GCN Circular 26908

LIGO/Virgo S200128d: No counterpart candidates in INTEGRAL SPI-ACS
prompt observation

Francesca Onori (IAPS, Italy), Alexis Coleiro
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 S200128d (GCN 26906
Not found
 
 
).

At the time of the event (2020-01-28 02:20:11 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 135 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (3.4% of optimal) response of ISGRI, somewhat suppressed
(42% of optimal) response of IBIS/Veto, and somewhat suppressed (43 %
of 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 2.6e-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 ~2.3e-07 (5.4e-08)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

For the mean reported distance 4031.0 Mpc this corresponds to the
limit on the total i sotropic equivalent energy in 1 s of 5e+50 erg
for the short GRB spectrum and for a long GRB spectrum isotropic
equivalent luminosity in 1 s (8 s) of 3.2e+50 erg/s (1e+50 erg/s)

We report for completeness and in order of FAP, all excesses
identified in the search region. We find: 1 possibly associated
excess:

T-T0 | scale | S/N | luminosity ( x 1e+51 erg/s) | FAP
4.73 | 0.5 | 4.1 | 16.5 +/- 5.64 +/- 14.1 | 0.0284

9 likely background excesses:

T-T0 | scale | S/N | luminosity ( x 1e+51 erg/s) | FAP
-201 | 5 | 4.3 | 5.28 +/- 1.77 +/- 4.5 | 0.0959
-128 | 1.5 | 5 | 11.3 +/- 3.25 +/- 9.64 | 0.128
6.65 | 0.75 | 3.1 | 9.85 +/- 4.59 +/- 8.38 | 0.175
-53.1 | 0.65 | 4 | 14.3 +/- 4.94 +/- 12.2 | 0.257
9.78 | 0.2 | 3.5 | 22.4 +/- 8.95 +/- 19 | 0.316
72.2 | 0.7 | 3.8 | 12.9 +/- 4.76 +/- 11 | 0.44
-9.6 | 0.15 | 3.4 | 2.48 +/- 1.03 +/- 2.11 | 0.566
271 | 3.3 | 3.5 | 5.06 +/- 2.18 +/- 4.31 | 0.577
45.6 | 1.15 | 3.1 | 8.14 +/- 3.7 +/- 6.93 | 0.646

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


-- 
Dr. Francesca Onori
Postdoctoral Researcher
IAPS, via Fosso del Cavaliere, 100, 00133 - Rome, Italy
e-mail: francesca.onori@inaf.it
Tel: +39 06 45488128