[vsnet-grb-info 24133] LIGO/Virgo S191110af: No counterpart candidates in INTEGRAL SPI-ACS, and IBIS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26232
SUBJECT: LIGO/Virgo S191110af: No counterpart candidates in INTEGRAL SPI-ACS, and IBIS prompt observation
DATE:    19/11/11 13:27:01 GMT
FROM:    Sergey Molkov at Space Research Inst., Moscow  <molkov at iki.rssi.ru>

Sergey Molkov (IKI, Russia), Maeve Doyle (UCD, Ireland),
V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland)
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

The GCN should be regarded as a substitute for 26225, which was not
processed correctly due to a technical issue.

Using combination of INTEGRAL all-sky detectors (following [1]):
SPI/ACS, IBIS/Veto, and IBIS we have performed a search for a prompt
gamma-ray counterpart of S191110af (GCN 26222).

At the time of the event (2019-11-10 23:06:44 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 178 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (4.1% of optimal) response of ISGRI, near-optimal
(86% of optimal) response of IBIS/Veto, and strongly suppressed
(34% of optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was very
stable (excess variance 1.2).

We have performed a search for any impulsive events in INTEGRAL
SPI-ACS (as described in [2]), IBIS, and IBIS/Veto data.

We do not detect any significant counterparts and estimate a 3-sigma
upper limit on the 75-2000 keV fluence of 5e-07 erg/cm^2 (within the
50% probability containment region of the source localization) for a
burst lasting less than 1s with a characteristic short GRB spectrum
(an exponentially cut off power law with alpha=-0.5 and Ep=600 keV)
occurring at any time in the interval within 300s around T0. For a
typical long GRB spectrum (Band function with alpha=-1, beta=-2.5,
and Ep=300 keV), the derived peak flux upper limit
is ~5.2e-07 (7.7e-08) erg/cm^2/s at 1s (8s) time scale
in 75-2000 keV energy range.

We report for completeness and in order of FAP, all excesses
identified in the search region. We find 1 possibly associated
excess:
scale | T     | S/N | flux ( x 1e-05 erg/cm2/s)  | FAP
0.1   | 0.877 | 3.6 | 2.07 +/- 0.739  +/- 1.46   | 0.0497

3 likely background excesses:
scale | T     | S/N | flux ( x 1e-05 erg/cm2/s)  | FAP
0.2   | -31   | 3.7 | 1.51 +/- 0.52   +/- 1.07   | 0.735
0.95  | 185   | 3.8 | 0.701 +/- 0.237 +/- 0.495  | 0.853
0.65  | 63.2  | 3.4 | 0.752 +/- 0.287 +/- 0.531  | 0.953

Note that FAP estimates (especially at timescales above 2s) may be
possibly further affected by enhanced non-stationary local background
noise. This list excludes any excesses for which FAP is close to
unity. All results quoted are preliminary.This circular is an official
product of the INTEGRAL Multi-Messenger team.

[1] Savchenko et al. 2017, A&A 603, A46
[2] Savchenko et al. 2012, A&A 541A, 122S