HAWC-211123A

The HAWC Collaboration  (http://www.hawc-observatory.org/collaboration/)
reports:

On 11 23, 2021, at 03:52:23.50 UTC, HAWC detected a burst signal
from its Burst Monitoring named HAWC-211123A. This monitor system looks
for excesses above the expected background in time windows of 0.2, 1, 10
and 100 seconds.
This event was found in the 1-second time window starting
at the reported trigger time.

The position of the alert is
RA (J200): 34.12 deg
Dec (J2000): -8.05 deg
Location uncertainty (68% containment): 0.60 deg (statistical only).

The monitor system found that this alert has a false alarm rate of 1.92
alert(s) per year.
We encourage follow-up observations of the HAWC alert region. We however
note that it is
consistent with background expectations based on the observation time.

Alexis Coleiro (APC/Universite de Paris) and Damien Dornic (CPPM/CNRS) on behalf of the ANTARES Collaboration. 

Using data from the ANTARES detector, we have performed a follow-up analysis of the recently reported HAWC-211123A alert (event 110400_42 <https://gcn.gsfc.nasa.gov/notices_amon_hawc/110400_42.amon>). 

No up-going muon neutrino candidate events were detected within the error box of the HAWC event during a +/- 1h time-window centered on the HAWC event time (T0), and over which the potential source remained visible 88% of time in the up-going field of view of ANTARES. At T0, the elevation of the alert was -7.3 degrees below the horizon for ANTARES. 

This leads to a preliminary 90% confidence level upper limit on the muon-neutrino radiant fluence from a point source of about 17 GeV.cm^-2 over the energy range 3 TeV ��� 3 PeV (the range corresponding to 5-95% of the detectable flux) for an E^-2 power-law spectrum, and about 70 GeV.cm^-2 (0.5 - 280 TeV) for an E^-2.5 spectrum. 
A search over an extended time window of +/-1 day has also yielded no detection (54% visibility). 

ANTARES <http://antares.in2p3.fr/> is the largest undersea neutrino detector (Mediterranean Sea) and it is primarily sensitive to astrophysical neutrinos in the TeV-PeV energy range. At 10 TeV, the median angular resolution for muon neutrinos is about 0.5 degrees. In the range 1-100 TeV ANTARES has a competitive sensitivity to this position in the sky.

C. Fletcher (USRA) reports on behalf of the Fermi-GBM team:

At the time of the burst candidate HAWC-211123A
(GCN 31106
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 ), Fermi was passing through the South Atlantic
Anomaly from 3.0 minutes prior until 18.6 minutes after the trigger
time; therefore the GBM detectors were disabled.

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 combination of INTEGRAL all-sky detectors (following [1]):
SPI/ACS, IBIS/Veto, and IBIS we have performed a search for a prompt
gamma-ray counterpart of HAWC-211123A (GCN 31106
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 ).

At the time of the event (2021-11-23 03:52:23 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 86 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(15% of optimal) response of ISGRI, strongly suppressed (30% of
optimal) response of IBIS/Veto, and somewhat suppressed (66% of
optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was rather
stable (excess variance 1.3).

We have performed a search for any impulsive events in INTEGRAL SPI-
ACS (as described in [2]), IBIS, and IBIS/Veto 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 ~2.2e-07 (7.7e-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: 2 possibly associated excesses:

T-T0  | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
-4.74 | 2.1   | 3.2 | 1.9 +/- 0.57 +/- 0.576    | 0.0349
13.1  | 1.1   | 4   | 3.25 +/- 0.79 +/- 0.987   | 0.0388

6 likely background excesses:

T-T0  | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
-42.6 | 1.4   | 3.9 | 2.83 +/- 0.7 +/- 0.859    | 0.111
18.8  | 0.95  | 3.1 | 2.78 +/- 0.849 +/- 0.844  | 0.377
-120  | 3.3   | 3.2 | 1.47 +/- 0.455 +/- 0.445  | 0.59
-95.4 | 0.85  | 3.5 | 3.31 +/- 0.899 +/- 1      | 0.787
-35.1 | 0.05  | 4.4 | 1.7 +/- 0.38 +/- 0.516    | 0.806
164   | 0.55  | 4.1 | 0.475 +/- 0.112 +/- 0.144 | 0.831

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

GCN pipeline version: 62a8c26