IceCube-200421A

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

On 21 April, 2020 at 00:35:24.24 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 threshold astrophysical neutrino purity for  Bronze alerts is 30% . This alert has an estimated false alarm rate of  2.8 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/133985_60770138.amon), more  sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 21 April 2020
Time:  00:35:24.24 UT
RA: 87.93 (+3.44/- 2.83 (+1.88/-1.64) deg  90% (50%) PSF containment) J2000
Dec: 8.23 (+2.09/-1.84 (+1.25/-1.19) deg 90% (50%) 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 4FGL or 3FHL catalog sources in the 50% uncertainty region. The nearest gamma-ray source in either catalog is 4FGL J0545.0+0613 at RA: 86.27 deg, Dec: 6.23 deg (2.59 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

Alexander Lutovinov (IKI, Russia),�� Enrico Bozzo,
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 unknown (GCN 27612
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 ).

At the time of the event (2020-04-21 00:35:24 UTC, hereafter T0),
INTEGRAL was operating in nominal mode. The peak of the event
localization probability was at an angle of 162 deg with respect to
the spacecraft pointing axis. This orientation implies strongly
suppressed (3.7% of optimal) response of ISGRI, near-optimal (85% of
optimal) response of IBIS/Veto, and strongly suppressed (38% of
optimal) response of SPI-ACS.

The background within +/-300 seconds around the event was rather
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 4.9e-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 ~5e-07 (9.2e-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: 2 possibly associated
excesses:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
3.07 | 1.15 | 3.6 | 0.553 +/- 0.152 +/- 0.186 | 0.0163
40 | 9.5 | 3.5 | 2.09 +/- 0.526 +/- 0.703 | 0.033

2 likely background excesses:

T-T0 | scale | S/N | flux ( x 1e-06 erg/cm2/s) | FAP
-36.6 | 1.5 | 3.3 | 0.433 +/- 0.133 +/- 0.146 | 0.307
-119 | 0.35 | 4.6 | 1.25 +/- 0.277 +/- 0.419 | 0.668
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

R. Hamburg (UAH) and P. Veres (UAH) report on behalf of the Fermi-GBM Team:

For the IceCube high-energy neutrino candidate event 200421A (GCN 27612
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 ),
the reported position:

RA: 87.93 (+3.44/-2.83 deg 90% PSF containment) J2000
Dec: 8.23 (+2.09/-1.84 deg 90% PSF containment) J2000

was occulted by the Earth for Fermi-GBM from approximately from 16.2
minutes prior until 18.1 minutes after event time. Therefore, the GBM
observations are not constraining for prompt gamma-ray emission.

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

On 2020/04/21 00:35:24 UTC, the IceCube collaboration reported a
track-like very-high-energy event  that has a high probability of
being an astrophysical neutrino, IceCube-200421A. Location is at
RA: 87.93 (+3.49/-2.83 deg  90% PSF containment) J2000
Dec: 8.23 (+2.09/-1.84 deg 90% PSF containment) J2000
(GCN circular 27612
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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 highest significance, 2.83 sigma (-0.43 post-trials),
is at RA 87.19 deg, Dec +7.37 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 = 2.82e-13 (E/TeV)^-0.3 TeV.cm^-2.s^-1

Search for a transient source.

Since the IceCube event fall inside the HAWC field of view,
we report on the result during the transit of the IceCube
position.

Data acquisition started on 2020/04/20 19:23:55 UTC and ended
2020/04/21 01:24:32 UTC.
The most significant location, with 3.28 sigma (1.53 post-trials),
is at RA 85.05 deg, Dec +8.5 (+-0.23 deg 68% containment) deg (J2000).
We set a time-integrated 95% CL upper limit at the position of
maximum significance of:

E^2 dN/dE = 1.3.e-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.

Alexis Coleiro (APC/Universite de Paris) and Damien Dornic (CPPM/CNRS) on behalf of the ANTARES Collaboration. <br> <br>  

Using data from the ANTARES detector, we have performed a follow-up analysis of the recently reported single track-like event IceCube-200117A (GCN 27612
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 < https://gcn.gsfc.nasa.gov/gcn3/27612.gcn3
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 >). The original reconstructed origin was 25.0 degrees below the horizon for ANTARES. <br>
No up-going muon neutrino candidate events were recorded at the location of the IceCube event coordinates (accounting for the reported uncertainties) during a +/- 1h time-window centered on the IceCube event time, and over which the potential source remained visible all time. A search over an extended time window of +/- 1 day has also yielded no detection (45% visibility). <br>

This leads to a preliminary 90% confidence level upper limit on the muon-neutrino fluence from a point source of 15 GeV.cm^-2 over the energy range 3.9 TeV ��� 4.0 PeV (the range corresponding to 5-95% of the detectable flux) for an E^-2 power-law spectrum, and 27 GeV.cm^-2 (720 GeV - 380 TeV) for an E^-2.5 spectrum. <br>

ANTARES <http://antares.in2p3.fr/ <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 
IC200421A neutrino event (GCN 27612
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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 2020-04-21 00:35:24.24 UT 
(T0) with J2000 position RA = 87.93 (+3.44, - 2.83) deg, Decl. = 8.23 
(+2.09, -1.84) deg 90% PSF containment. No cataloged >100 MeV gamma-ray 
sources are located within the 90% IC200421A localization error.

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 (> 100 
MeV) within the IC200421A 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 < 2e-9 ph cm^-2 s^-1 for ~11-years (2008-08-04 / 
2020-04-21 UTC), < 8e-9 (< 5e-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 this source 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.

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-200421A (https://gcn.gsfc.nasa.gov/gcn3/27612.gcn3
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 ) in a time
range of 2 days centered on the alert event time (2020-04-20 00:35:24.24 UTC to 2020-04-22 00:35:24.240 UTC) during which IceCube was collecting good quality data. Excluding the
event that prompted the alert, one additional track-like event is found in spatial coincidence
with the 90% containment region of IceCube-200421A. 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 2 PeV.

A subsequent search was performed to include the month of data prior to the alert event (2020-03-21 00:35:24.24 UTC to 2020-04-22 00:35:24.240 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
 8.4 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>.