IceCube-220421A

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

On 2022-04-21 at 05:20:18.26 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 2.281 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/136556_43687362.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 2022-04-21
Time:  05:20:18.26 UT
RA: 113.64 (+3.17, -2.74 deg  90% PSF containment) J2000
Dec: 5.83 (+2.18, -1.69 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.

One gamma-ray source listed in the 4FGL-DR3 Fermi-LAT catalog is located within the 90% containment region for the event: the source 4FGL J0733.8+0455 (RA: 113.47 deg, Dec: 4.93 deg J2000), 0.92 deg from the best-fit neutrino candidate 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

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 
IC220421A neutrino event (GCN 31931
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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 2022-04-21 at 05:20:18.26�� 
UT (T0) with J2000 position RA = +113.64 (+3.17, -2.74) deg, Decl. = 
+5.83 (+2.18, -1.69) deg (90% PSF containment). One cataloged gamma-ray 
(>100 MeV) source is located within the 90% IC220421A localization 
region (4FGL-DR3; arXiv:2201.11184
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 ; The Fermi-LAT collaboration 2020, 
ApJS, 247, 33). This is 4FGL J0733.8+0455, associated with the 
high-redshift blazar CGRaBS J0733+0456 (z =3.01; Healey et al. 2008 
ApJS, 175, 97).

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 IC220421A 
best-fit position. Assuming a power-law spectrum (photon index = 2.0 
fixed) for a point source at the IC220421A best-fit position, the >100 
MeV flux upper limit (95% confidence) is < 2.0e-10 ph cm^-2 s^-1 for 
~13-years (2008-08-04 to 2022-04-21 UTC), and < 5.3e-9 (< 4.8e-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.

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

IceCube has performed a search [1] for additional track-like muon neutrino events arriving from the direction of IceCube-220421A (https://gcn.gsfc.nasa.gov/gcn3/31931.gcn3
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 ) in a time range of 1000 seconds centered on the alert event time (2022-04-21 05:11:58.260 UTC to 2022-04-21 05:28:38.260 UTC) during which IceCube was collecting good quality data. Excluding the event  that prompted the alert, zero track-like events are found within the 90% containment region of IceCube-220421A. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-220421A ranges from 1.3e-01 to 1.4e-01 GeV cm^-2 in a 1000 second time window. 90% of events IceCube would detect from a source at this declination with an E^-2.5 spectrum have energies in the approximate energy range between 3e+02 GeV and 1e+05 GeV.

A subsequent search was performed including 2 days of data centered on the alert event time (2022-04-20 05:20:18.260 UTC to 2022-04-22 05:20:18.260 UTC). In this case, we report a p-value of 0.06, consistent with no significant excess of track events. The IceCube sensitivity to neutrino point sources with an E^-2.5 spectrum (E^2 dN/dE at 1 TeV) within the locations spanned by the 90% spatial containment region of IceCube-220421A ranges from 1.5e-01 to 1.6e-01 GeV cm^-2 in a 2 day time window.

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>.

[1] IceCube Collaboration, R. Abbasi  et al., ApJ 910 4 (2021)