IceCube-201221A

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

On 20/12/21 at 12:36:53.45 UT IceCube detected a track-like event with a high probability of being of astrophysical origin. The event was selected by the ICECUBE_Astrotrack_GOLD alert stream.  The average astrophysical neutrino purity for Gold alerts is 50%. This alert has an estimated false alarm rate of 0.147 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/134817_29175858.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 20/12/21
Time: 12:36:53.45 UT
RA:  261.69 (+2.29  - 2.50 deg  90% PSF containment) J2000
Dec: 41.81 (+1.29  -1.20  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 no Fermi-LAT 4FGL or 3FHL sources inside the 90% localization region. The closest source is 4FGL J1724.2+4005 located at RA 261.05 deg and Dec 40.09 deg (J2000), at a distance of 1.78 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

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-201221A (GCN 29102
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 ).

At the time of the event (2020-12-21 12:36:53 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 63 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(13% of optimal) response of ISGRI, strongly suppressed (33% of
optimal) response of IBIS/Veto, and near-optimal (79% of optimal)
response of SPI-ACS.

The background within +/-300 seconds around the event was very stable
(excess variance 1.1).

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 2.2e-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 ~1.9e-07 (6.1e-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: 1 possibly associated
excess:

T-T0   | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
0.588  | 0.4   | 3.1 | 0.379 +/- 0.119 +/- 0.127 | 0.0246

3 likely background excesses:

T-T0   | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
-1.39  | 0.05  | 3.1 | 1.06 +/- 0.342 +/- 0.355  | 0.382
-18.2  | 0.65  | 3.1 | 2.97 +/- 0.935 +/- 0.991  | 0.532
-296   | 1.5   | 3.7 | 2.28 +/- 0.614 +/- 0.76   | 0.995

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


--

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

For the IceCube high-energy neutrino candidate event IceCube-201221A
(GCN 29102
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 ), the reported position:

RA:  261.69 (+2.29  -2.50 deg  90% PSF containment) J2000
Dec: 41.81 (+1.29  -1.20  deg 90% PSF containment) J2000

was occulted by the Earth for Fermi-GBM from approximately 16.7
minutes prior until 15.2 minutes after event time. Therefore, the GBM
observations are not constraining for prompt gamma-ray emission.

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

We report an analysis of observations of the vicinity of the high-energy 
IC201221A neutrino event (GCN 29102
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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 2020-12-21 at 12:36:53.45 
UT (T0) with J2000 position RA = 261.69 (+2.29, -2.50) deg, Decl. = 
41.81 (+1.29, -1.20) deg (90% PSF containment). No cataloged >100 MeV 
gamma-ray source is located within the 90% IC201221A localization region.

We searched for intermediate (days 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 IC201221A 
best-fit position. Assuming a power-law spectrum (photon index = 2.0 
fixed) for a point source at the IC201221A best-fit position, the >100 
MeV flux upper limit (95% confidence) is < 3.3e-10 ph cm^-2 s^-1 for 
~12-years (2008-08-04 to 2020-12-21 UTC), and < 3.6e-9 (< 4.1e-8) 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 source 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.

C. Cai, C. Zheng, J. C. Liu, Q. Luo, S. Xiao, 
W. C. Xue, Q. B. Yi, Y. Q. Zhang, Y. Huang, C. K. Li, 
G. Li, X. B. Li, J. Y. Liao, X. Y. Song, S. L. Xiong, C. Z. Liu, 
X. F. Li, Z. W. Li, Z. Chang, A. M. Zhang, Y. F. Zhang, 
X. F. Lu, C. L. Zou (IHEP), Y. J. Jin, Z. Zhang (THU), 
T. P. Li (IHEP/THU), F. J. Lu, L. M. Song, 
M. Wu, Y. P. Xu, S. N. Zhang (IHEP), 
report on behalf of the Insight-HXMT team:

Insight-HXMT was taking data normally around the trigger time
(T0=2020-12-21T12:36:53.00 UTC) of this high-energy neutrino event 
(GCN #29102
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 ), which was monitored by Insight-HXMT without any 
occultation by the Earth.

There is no significant excess event (SNR > 3 sigma) 
found in the search of the Insight-HXMT/HE raw light curve
(within T0 +/- 100 s).

Assuming the counterpart GRB with three typical GRB Band spectral 
models, two typical duration timescales(1 s, 10 s) coming from the position
of this neutrino event, the 5-sigma upper-limits fluence 
(0.2 - 5 MeV, incident energy) are reported below:

Band model 1 (alpha=-1.9, beta=-3.7, Ep=70 keV):
1s:  3.9e-08 erg cm^-2   
10s: 2.4-07 erg cm^-2 

Band model 2 (alpha=-1.0, beta=-2.3, Ep=230 keV):
1s:  6.8e-08 erg cm^-2   
10s: 3.6e-07 erg cm^-2

Band model 3 (alpha=-0.0, beta=-1.5, Ep=1000 keV):
1s:  2.0e-07 erg cm^-2  
10s: 1.1e-06 erg cm^-2 

Further analysis will be reported in the following circulars.

All measurements above are made with the CsI detectors operating in the
regular mode with the energy range of about 80-800 keV (record energy).
Only gamma-rays with energy greater than about 200 keV can penetrate
the spacecraft and leave signals in the CsI detectors installed inside
of the telescope.

Insight-HXMT is the first Chinese space X-ray telescope, which was 
funded jointly by the China National Space Administration (CNSA) and 
the Chinese Academy of Sciences (CAS). 
More information could be found at: http://www.hxmt.org.

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

On 20/12/21 12:36:53.45 UTC, the IceCube collaboration reported a
track-like very-high-energy event that has a high probability of
being an astrophysical neutrino, IceCube-201221A. Location is at
RA: 261.69 (+2.29/-2.50 deg  90% PSF containment) J2000
Dec: 41.81 (+1.29/-1.20 deg 90% PSF containment) J2000
(GCN circular 29102
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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 2.02e-02 (7.10e-01 post-trials),
is at RA 263.01 deg, Dec 43.11 deg (�0.19 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 = 1.78e-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 2020/12/20 20:45:38 UTC and ended
2020/12/22 19:00:34 UTC.
The most significant location, with p-value 4.02e-03 (2.17e-01 post-trials),
is at RA 260.326 deg, Dec 40.9502 deg (�0.26 deg 68% containment) J2000.
We set a time-integrated 95% CL upper limit at the position of
maximum significance of:

E^2 dN/dE = 6.09e-12 (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.



----------------------------------------------------------------------------------------------------------------------
Mateo Fern�ndez Alonso
Postdoctoral Scholar
Pennsylvania State University
Mail:  mkf5479@psu.edu

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

IceCube has performed a search for additional track-like muon neutrino events arriving
from the direction of IceCube-201221A (https://gcn.gsfc.nasa.gov/gcn3/29102.gcn3
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 ) in a time
range of 2 days centered on the alert event time (2020-12-20 12:36:53.45 UTC to 2020-12-22 12:36:53.45 UTC) during which IceCube was collecting good quality data. Excluding the
event that prompted the alert, two additional track-like events are found in spatial coincidence
with the 90% containment region of IceCube-201221A. We find that these data are consistent with atmospheric background expectations, with a p-value of 1.0. We accordingly derive a time-integrated muon-neutrino flux upper limit at the alert position of E^2 dN/dE = 5.5 x 10^-5 TeV cm^-2 at 90% CL, under the assumption of an E^-2 power law. 90% of events IceCube would detect from a source at this declination with an E^-2 spectrum are approximately between 500 GeV and 200 TeV.

A subsequent search was performed to include the month of data prior to the alert event (2020-11-21 12:36:53.45 UTC to 2020-12-22 12:36:53.45 UTC). In this case, we report a p-value of 1.0, consistent with no significant excess of track-like events, and a corresponding time-integrated muon-neutrino flux upper limit assuming an E^-2 spectrum (E^2 dN/dE) of
6.0 x 10^-5 TeV cm^-2 at the 90% CL.

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>.