[vsnet-grb-info 25437] IceCube-200410A: Possible associated excess in INTEGRAL SPI-ACS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  27537
SUBJECT: IceCube-200410A: Possible associated excess in INTEGRAL SPI-ACS prompt observation
DATE:    20/04/11 06:53:15 GMT
FROM:    Sandro Mereghetti at IASF-Milano/INAF  <sandro.mereghetti at inaf.it>

S. Mereghetti (INAF IASF-Milano, Italy), E. Bozzo, V. Savchenko, C.
Ferrigno (ISDC/UniGE, Switzerland), A. Martin-Carrillo (UCD, Ireland), J.
Rodi, F.Onori (IAPS-Roma, Italy), A. Coleiro (APC, France)

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 IceCube-200410A (GCN 27534).

At the time of the event (2020-04-10 23:19:55 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 (3.7% of optimal) response of ISGRI, near-optimal (72% of
optimal) response of IBIS/Veto, and strongly suppressed (33% 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 data (as described in [2].

On a timescale of 1s we found an event with S/N=4 and a false alarm
probability of 0.0124, about 4 s before T0.
Its properties are the following

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
-4.09 | 1 | 4 | 0.728 +/- 0.209 +/- 0.393 | 0.0124

Besides this possible associated excess, we do not detect any significant
counterparts
and estimate a 3-sigma
upper limit on the 75-2000 keV fluence of 5.2e-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 ~5e-07 (1.4e-07)
erg/cm^2/s at 1 s (8 s) time scale in 75-2000 keV energy range.

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

3 likely background excesses:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
96.6 | 1.6 | 3.3 | 0.479 +/- 0.164 +/- 0.259 | 0.613
240 | 1.25 | 3.9 | 0.614 +/- 0.186 +/- 0.332 | 0.74
204 | 0.85 | 3.9 | 0.753 +/- 0.226 +/- 0.407 | 0.899

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