[vsnet-grb-info 23401] LIGO/Virgo S190828j: No counterpart candidates in INTEGRAL SPI-ACS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25500
SUBJECT: LIGO/Virgo S190828j: No counterpart candidates in INTEGRAL SPI-ACS prompt observation
DATE:    19/08/28 08:37:47 GMT
FROM:    Enrico Bozzo at ISDC  <Enrico.Bozzo at unige.ch>

E. Bozzo (ISDC/UniGE, Switzerland), S. Schanne (CEA, France)
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 S190828j (GCN 25497).

At the time of the event (2019-08-28 06:34:05 UTC, hereafter T0), 
INTEGRAL was operating in nominal mode. The peak of the event 
localization probability was at an angle of 83 deg with respect to the 
spacecraft pointing axis. This orientation implies strongly suppressed 
(11% of optimal) response of ISGRI, strongly suppressed (31% of optimal) 
response of IBIS/Veto, and near-optimal (80% 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]) data.

We do not detect any significant counterparts and estimate a 3-sigma 
upper limit on the 75-2000 keV fluence of 2e-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 ~1.7e-07 (5.6e-08) 
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

For the mean reported distance 2276.0 Mpc this corresponds to the limit 
on the total isotropic equivalent energy in 1 s of 1.2e+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 (3.5e+49 erg/s)

We report for completeness and in order of FAP, all excesses identified 
in the search region. We find 9 background excesses:

scale | T | S/N | luminosity ( x 1e+50 erg/s) | FAP
5.2 | -276 | 3.9 | 9.22 +/- 3.03 +/- 5.34 | 0.2
0.55 | -20.1 | 3.6 | 27.9 +/- 9.38 +/- 16.2 | 0.223
0.1 | -1.25 | 3.1 | 5.75 +/- 2.22 +/- 3.33 | 0.249
0.2 | 10.7 | 3.5 | 4.58 +/- 1.56 +/- 2.65 | 0.393
1.6 | 37.3 | 3.1 | 13.9 +/- 5.48 +/- 8.05 | 0.46
1.4 | -135 | 3.6 | 17.3 +/- 5.86 +/- 10 | 0.547
0.05 | -7.63 | 3.6 | 9.67 +/- 3.17 +/- 5.6 | 0.564
0.55 | 63.4 | 3.5 | 27.3 +/- 9.38 +/- 15.8 | 0.807
0.95 | -60.6 | 3.2 | 19.3 +/- 7.12 +/- 11.2 | 0.83

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