[vsnet-grb-info 24951] LIGO/Virgo S200213t: No counterpart candidates in INTEGRAL SPI-ACS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  27050
SUBJECT: LIGO/Virgo S200213t: No counterpart candidates in INTEGRAL SPI-ACS prompt observation
DATE:    20/02/13 07:25:04 GMT
FROM:    James Rodi at IAPS-INAF  <james.rodi at inaf.it>

James Rodi (IAPS-Roma, Italy)
V. Savchenko, C. Ferrigno (ISDC/UniGE, Switzerland)
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 S200213t (GCN 27042)

At the time of the event (2020-02-13 04:10:40 UTC, hereafter T0), INTEGRAL
was operating in nominal mode. The peak of the event localization
probability was at an angle of 139 deg with respect to the spacecraft
pointing axis. This orientation implies strongly suppressed (2.8% of
optimal) response of ISGRI, somewhat suppressed (60% of optimal) response
of IBIS/Veto, and strongly suppressed (38% of optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was rather stable
(excess variance 1.5).

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 4.8e-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 ~4.6e-07 (1.9e-07) erg/cm^2/s at 1 s (8 s)
time scale in 75-2000 keV energy range.

For the mean reported distance 224.0 Mpc this corresponds to the limit on
the total isotropic equivalent energy in 1 s of 2.9e+48 erg for the short
GRB spectrum and for a long GRB spectrum isotropic equivalent luminosity in
1 s (8 s) of 2.5e+48 erg/s (1.1e+48 erg/s)

We report for completeness and in order of FAP, all excesses identified in
the search region. We find: 5 likely background excesses:

T-T0 | scale | S/N | luminosity ( x 1e+48 erg/s) | FAP
-3.11 | 0.3 | 3.5 | 6.47 +/- 1.85 +/- 2.45 | 0.0691
-3.88 | 0.05 | 4 | 17.7 +/- 4.62 +/- 6.7 | 0.147
61.7 | 1.7 | 3.3 | 27.7 +/- 7.74 +/- 10.5 | 0.368
-148 | 1.1 | 3.7 | 36.7 +/- 9.64 +/- 13.9 | 0.676
254 | 0.75 | 4.1 | 4.88 +/- 1.17 +/- 1.85 | 0.974

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