IceCube-201021A

D.F. Cowen (PSU), J. DeLaunay (PSU), D. B. Fox (PSU),
A. Keivani (Columbia U.), J.P. Osborne (U. Leicester),
F. Krauss (PSU), T. Gregoire (PSU), P.A. Evans (U. Leicester),
J.A. Kennea (PSU) and H. A. Ayala Solares (PSU) report:

We have analysed 2.7 ks of XRT data for Fermi J1725.51312 (Buson et al.
GCN Circ. 28751
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 ). The data were taken in Photon Counting (PC) mode.
XRT observations establish a refined position for the
source: R.A. 17:23:14.09, Dec. +14:20:59.1, with a 90% containment
radius r_90 = 2.9".
The best-fit power-law spectrum has a photon index alpha = 1.5 (+0.8, -0.4)
and observed flux f_X = 6.5 (+3.1, -2.5) x 10-13 erg cm-2 s-1 (0.3-10 keV)
The observed flux is lower by a factor of 4 to 10 by comparison to its XMM
slew observation on 2006-09-04 (XMMSL2 J172313.9+142100, OsID:9123400002)

We do not claim this source as a counterpart to IceCube 201021A from these
observations.

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

On 2020/10/21 06:37:47 UTC, the IceCube collaboration reported a
track-like very-high-energy event  that has a high probability of
being an astrophysical neutrino, IceCube-201021A. Location is at
RA: 260.82 (+ 1.73 - 1.68 deg  90% PSF containment) J2000
Dec: 14.55 (+ 1.35 - 0.74 deg 90% PSF containment) J2000
(GCN circular 28715
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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 5.54e-03 (1.83e-01
post-trials),
is at RA 259.41deg, Dec +15.79 deg (��0.21 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 = 2.18e-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 2020/10/20 00:44:12 UTC and ended
2020/10/22 01:01:46 UTC.
The most significant location, with p-value 2.33e-03  (8.11e-02
post-trials),
is at RA 259.41 deg, Dec +14.71 deg (��0.17 deg 68% containment) J2000.
We set a time-integrated 95% CL upper limit at the position of
maximum significance of:

E^2 dN/dE = 8.99e-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.

Robert Stein (DESY), Simeon Reusch (DESY), Sven Weimann (Ruhr University Bochum) and Michael Coughlin (University of Minnesota) report,

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: 

We observed the localization region of the neutrino event IceCube-201021A (Lagunas et. al, GCN 28715
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 ) with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2020-10-23 02:21 UTC, approximately 43.7 hours after event time. We covered 6.3 sq deg at least twice, corresponding to 95.7% of the reported localization region. This estimate accounts for chip gaps. Each exposure was 300s with a typical depth of 21.0 mag. 
 
The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2020) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019). We are left with one high-significance transient candidate by our pipeline, lying within the 90.0% localization of the skymap.

+---------------------------------------------------------------------------------+
| ZTF Name     | IAU Name   | RA (deg)    | DEC (deg)   | Filter | Mag   | MagErr |
+---------------------------------------------------------------------------------+
| ZTF20acmxnpa |  AT2020ybb
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  | 260.4617133 | +14.7743083 | g      | 20.94 | 0.17   |
+---------------------------------------------------------------------------------+

ZTF20acmxnpa/AT2020ybb
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  was first detected on 2020-10-13 02:37 UTC, and is a candidate supernova around peak. This would be consistent with a CSM-interaction supernova neutrino production model. We encourage spectroscopic follow-up of this object to confirm its nature.

We were unable to retrieve the optical source reported by Im et al. (GCN 28756
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 ) with forced photometry of the ZTF science images (at SNR=3). However, our limiting magnitude is comparable to the reported flux level.

ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; IPAC, USA, WIS, Israel; OKC, Sweden; JSI/UMd, USA; U Washington, USA; DESY, Germany; MOST, Taiwan; UW Milwaukee, USA; LANL USA; Tokyo Tech, Japan; IITB, India; IIA, India; LJMU, UK; TTU, USA; SDSU, USA and USyd, Australia.

ZTF acknowledges the generous support of the NSF under AST MSIP Grant No 1440341. 
GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949.
Alert distribution service provided by DIRAC@UW (Patterson et al. 2019).
Alert database searches are done by AMPEL (Nordin et al. 2019).
Alert filtering is performed with the AMPEL Follow-up Pipeline (Stein et al. 2020).

Myungshin Im, Gregory S. Paek, Gu Lim, Sungyong Hwang (CEOU/SNU) and H.-I.
Sung (KASI), on behalf of a larger collaboration

Using the LOAO 1m telescope at Mt. Lemmon, Arizona, USA, we observed the
field centered on WISEA J172314.13+142101.5, an IR source suggested as a
possible counterpart of the gamma ray source Fermi J1725.5+1312 (Buson et
al., GCN 28751
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 ). Note that  Fermi J1725.5+1312 is a new gamma ray source
that appeared recently within the 90% localization of the neutrino event,
IC201021A (IceCube collaboration, GCN 28715
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 ). The observation started at
2020-10-23 01:51:54 (UT). The images were taken in R-band with on-source
integration time of 27 min.

We find an optical counterpart of WISEA J172314.13+142101.5 near the
detection limit of the stacked LOAO image with a tentative magnitude of
R=21.0 +- 0.3 AB mag. The PS1 DR2 archive (Chambers, K. C. et al.) shows
that this source has a mean magnitude of r=21.22 +- 0.04 AB mag (PSF
magnitude). Examination of the PS1 data also shows the variability of this
object at the level of a few tenths of magnitude in the past. Our result
indicates that the object has not brightened significantly (<0.2 mag)
compared to the historic mean. A deeper observation is desirable to better
constrain the current optical variability state of this source.

We thank the staff of LOAO, Jaehyuk Yoon, for performing the observation.

S. Buson (Univ. of Wuerzburg), S. Garrappa (DESY-Zeuthen), C. C. Cheung 
(Naval Research Laboratory) and M. Ajello (Clemson Univ.) on behalf of 
the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the high-energy 
IC201021A neutrino event (GCN�28715) 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 2020-10-21 at 
06:37:47.48�UT (T0) with J2000 position RA = 260.82 (+1.73, -1.68) deg, 
Decl. = 14.55 (+1.35, -0.74) deg (90% PSF containment). No cataloged 
 >100 MeV gamma-ray sources (The Fourth Fermi-LAT catalog, 4FGL; The 
Fermi-LAT collaboration 2020, ApJS, 247, 33)�are located within the 90% 
IC201021A localization error.

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 
IC201021A�best-fit position. Assuming a power-law spectrum (photon index 
= 2.0 fixed) for a point source at the IC201021A�best-fit position, the 
 >100 MeV flux upper limit (95% confidence) is < 4.2e-10 ph cm^-2 s^-1 
for ~12-years (2008-08-04 to 2020-10-21 UTC), and < 1.3e-8 (< 4.3e-8) ph 
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

Within the 90% confidence localization of the neutrino, 10�arcmin offset 
from the best-fit IC201021A position, an excess of gamma rays, Fermi 
J1725.5+1312, was detected in an analysis of the integrated LAT data 
(0.1 - 300 GeV) between 2008-08-04 and 2020-10-21. This putative new 
source is detected at a statistical significance >3 sigma (calculated 
following the prescription adopted in the 4FGL). Assuming a power-law 
spectrum, the candidate gamma-ray source has best-fit localization of RA 
= 260.76 deg, Decl. = 14.39 deg (5�arcmin 68% containment, 11�arcmin 99% 
containment) with best-fit spectral parameters, flux = (1.8 +/- 0.5)e-9 
ph cm^-2 s^-1, index = 2.2 +/- 0.2.�In a preliminary analysis of the LAT 
data over one day and one month prior T0, Fermi J1725.5+1312 is not 
significantly detected in the LAT data. All values include the 
statistical uncertainty only.�The highest-energy photon likely 
associated to the source is a ~52 GeV event (90% prob), detected on 2009 
November 20.

A possible counterpart of Fermi J1725.5+1312 is 1RXS J172314.4+142103 
(RA = 260.812500 deg, Decl. = 14.350556 deg; Voges et al. 1999, A&A, 
349, 389) of unknown redshift. It is located 4�arcmin�from the Fermi 
J1725.5+1312 best-fit position, and within the gamma-ray 68% positional 
uncertainty. This X-ray source has been previously reported by 
XMM-Newton, as XMMSL1 J172315.0+142102 (observed on 2006-09-04; Saxton 
et al. 2008 A&A, 480, 611). A Swift follow up observation of the field 
of IC201021A was performed on 2020-10-21 and detected 1RXS 
J172314.4+142103 at a flux consistent with catalogued values (GCN 
28724
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 ). A possible infrared�counterpart of 1RXS J172314.4+142103 is 
WISEA J172314.13+142101.5 (Cutri et al.�2013 wise.rept, 1). Coincident 
with this WISE source there is�a faint radio source (~1.1 mJy peak at 3 
GHz), as seen in the�the NRAO VLASS quick-look 
image�(https://science.nrao.edu/vlass/data-access/vlass-epoch-1-quick-look-users-guide) 
from data obtained on 2019-03-30.

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 
<http://desy.de/>) and S. Buson (sara.buson at uni-wuerzburg.de 
<http://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.

H. A. Ayala Solares (PSU), D.F. Cowen (PSU), J. DeLaunay (PSU),
D. B. Fox (PSU), A. Keivani (Columbia U.), J.P. Osborne (U. Leicester),
F. Krauss (PSU), T. Gregoire (PSU), P.A. Evans (U. Leicester) and
J.A. Kennea (PSU) report:

The Swift observation of the field of IceCube 201021A (GCN Circ. 28715
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 )
reported in GCN Circ. 28724
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  (https://gcn.gsfc.nasa.gov/gcn3/28724.gcn3
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 )
had not finished, there has been 300s more observation time and a fifth
source has been detected. We report it here for completeness.
  
A fifth X-ray source has been found, which is unknown but with count rate
consistent with the previous non-detections. We therefore do not claim it
as the likely counterpart to IceCube 201021A.

The fifth detected source was:

Source no:   5
RA (J2000):  260.56090 [degrees] = 17h 22m 14.62s
Dec (J2000): +14.5899 [degrees] = +14d 35' 23.6"
Error:       +6.5 [arcsec, 90% conf. radius]
Count rate (0.3-10 keV): 4.3 (+2.3, -1.7) x 10-3 ct s-1
Flux (0.3-10 keV):       1.8 (+1.0, -0.7) x 10-13 erg cm-2 s-1

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-201021A (https://gcn.gsfc.nasa.gov/gcn3/28715.gcn3
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 ) in a time
range of 2 days centered on the alert event time (2020-10-20 06:37:47.48 UTC to 2020-10-22 06:37:47.48 UTC) during which IceCube was collecting good quality data. Excluding the
event that prompted the alert, three additional track-like events are found in spatial coincidence
with the 90% containment region of IceCube-201021A. 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 = 3.8 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 1 TeV and 1 PeV.

A subsequent search was performed to include the month of data prior to the alert event (2020-09-21 06:37:47.48 UTC to 2020-10-22 06:37:47.48 UTC). In this case, we report a p-value of 0.08, 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
6.4 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>.

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 single track-like event IceCube-201021A (GCN 28715
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  <https://gcn.gsfc.nasa.gov/gcn3/28715.gcn3
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 >). The reconstructed origin was -14 degrees below the horizon for ANTARES.

No up-going 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 all time. This leads to a preliminary 90% confidence level upper limit on the muon-neutrino fluence from a point source of 19 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 48 GeV.cm^-2 (850 GeV - 430 TeV) for an E^-2.5 spectrum. A search over an extended time window of +/- 1 day has also yielded no detection (42% 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. Lesage (UAH) reports on behalf of the Fermi-GBM team:

For the IceCube high-energy neutrino candidate event IceCube-201021A
(GCN 28715
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 ), at the event time Fermi-GBM was observing the reported
neutrino location at:

RA: 260.82 (+1.73 -1.68 deg 90% PSF containment) J2000
Dec: 14.55 (+1.35 -0.74 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-201021A.

We set upper limits on impulsive gamma-ray emission. Using the
representative soft, normal, and hard GRB-like templates described in
arXiv:1612.02395
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 , 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:    5.2      8.1      16.0
1.024 s:    2.5      3.5      5.9
8.192 s:    0.6      0.8      1.7

These results are preliminary.

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-201021A (GCN 28715
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 ).

At the time of the event (2020-10-21 06:37:47 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 107 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (5.3% of optimal) response of ISGRI, strongly suppressed
(36% 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 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 2.7e-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 (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: 7 likely background
excesses:

T-T0  | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
145   | 1.5   | 5   | 3.74 +/- 0.804 +/- 1.5    | 0.145
42    | 1.1   | 3.7 | 3.22 +/- 0.938 +/- 1.29   | 0.182
-46.5 | 3     | 3   | 1.5 +/- 0.567 +/- 0.6     | 0.329
46.7  | 0.6   | 3.5 | 0.412 +/- 0.127 +/- 0.165 | 0.564
10.9  | 0.15  | 3.4 | 0.802 +/- 0.256 +/- 0.322 | 0.669
9.12  | 0.15  | 3.2 | 0.779 +/- 0.256 +/- 0.312 | 0.818
-30.1 | 0.2   | 3.6 | 0.75 +/- 0.221 +/- 0.301  | 0.82

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


--

H. A. Ayala Solares (PSU), D.F. Cowen (PSU), J. DeLaunay (PSU),
D. B. Fox (PSU), A. Keivani (Columbia U.), J.P. Osborne (U. Leicester),
F. Krauss (PSU), T. Gregoire (PSU), P.A. Evans (U. Leicester) and
J.A. Kennea (PSU) report:

Swift observed the field of IceCube 201021A (GCN Circ. 28715
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 ) between 08:56:50
2020 October 21 and 12:05:31 on 2020 October 21, collecting a total of 3.4 ks of
cleaned photon counting (PC) mode data. The observations used a 4-point tiling
pattern with a radius of ~0.3 degrees.

We found 4 X-ray sources, as detailed below. All of these are either known X-ray
sources, consistent with catalogued fluxes, or are unknown but with count rate
consistent with the previous non-detections. We therefore do not claim any of
them as the likely counterpart to IceCube 201021A.

The 3-sigma upper limit in the field was in the range 5-8 x 10^-3 ct/sec.

The detected sources were:

Source no:   1
RA (J2000):  260.80883 [degrees] = 17h 23m 14.12s
Dec (J2000): +14.3492 [degrees] = +14d 20' 57.2"
Error:       +4.5 [arcsec, 90% conf. radius]
Count rate (0.3-10 keV): 1.2 (+0.7, -0.5) x 10-2 ct s-1
Flux (0.3-10 keV):       5.3 (+3.2, -2.3) x 10-13 erg cm-2 s-1
Notes: This source is consistent with 1RXS J172314.4+142103


Source no:   2
RA (J2000):  261.04703 [degrees] = 17h 24m 11.29s
Dec (J2000): +14.7320 [degrees] = +14d 43' 55.3"
Error:       +5.2 [arcsec, 90% conf. radius]
Count rate (0.3-10 keV): 7 (+4, -3) x 10-3 ct s-1
Flux (0.3-10 keV):       3.1 (+1.9, -1.3) x 10-13 erg cm-2 s-1


Source no:   3
RA (J2000):  260.59538 [degrees] = 17h 22m 22.89s
Dec (J2000): +14.7598 [degrees] = +14d 45' 35.3"
Error:       +6.3 [arcsec, 90% conf. radius]
Count rate (0.3-10 keV): 7.2 (+2.9, -2.3) x 10-3 ct s-1
Flux (0.3-10 keV):       3.10 (+1.27, -1.00) x 10-13 erg cm-2 s-1


Source no:   4
RA (J2000):  260.84848 [degrees] = 17h 23m 23.63s
Dec (J2000): +14.7720 [degrees] = +14d 46' 19.3"
Error:       +5.0 [arcsec, 90% conf. radius]
Count rate (0.3-10 keV): 7.4 (+3.6, -2.7) x 10-3 ct s-1
Flux (0.3-10 keV):       3.2 (+1.5, -1.2) x 10-13 erg cm-2 s-1

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

On 20/10/21 at 06:37:47.48 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 2.987 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/134621_31008065.amon), more 
sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 20/10/21
Time: 06:37:47.48  UT
RA: 260.82 (+ 1.73 - 1.68  deg 90% PSF containment) J2000
Dec: 14.55 (+ 1.35 - 0.74  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 J1728.0+1216 located at RA 262.02 deg and Dec 12.28 deg (J2000), at a distance of 2.56 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