IceCube-200926B

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

On 2020-09-26 22:35:29.22 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 1.29 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/134535_41069485.amon), more 
sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 20/09/26
Time: 22:35:29.22  UT
RA: 184.75 (+3.64 -1.55  deg 90% PSF containment) J2000
Dec: 32.93 (+1.15   -0.91  deg 90% PSF containment) J2000 

We do note a significant shift and increased error estimate in the refined RA position with respect to the automated alert.  This event had a relatively short track within the instrumented region of our detector, leading to some ambiguity in the final direction.  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 J1220.1+3432 located at RA 185.05 deg and Dec 34.54 deg (J2000), at a distance of 1.63 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 single track-like event IceCube-200926B (GCN 28509
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  <https://gcn.gsfc.nasa.gov/gcn3/28509.gcn3
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 >). The reconstructed origin was -13.3 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 all time. This leads to a preliminary 90% confidence level upper limit on the muon-neutrino fluence from a point source of 15.5 GeV.cm^-2 over the energy range 8 TeV - 7 PeV (the range corresponding to 5-95% of the detectable flux) for an E^-2 power-law spectrum, and 35.5 GeV.cm^-2 (1 - 690 TeV) for an E^-2.5 spectrum. A search over an extended time window of +/- 1 day has also yielded no detection (29% 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) 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 
IC200926B neutrino event (GCN 28509
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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 2020-09-26 at 22:35:29.22� 
UT (T0) with J2000 position RA = 184.75 (+3.64, -1.55) deg, Decl. = 
32.93 (+1.15, -0.91) deg 90% PSF containment. No cataloged >100 MeV 
gamma-ray sources (The Fourth Fermi-LAT catalog; The Fermi-LAT 
collaboration 2020, ApJS, 247, 33) are located within the 90% IC200926B 
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 IC200926B 
best-fit position. Assuming a power-law spectrum (photon index = 2.0 
fixed) for a point source at the IC200926B best-fit position, the >100 
MeV flux upper limit (95% confidence) is < 1.4e-10 ph cm^-2 s^-1 for 
~12-years (2008-08-04 / 2020-09-26 UTC), < 6.6e-9 (<1e-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 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-200926B (https://gcn.gsfc.nasa.gov/gcn3/28509.gcn3
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 ) in a time
range of 2 days centered on the alert event time (2020-09-25 22:35:29.22 UTC to 2020-09-27 22:35:29.22 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-200926B. 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 = 4.5 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 700 GeV and 500 TeV.

A subsequent search was performed to include the month of data prior to the alert event (2020-08-27 22:35:29.22 UTC to 2020-09-27 22:35:29.22 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
6.2 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>.

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

For the IceCube high-energy neutrino candidate event IceCube-200926B
(GCN 28509
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 ), at the event time Fermi-GBM was observing the reported
neutrino location at:

RA: 184.75 (+3.64 -1.55  deg 90% PSF containment) J2000
Dec: 32.93 (+1.15 -0.91  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-200926B.

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:    7.2      10.      23.
1.024 s:    2.1      3.6      6.5
8.192 s:    0.7      1.0      2.4

These results are preliminary.

Alberto Carrami��ana (INAOE) reports on behalf of the HAWC 
collaboration (http://www.hawc-observatory.org/collaboration):

On 2020/09/26 22:35:29 UTC, the IceCube collaboration reported a 
track-like very-high-energy event  that has a moderate probability of 
being an astrophysical neutrino, IceCube-200926B. Location is at 
RA: 184.75 (+3.64/-1.55 deg  90% PSF containment) J2000
Dec: 32.93 (+1.15/-0.91 deg 90% PSF containment) J2000  
(GCN circular 28509
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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 2.72e-03 (1.38e-01 post-trials), 
is at RA 185.24 deg, Dec +32.58 deg (��0.16 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 = 2.78e-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 2020/09/25 21:30:27 UTC and ended
2020/09/27 21:45:47 UTC.
The most significant location, with p-value 1.66e-02 (5.98e-01 post-trials),
is at RA 185.45 deg, Dec +32.49 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 = 7.79e-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.

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-200926B (GCN 28509
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 ).

At the time of the event (2020-09-26 22:35:29 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 85 deg with respect to the
spacecraft pointing axis. This orientation implies strongly suppressed
(12% of optimal) response of ISGRI, strongly suppressed (30% of
optimal) response of IBIS/Veto, and somewhat suppressed (52% of
optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was 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 3.4e-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 ~3.2e-07 (8.9e-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: 8 likely background
excesses:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
-14.6 | 0.5 | 3.6 | 0.573 +/- 0.187 +/- 0.261 | 0.178
-17.7 | 0.5 | 3.5 | 0.59 +/- 0.187 +/- 0.269 | 0.257
-255 | 6.5 | 3.5 | 1.75 +/- 0.517 +/- 0.795 | 0.28
-24.7 | 0.05 | 4.7 | 2.52 +/- 0.606 +/- 1.15 | 0.465
-15.4 | 0.55 | 3 | 0.47 +/- 0.178 +/- 0.214 | 0.592
-62.2 | 0.5 | 3.5 | 0.594 +/- 0.187 +/- 0.271 | 0.853
38.2 | 0.9 | 3 | 0.364 +/- 0.139 +/- 0.166 | 0.928
45.3 | 0.55 | 3.3 | 0.523 +/- 0.179 +/- 0.238 | 0.936

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

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