IceCube-211216A

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

On 2021-12-16 at 07:07:38.13 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 2.37 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/136055_348073.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 2021-12-16
Time: 07:07:38.13

RA: 316.05 (+2.58/-1.95 deg 90% PSF containment) J2000
Dec: 15.79 (+1.29/-1.63 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.

Two gamma-ray sources listed in the 4FGL Fermi-LAT catalog are located in the 90% containment region. The sources are 4FGL J2100.0+1445 at RA: 315.02 deg, Dec: 14.76 deg (1.43 deg away from the best-fit event position) and 4FGL J2108.5+1434 at RA: 317.15, Dec: 14.58 (1.61 deg away from the best-fit event position).

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

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-211216A (GCN 31241
Loading...
 
 ).

At the time of the event (2021-12-16 07:07:38 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 153 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (2.7% of optimal) response of ISGRI, somewhat suppressed
(54% of optimal) response of IBIS/Veto, and somewhat suppressed (44%
of optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was somewhat
unstable (excess variance 1.8).

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 3.9e-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 ~3.9e-07 (1.2e-07)
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: 1 tentatively associated
excess:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
0.0226 | 0.7 | 3.4 | 0.521 +/- 0.15 +/- 0.13 | 0.00936

1 possibly associated excess:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
0.748 | 0.05 | 3.7 | 2.23 +/- 0.571 +/- 0.557 | 0.0466

3 likely background excesses:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
-14.9 | 1.15 | 3.4 | 0.405 +/- 0.117 +/- 0.101 | 0.126
-159 | 3.3 | 3.2 | 2.14 +/- 0.687 +/- 0.536 | 0.645
220 | 0.75 | 4.2 | 0.613 +/- 0.145 +/- 0.153 | 0.736

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.

We note that combination of two low-FAP excesses (at T0+0.02s and 
T0+0.74s)
is unusual, but accurate assessment of the chance of this coincidence
would require much more detailed analysis.


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

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 track event IceCube-211216A (GCN #31241
Loading...
 
  <https://gcn.gsfc.nasa.gov/gcn3/31241.gcn3
Loading...
 
 >). At the time of the alert, the reconstructed origin was -8.4 degrees below the horizon for ANTARES.

No up-going muon neutrino candidate events were recorded within the 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 from [T-60min, T+51min] (95%). This leads to a preliminary 90% confidence level upper limit on the muon-neutrino fluence from a point source of 16 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 49 GeV.cm^-2 (1 TeV - 450 TeV) for an E^-2.5 spectrum.

A search over an extended time window of +/- 1 day has also yielded no detection (41% 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.

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-211216A (https://gcn.gsfc.nasa.gov/gcn3/31241.gcn3
Loading...
 
 )
in a time range of 1000 seconds centered on the alert event time (2021-12-16 06:59:18.130 UTC to 2021-12-16 07:15:58.130 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-211216A. 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-211216A 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 energy range between 2e+02 GeV and 1e+05 GeV.

A subsequent search was performed including 2 days of data centered on the alert event time (2021-12-15 07:07:38.130 UTC to 2021-12-17 07:07:38.130 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-211216A is 1.6e-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)

J. Wood (NASA/MSFC) reports on behalf of the Fermi-GBM team:

For the IceCube high-energy neutrino candidate event IceCube-211216A
(GCN 31241
Loading...
 
 ), at the event time Fermi-GBM was observing the reported
neutrino location at:

RA: 316.05 (+2.58/-1.95 deg 90% PSF containment) J2000
Dec: 15.79 (+1.29/-1.63 deg 90% PSF containment) J2000

There was no Fermi-GBM onboard trigger around the event time of the
neutrino candidate. An automated, blind search for short gamma-ray bursts
below the onboard triggering threshold in Fermi-GBM also identified no
counterpart candidates.

The GBM targeted search, the most sensitive, coherent search for GRB-like
signals, was run from +/-30 s around the neutrino candidate time.
From this search, no significant signal was found related
to IceCube-211216A.

We set upper limits on impulsive gamma-ray emission. Using the
representative soft, normal, and hard GRB-like templates described in
arXiv:1612.02395
Loading...
 
 , we set the following 3 sigma flux upper limits over
10-1000 keV (in units of 10^-7 erg/s/cm^2):

Timescale   Soft   Normal   Hard
-------------------------------------------
0.128 s:    7.9    14.      23.
1.024 s:    2.1    4.1      9.3
8.192 s:    0.6    1.2      2.3

These results are preliminary.

S. Garrappa (DESY-Zeuthen), S. Buson (Univ. of Wuerzburg) and J. 
Sinapius (DESY-Zeuthen) on behalf of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the high-energy 
IC211216A neutrino event (GCN 31241
Loading...
 
 ) 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 2021-12-16 07:07:38.13 UTC 
(T0) with J2000 position RA = 316.05 (+2.58, -1.95) deg, Decl. = 15.79 
(+1.29, -1.63) deg 90% PSF containment. Two cataloged >100 MeV gamma-ray 
sources are located within the 90% IC211216A localization error. These 
are 4FGL J2100.0+1445 at a distance of roughly 1.5 deg and 4FGL 
J2108.5+1434 at a distance of roughly 1.6 deg (The Fourth Fermi-LAT 
catalog DR2; The Fermi-LAT collaboration 2020, ApJS, 247, 33). 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 (>5sigma) new excess emission (0.1 - 
300 GeV) within the IC211216A 90% confidence localization. Assuming a 
power-law spectrum (photon index = 2.0 fixed) for a point source at the 
IceCube best-fit position, the >100 MeV flux upper limit (95% 
confidence) is < 6.3e-10 ph cm^-2 s^-1 for ~13-years (2008-08-04 / 
2021-12-16 UTC), < 4.2e-9 (< 2.3e-7) ph cm^-2 s^-1 for a 1-month (1-day) 
integration time before T0.

Since Fermi normally operates in an all-sky scanning mode, regular 
monitoring of this region will continue. 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.

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

On 2021/12/16 07:07:38.13 UTC, the IceCube collaboration reported a
track-like very-high-energy event  that has a high probability of
being an astrophysical neutrino, IceCube-211216A. Location is at
RA: 316.05 (+2.58/-1.95 deg  90% PSF containment) J2000
Dec: 15.74 (+1.29/-1.63 deg 90% PSF containment) J2000
(GCN circular 31241
Loading...
 
 ).

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 2.5e-3 (1.5e-1 post-trials),
is at RA 316.27 deg, Dec +15.33 deg (��0.7 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 = 1.33e-13 (E/TeV)^-0.3 TeV.cm^-2.s^-1

Search for a transient source.

Since the event was not in our field of view at the time reported,
we report the combined result for the transits before and after the
IceCube event.

Data acquisition started on 2021/12/15 18:23:07 UTC and ended
2021/12/17 01:00:21 UTC.
The most significant location, with p-value 9.7e-4  (6.3e-2 post-trials),
is at RA 318.38 deg, Dec +16.57 deg (��0.20 deg 68% containment) J2000.
We set a time-integrated 95% CL upper limit at the position of
maximum significance of:

E^2 dN/dE = 2.16e-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.