IceCube-211125A

The IceCube Collaboration (http://icecube.wisc.edu/) reports:

On 21/11/25 at 06:22:21.56 UT IceCube detected a track-like event with a moderate probability of being of astrophysical origin. The event was selected by the ICECUBE_Astrotrack_Bronze alert stream. The average astrophysical neutrino purity for Bronze alerts is 30%. This alert has an estimated false alarm rate of 1.973 events per year due to atmospheric backgrounds. The IceCube detector was in a normal operating state at the time of detection. 

After the initial automated alert (https://gcn.gsfc.nasa.gov/notices_amon_g_b/135936_74588253.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 21/11/25
Time: 06:22:21.56 UT
RA: 43.59 (+ 3.13 - 2.71 deg 90% PSF containment) J2000
Dec: 22.59 (+ 1.54 - 2.53 deg 90% PSF containment) J2000 

We encourage follow-up by ground and space-based instruments to help identify a possible astrophysical source for the candidate neutrino.

There are two Fermi-LAT 4FGL sources inside the 90% localization region. The nearest source is 4FGL J0248.0+2232, located at RA 42.01 deg and Dec 22.54 deg (J2000), at a distance of 1.46 degrees from the best-fit location.

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu

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 bronze track event IceCube-211125A (GCN #31126
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  <https://gcn.gsfc.nasa.gov/gcn3/31126.gcn3
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 >). The reconstructed origin was 6 degrees below the horizon for ANTARES at the time of the alert.

No muon neutrino candidate events were recorded within 90% error box of the IceCube event during a +/- 1h time-window centered on the IceCube event time, and over which the potential source remained visible 82.5% of the time window. This leads to a preliminary 90% confidence level upper limit on the muon-neutrino radiant fluence from a point source of about 12 GeV.cm^-2 over the energy range 5 TeV ��� 5 PeV (the range corresponding to 5-95% of the detectable flux) for an E^-2 power-law spectrum, and 35 GeV.cm^-2 (1 - 500 TeV) for an E^-2.5 spectrum. 

A search over an extended time window of +/- 1 day has also yielded no detection (37% 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.

S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and R. de 
Menezes (Univ. of Wuerzburg, Univ. of Sao Paulo) on behalf of the 
Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the high-energy 
IC211125A neutrino event (GCN 31126
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 ) with all-sky survey data from the 
Large Area Telescope (LAT), on board the Fermi Gamma-ray Space 
Telescope. The IceCube event was detected on 2021-11-25 at 06:22:21.56 
UT (T0) with J2000 position RA = 43.59 (+ 3.13, - 2.71) deg, Decl. = 
22.59 (+ 1.54, - 2.53) deg (90% PSF containment). Two cataloged 
gamma-ray (>100 MeV) sources are located within the 90% IC211125A 
localization region (4FGL-DR2, The Fermi-LAT collaboration 2020, ApJS, 
247, 33). These are 4FGL J0248.0+2232 at 1.46 deg distance, which is 
associated with the blazar of uncertain type 1RXS J024800.1+223136, and 
4FGL J0258.1+2030 at 2.26 deg distance, which is associated with the BL 
Lac object MG3 J025805+2029. Based on a preliminary analysis of the LAT 
data over the 1-month timescale before T0, these objects are not 
significantly detected at gamma-rays.

We searched for the existence of intermediate (months to years) 
timescale emission from a new gamma-ray transient source. Preliminary 
analysis indicates no significant (> 5 sigma) new excess emission (> 100 
MeV) at the IC211125A best-fit position. Assuming a power-law spectrum 
(photon index = 2.0 fixed) for a point source at the IC211125A best-fit 
position, the >100 MeV flux upper limit (95% confidence) is < 2.0e-10 ph 
cm^-2 s^-1 for ~13-years (2008-08-04 to 2021-11-25 UTC), and < 1.1e-8�� 
ph cm^-2 s^-1 for a 1-month integration time before T0.

Due to the current Fermi-LAT observing profile 
(https://fermi.gsfc.nasa.gov/ssc/observations/types/post_anomaly/) the 
region of IC211125A is not observable by the LAT since November 12, 
2021. Regular monitoring of this region will be resumed at the beginning 
of December 
(https://fermi.gsfc.nasa.gov/ssc/observations/timeline/posting/). For 
these observations the Fermi-LAT contact persons are S. Garrappa 
(simone.garrappa at desy.de) and S. Buson (sara.buson at uni-wuerzburg.de).

The Fermi-LAT is a pair conversion telescope designed to cover the 
energy band from 20 MeV to greater than 300 GeV. It is the product of an 
international collaboration between NASA and DOE in the U.S. and many 
scientific institutions across France, Italy, Japan and Sweden.

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 IceCube-211125A (GCN 31126
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 ).

At the time of the event (2021-11-25 06:22:21 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 72 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(15% of optimal) response of ISGRI, strongly suppressed (23% of
optimal) response of IBIS/Veto, and near-optimal (68% 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 2.8e-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 ~2.3e-07 (8.2e-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: 4 likely background
excesses:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
-250 | 7.5 | 3.9 | 1.3 +/- 0.282 +/- 0.291 | 0.117
-9.3 | 0.2 | 4 | 0.778 +/- 0.175 +/- 0.175 | 0.147
16.4 | 0.2 | 3.4 | 0.646 +/- 0.175 +/- 0.145 | 0.715
-211 | 0.05 | 7.8 | 3.07 +/- 0.364 +/- 0.689 | 0.979

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.


AT2021afpi
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  (a possible counterpart to IceCube-211125A) was not 
recently in INTEGRAL instrument FoV and can not be followed-up due
to solar angle constrains.


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

The IceCube Collaboration (http://icecube.wisc.edu/) reports:

IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-211125A (https://gcn.gsfc.nasa.gov/gcn3/31126.gcn3
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 ) in a time range of 1000 seconds centered on the alert event time (2021-11-25 06:14:01.550 UTC to 2021-11-25 06:30:41.550 UTC) during which IceCube was collecting good quality data. Excluding the event  that prompted the alert, zero track-like events are found within the 90% containment region of IceCube-211125A. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-211125A is 1.4e-01 GeV cm^-2 in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate range between 2e+02 GeV and 8e+04 GeV.

A subsequent search was performed including 2 days of data centered on the alert event time (2021-11-24 06:22:21.550 UTC to 2021-11-26 06:22:21.550 UTC). In this case, we report a p-value of 1.00, consistent with no significant excess of track events. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-211125A is 1.7e-01 GeV cm^-2 in a 2 day time window.

The IceCube Neutrino Observatory is a cubic-kilometer neutrino detector operating at the geographic South Pole, Antarctica. The IceCube realtime alert point of contact can be reached at roc@icecube.wisc.edu<mailto:roc@icecube.wisc.edu>.

[1] IceCube Collaboration, R. Abbasi  et al., ApJ 910 4 (2021)

Hugo Ayala (Penn State) reports on behalf of the HAWC
collaboration (http://www.hawc-observatory.org/collaboration):

On 2021/11/25 06:22:21 UTC, the IceCube collaboration reported a
track-like very-high-energy event that has a high probability of
being an astrophysical neutrino, IceCube-211125A. Location is at
RA: 43.59 (+3.13/-2.71 deg  90% PSF containment) J2000
Dec: 22.59 (+1.54/-2.53 deg 90% PSF containment) J2000
(GCN circular 31126
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 ).

We performed two types of analyses for the follow-up. The first is for
a steady source in archival data and the second is a search for a
transient source. We assume a power-law spectrum with an index of -2.3
for both analyses.

Search for a steady source in archival data:
The archival data spans from November 2014 to June 2019. We searched
inside the reported IceCube error region.
The most significant location, with p-value 3.36e-3 (0.34 post-trials),
is at RA 44.03 deg, Dec +20.62 deg (��0.10 deg 68% containment) J2000.
We set a time-integrated 95% CL  upper limit on gamma rays at the
maximum position of:

E^2 dN/dE = 2.66e-13 (E/TeV)^-0.3 TeV.cm^-2.s^-1

Search for a transient source.

Since the IceCube event fall inside the HAWC field of view,
we report on the result for the transit of the IceCube
position.

Data acquisition started on 2021/11/25 01:54:48 UTC and ended
2021/11/05 08:19:21 UTC.
The most significant location, with p-value 3.8e-3  (0.34 post-trials),
is at RA 43.07 deg, Dec +23.52 deg (��0.15 deg 68% containment) J2000.
We set a time-integrated 95% CL upper limit at the position of
maximum significance of:

E^2 dN/dE = 1.36e-11 (E/TeV)^-0.3 TeV.cm^-2.s^-1

HAWC is a very-high-energy gamma-ray observatory operating in Central
Mexico at latitude 19 deg. north. Operating day and night with over
95% duty cycle, HAWC has an instantaneous field of view of 2 sr and
surveys 2/3 of the sky every day. It is sensitive to gamma rays from
300 GeV to 100 TeV.