[vsnet-grb-info 25552] IceCube-200425A: No counterpart candidates in INTEGRAL SPI-ACS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  27652
SUBJECT: IceCube-200425A: No counterpart candidates in INTEGRAL SPI-ACS prompt observation
DATE:    20/04/26 07:43:15 GMT
FROM:    Diego Gotz at CEA  <diego.gotz at cea.fr>

D. Gotz (CEA-AIM, France), A. Lutovinov (IKI, Russia),
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 IceCube-200425A (GCN 27651).

At the time of the event (2020-04-25 23:26:46 UTC, hereafter T0), INTEGRAL was operating in nominal mode. The peak of the event localization probability was at an angle of 123 deg with respect to the spacecraft pointing axis. This orientation implies strongly suppressed (5.3% of optimal) response of ISGRI, strongly suppressed (36% of optimal) response of IBIS/Veto, and somewhat suppressed (61% 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 data  (as described in [2]).

We do not detect any significant counterparts and estimate a 3-sigma upper limit on the 75-2000 keV fluence of 2.8e-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 ~2.5e-07 (7.8e-08) 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. We find: 7 likely background excesses:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP  
69.3 | 7 | 3.2 | 1.17 +/- 0.379 +/- 0.437 | 0.149 
13.7 | 1 | 3.1 | 0.302 +/- 0.101 +/- 0.113 | 0.226 
-2.22 | 0.15 | 3.1 | 0.772 +/- 0.261 +/- 0.289 | 0.272 
-18.9 | 0.3 | 3.4 | 0.612 +/- 0.184 +/- 0.229 | 0.517 
-264 | 2.5 | 3.5 | 2.13 +/- 0.635 +/- 0.799 | 0.708 
-295 | 6.5 | 3.1 | 1.19 +/- 0.393 +/- 0.447 | 0.755 
7.48 | 0.05 | 3.4 | 1.5 +/- 0.458 +/- 0.562 | 0.936 

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