[vsnet-grb-info 23210] HAWC-190806A: upper limits from INTEGRAL SPI-ACS and IBIS prompt observation

[GCN Circulars]

TITLE:   GCN CIRCULAR
NUMBER:  25309
SUBJECT: HAWC-190806A: upper limits from INTEGRAL SPI-ACS and IBIS prompt observation
DATE:    19/08/09 22:44:12 GMT
FROM:    Volodymyr Savchenko at ISDC,U of Geneve  <savchenk at in2p3.fr>

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, and following [1], we have performed
a search for prompt gamma-ray counterparts of HAWC-190806A (GCN 25284).
   
At the time of the event (2019-08-06 13:20:48 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The event localization was at an
angle of 33 deg with respect to thespacecraft pointing axis.
This orientation implies a strongly suppressed(17% of optimal)
response of ISGRI, a strongly suppressed (31% ofoptimal) response of
IBIS/Veto, and a near-optimal (75% of optimal)response of SPI-ACS.
This orientation implies a favorableresponse of IBIS/PICsIT.

Thebackground within +/-300 seconds around  the event was ratherstable
(excess variance 1.3, slightly higher than usual).

We have performed a search for any impulsive events in INTEGRAL SPI-ACS
(as described in [2]) data. We detect a marginal event (S/N 3.3) at a 3.5s
time scale atT0-1.8s. The peak count rate of the signal in SPI-ACS is
713 cts/s,
which corresponds to a flux between 1.2e-07 and 1.7e-07 erg/cm2/s,
depending on thelocation within the source localization region, and
assuming a typical short GRB spectrum (an exponentially cut offpower law
with
alpha=-0.5 and Ep=600 keV). This estimate does not take into account
the uncertaintyrelated to the unknown event spectrum, the 20% systematic
uncertainty on theresponse, or any dead-time correction. We derive
preliminary
estimate of the association FAP at the level of 0.011 (2.3 sigma).
This tentatively indicates a random association.

Further analysis, taking into account accurate FAR measured on
the basis of the study ofthe background during days surrounding
the event might be reported in aforthcoming circulars.

In the IBIS/PICsIT analysis, we do not detect any relevant signal,
despite an
orientation favorable for this instrument. This disfavours a cosmic origin
of the observed SPI-ACS excess.

Otherwise, we estimate a 3-sigma upper limit on the 75-2000 keV
fluence of 2e-07 erg/cm^2 for a burst lasting less than 1 s
with a characteristic short GRB spectrum  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 ~1.7e-07 (6.9e-08) erg/cm^2/s at 1 s (8 s) time
scale in 75-2000 keV energy range.

In addition, we identify the following low-S/N excesses,
which likely belong to thebackground:

scale | T     | S/N | flux ( x 1e-7 erg/cm2/s) | FAP
2.9   | -135  | 3.8 | 1.9 ± 0.5 ± 0.8          | 0.2
2.7   | -238  | 4.2 | 2.1 ± 0.5 ± 0.8          | 0.231
0.1   | -9.26 | 4.2 | 10.6 ± 2.7 ± 4.4         | 0.248
0.8   | 92.7  | 4.1 | 3.6 ± 0.9 ± 1.5          | 0.341
0.5   | -113  | 3.9 | 4.5 ± 1.2 ± 1.8          | 0.786
1.1   | -275  | 3.9 | 3.0 ± 0.8 ± 1.2          | 0.896

Note that FAP estimates (especially at timescales above 2s) may be
further affected by possibly 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