL/IBAS in realtime.
Furthermore, a location close to the FoV (within ~30 deg
from the center of the pointing) is discouraged by the lack
of INTEGRAL IBIS/PICsIT detection.

The south-west arc of the LIGO/Virgo localization of S190425z
is slightly unfavorable for the origin of the low-S/N excess,
if it is associated with the GW signal due to the absence
of any signal in IBIS/Veto shield.

Further analysis will be reported in the following circulars.

INTEGRAL has initiated a target of opportunity observation
of S190425z centered near peak of the LIGO/Virgo localization.
The results of these observations will be reported in
upcoming publications.