[vsnet-grb-info 23610] LIGO/Virgo S190910h: No counterpart candidates in INTEGRAL SPI-ACS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25709
SUBJECT: LIGO/Virgo S190910h: No counterpart candidates in INTEGRAL SPI-ACS prompt observation 
DATE:    19/09/10 09:47:10 GMT
FROM:    Alexis Coleiro at APC/U. Paris Diderot  <coleiro at apc.in2p3.fr>

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

on behalf of the INTEGRAL multi-messenger collaboration:
https://www.astro.unige.ch/cdci/integral-multimessenger-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 S190910h (GCN 25707).

At the time of the event (2019-09-10 08:29:58 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 129 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (4% of optimal) response of ISGRI, somewhat suppressed (57%
of optimal) response of IBIS/Veto, and strongly suppressed (30% o f
optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was very stable
(excess variance 1.1).

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 3.1e-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 ~2.8e-07 (5.2e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

For the mean reported distance 241.0 Mpc this corresponds to the limit
on the total isotropic equivalent energy in 1 s of 2.2e+48 erg for
the short GRB spectrum and for a long GRB spectrum isotropic
equivalent luminosity in 1 s (8 s) of 1.2e+48 erg/s (3.7e+47 erg/s).

We report for completeness and in order of FAP, all excesses
identified in the search region. We find: 4 likely background
excesses:

scale | T | S/N | luminosity ( x 1e+48 erg/s) | FAP  
0.3 | 1.48 | 3.1 | 7.71 +/- 3.62 +/- 7.58 | 0.082 
1.95 | -27.4 | 3.5 | 3.25 +/- 1.41 +/- 3.19 | 0.105 
0.95 | 107 | 3.6 | 4.79 +/- 2.03 +/- 4.71 | 0.69  
1.5 | -85.7 | 3.2 | 3.24 +/- 1.61 +/- 3.18 | 0.773 

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. In addition we note that the most likely excess in the search region, despite
high FAP, occurred at the time delay previously found in BNS GRB association. 
The FAP is computed with respect to the GW T0 and not to the likely GRB offset (1.7s). 
Using GW T0 + 1.7s as the reference for the FAP would slightly improve the 
association significance, but not enough to claim significant association. 
Furthermore changing the reference for FAP would not be compliant with the 
predefined procedure.

All results quoted are preliminary.

This circular is an official product of the IN TEGRAL Multi-Messenger
team.

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