[vsnet-grb-info 24343] LIGO/Virgo S191215w: No counterpart candidates in INTEGRAL SPI-ACS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  26442
SUBJECT: LIGO/Virgo S191215w: No counterpart candidates in INTEGRAL SPI-ACS prompt observation
DATE:    19/12/15 23:18:33 GMT
FROM:    Diego Gotz at CEA  <diego.gotz at cea.fr>

Diego Gotz (AIM/DAp CEA Saclay, France), Alexander Lutovinov (IKI, Russia)
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

Using INTEGRAL/SPI-ACS realtime data (following [1]) we have performed
a search for a prompt gamma-ray counterpart of S191215w (GCN 26441).

At the time of the event (2019-12-15 22:30:52 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 37 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(14% of optimal) response of ISGRI, strongly suppressed (30% of
optimal) response of IBIS/Veto, and near-optimal (74% of optimal)
response of SPI-ACS.

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

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

We do not detect any significant counterparts and estimate a 3-sigma
upper limit on the 75-2000 keV fluence of 3.3e-07 erg/cm^2 (within the
50% probability containment region of the source localization) for a
burst lasting less than 1 s 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 300 s 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 ~2.9e-07 (7.2e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

For the mean reported distance 2216.0 Mpc this corresponds to the
limit on the total isotropic equivalent energy in 1 s of 1.9e+50 erg
for the short GRB spectrum and for a long GRB spectrum isotropic
equivalent luminosity in 1 s (8 s) of 1.1e+50 erg/s (4.2e+49 erg/s)

We report for completeness and in order of FAP, all excesses
identified in the search region. We find: 8 likely background
excesses:

T-T0 | scale | S/N | luminosity ( x 1e+50 erg/s) | FAP
172 | 0.1 | 12 | 35.9 +/- 4.25 +/- 31 | 0.271
20.6 | 0.15 | 3.9 | 9.44 +/- 3.33 +/- 8.14 | 0.492
-23.3 | 0.15 | 3.9 | 9.47 +/- 3.33 +/- 8.16 | 0.58
-55.7 | 0.35 | 3.9 | 6.08 +/- 2.17 +/- 5.24 | 0.608
-18 | 0.4 | 3.2 | 4.76 +/- 2.02 +/- 4.1 | 0.617
236 | 3.55 | 3.3 | 17.1 +/- 6.75 +/- 14.8 | 0.666
120 | 1.1 | 3.5 | 3.16 +/- 1.22 +/- 2.73 | 0.719
175 | 0.95 | 3.8 | 3.62 +/- 1.31 +/- 3.12 | 0.725

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.

We note that despite a quite high S/N ratio the excess at T-T0=172 s is not considered as associated with the GW event.

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