IceCube-200921A

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

On 20/09/21 at 19:07:12.89 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 1.586 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/134512_71996695.amon), more 
sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 20/09/21
Time: 19:07:12.89  UT
RA: 195.29 (+ 2.35 - 1.73  deg 90% PSF containment) J2000
Dec:  26.24 (+ 1.51  - 1.77  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 is one  Fermi 4FGL/3FHL source located within the 90% localization region, 4FGL J1303.0+2434, located at RA: 195.76 deg, Dec: 24.58 deg (J2000), at a distance of 1.71 deg 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

R. Hamburg (UAH) reports on behalf of the Fermi-GBM team:

For the IceCube high-energy neutrino candidate event IceCube-200921A
(GCN 28468
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 ), at the event time Fermi-GBM was observing the reported
neutrino location at:

RA: 195.29 (+2.35  -1.73 deg 90% PSF containment) J2000
Dec: 26.24 (+1.51  -1.77 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-200921A.

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^-6 erg/s/cm^2):

Timescale   Soft     Normal   Hard
--------------------------------------
0.128 s:    17.      34.      73.
1.024 s:    6.4      13.      21.
8.192 s:    2.2      2.5      1.4

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-200921A (GCN 28468
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 ).

At the time of the event (2020-09-21 19:07:12 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 87 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(8.6% of optimal) response of ISGRI, strongly suppressed (31% of
optimal) response of IBIS/Veto, and somewhat suppressed (63% 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 2.9e-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 ~2.6e-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: 6 likely background
excesses:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
-3.86 | 0.25 | 3.8 | 0.752 +/- 0.217 +/- 0.316 | 0.0625
-14 | 0.9 | 3.7 | 0.381 +/- 0.114 +/- 0.16 | 0.0812
16.5 | 0.05 | 4.7 | 2.17 +/- 0.494 +/- 0.911 | 0.295
-43.9 | 0.7 | 3.2 | 0.379 +/- 0.129 +/- 0.159 | 0.832
239 | 0.95 | 3.9 | 0.412 +/- 0.111 +/- 0.173 | 0.858
-19.8 | 0.15 | 3.5 | 0.891 +/- 0.28 +/- 0.375 | 0.964

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
--

Alberto Carrami��ana (INAOE) and Hugo Ayala (PSU) report on behalf of 
the HAWC collaboration (http://www.hawc-observatory.org/collaboration):

On 2020/09/21 19:07:13 UTC, the IceCube collaboration reported a track-like 
very-high-energy event that has a moderate probability of being an astrophysical 
neutrino, IceCube-200921A. Location is at 
RA: 195.29 (+2.35/-1.73 deg  90% PSF containment) J2000
Dec: 26.24 (+1.51/-1.77 deg 90% PSF containment) J2000
(GCN circular 28468
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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 May 2018. We searched
inside the reported IceCube error region.
The most significant location, with p-value  2.51e-04 (2.85e-02 post-trials),
is at RA 193.67 deg, Dec +25.86 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 = 3.03e-13 (E/TeV)^-0.3 TeV.cm^-2.s^-1

- Search for a transient source:

The event was in our field of view at the time reported. Data acquisition 
started on 2020/09/21 16:13:38 UTC and ended 2020/09/21 22:39:47 UTC.
The most significant location, with p-value 2.02e-04  (2.30e-02 post-trials),
is at RA 197.67 deg, Dec +25.82 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 = 1.63e-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.

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 
IC200921A neutrino event (GCN 28468
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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 2020-09-21 19:07:12.89 UT 
(T0) with J2000 position RA = 195.29 (+2.35, -1.73) deg, Decl. = 26.24 
(+1.51, -1.77) deg 90% PSF containment. Three cataloged >100 MeV 
gamma-ray sources are located within the 90% IC200921A localization 
error (The Fourth Fermi-LAT catalog; The Fermi-LAT collaboration 2020, 
ApJS, 247, 33). These are 4FGL J1303.0+2434 (associated with the BL Lac 
object MG2 J130304+2434),�4FGL J1256.9+2736�(associated with the AGN 
 �NGC 4839) and�4FGL J1310.6+2449�(associated with the BL Lac object 
CRATES J131038.52+244822.1). Based on a preliminary analysis of LAT data 
at timescales of 1-month and 1-day prior to T0, these objects are not 
significantly detected (>5sigma).

We searched for the existence of intermediate (days to years) timescale 
emission from a new gamma-ray transient source. Preliminary analysis 
indicates no significant (>5sigma) new excess emission (>100 MeV) at the 
IC200921A best-fit position. 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 < 2.8e-10 ph cm^-2 s^-1 
for ~12-years (2008-08-04 / 2020-09-21 UTC), < 3.7e-9 (< 6.1e-8) ph 
cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

In the analysis of the ~12-years integrated LAT data (0.1 - 800 GeV), a 
4.7 sigma new excess of gamma rays, Fermi J1256.9+2630, was detected 1.0 
deg offset from the best-fit IC200921A position and within the 90% 
confidence localization of the direction of the neutrino. Assuming a 
power-law spectrum, the best-fit localization is (J2000) RA:�194.23 deg, 
Dec: 26.51 deg (7.4 arcmin 99% containment, 3.6 arcmin 68% containment). 
The gamma-ray best-fit spectral parameters are flux = (4+/-3)e-10 ph 
cm^-2 s^-1 and index = 1.8+/-0.2. In a preliminary analysis of the LAT 
data over 1-day and 1-month prior T0, Fermi J1256.9+2630�is not 
significantly detected in the LAT data. All values include 
the�statistical uncertainty only.

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.

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-200921A (https://gcn.gsfc.nasa.gov/gcn3/28433.gcn3
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 ) in a time
range of 2 days centered on the alert event time (2020-09-20 19:07:12.89 UTC to 2020-09-22 19:07:12.89 UTC) during which IceCube was collecting good quality data. Excluding the
event that prompted the alert, zero additional track-like events are found in spatial coincidence
with the 90% containment region of IceCube-200921A. We accordingly derive a time-integrated muon-neutrino flux upper limit at the alert position of E^2 dN/ dE = 4.8 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 (2020-08-22 19:07:12.89 UTC to 2020-09-22 19:07:12.89 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
7.6  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>.