[vsnet-grb-info 23133] IceCube 190730A: one weakly associated counterpart candidates in INTEGRAL SPI-ACS, and IBIS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25232
SUBJECT: IceCube 190730A: one weakly associated counterpart candidates in INTEGRAL SPI-ACS, and IBIS prompt observation
DATE:    19/07/31 10:10:41 GMT
FROM:    Carlo Ferrigno at IAAT/ISDC  <carlo.Ferrigno at unige.ch>

C. Ferrigno, V. Savchenko (ISDC/UniGE, Switzerland)
Maeve Doyle (UCD, Ireland), Alexander Lutovinov (IKI, Moscow, Russia)
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 the INTEGRAL all-sky detectors
SPI/ACS (following [1]), IBIS/Veto, and IBIS, we have performed a search 
for a prompt
gamma-ray counterpart of IceCube 190730A, a probable high-energy 
neutrino (GCN 25225).

At the time of the event (2019-07-30 20:50:41 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The most likely event
localization was at an angle of 81 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(15% of optimal) response of ISGRI, strongly suppressed (31% of
optimal) response of IBIS/Veto, and near-optimal (90% of optimal)
response of SPI-ACS.

The background within +/-300 seconds around the event was very stable
(excess variance 1.1). However, we note the presence of excessive background
variations in the hours surrounding the event, increasing the chance of
spurious counterpart associations, and complicating the preliminary 
background estimation reported here.

We have performed a search for impulsive events in INTEGRAL SPI-ACS
(as described in [2]), IBIS, and IBIS/Veto data. We detect a moderately
significant event (S/N 4.7) at 30s time scale at T0+10 s.
The peak count rate (30s time scale) of the signal in SPI-ACS is 314 cts/s,
which corresponds to 4.7e-8 erg/cm2/s in the 75-2000 keV energy range,
assuming coordinates within the localization region and a typical
long GRB spectrum (Band function with alpha=-1, beta=-2.5, and Ep=300 keV).
This estimate does not account for uncertainties related to the unknown 
event spectrum,
systematic uncertainty on the response (20%), or any dead-time correction.
The non-detection of the event in IBIS would be compatible with a source
at the location of the IceCube neutrino.
We derive a preliminary estimate of the association False Alarm 
Probability (FAP)
at the level of 0.046 (2 sigma), which suggests a likely random 
coincidence.
Further analysis, taking into account accurate FAR measured on the basis 
of the study of the background during days surrounding the event will be 
reported elsewhere.


Given the high chance of a random coincidence of the above event,
we estimate a 3-sigma upper limit on the 75-2000 keV
fluence of 1.8e-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 the typical long GRB spectrum used 
also above,
the derived peak flux upper limit is ~1.6e-07 (5.4e-08) erg/cm^2/s at 1 
s (8 s) time
scale in 75-2000 keV energy range.

 From now on, we will report all the low-S/N excesses detected by our 
pipelines
with preliminary FAP below unity, which are likely background fluctuations.
In addition to the event described above, we find:

       scale (s) | T-T0 (s) | S/N | flux ( x 1e-7 erg/cm2/s) |   FAP
        0.25     | -6.6     | 3.1 | 4.11 +/-  1.22 +/-   1.1 | 0.433
        0.95     |  101     | 3.5 | 2.31 +/- 0.625 +/- 0.618 | 0.785

In this list "flux" is the derived flux assuming a short-GRB typical 
spectrum
with statistical and systematic uncertainties.
Note that FAP estimates (especially at timescales above 2s) may be 
underestimated
due to non-stationary local background noise.

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