IceCube-250421A

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

On 25-04-21 at 17:06:08.07 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 1.0254 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/140824_2517159.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 25-04-21
Time: 17:06:08.07 UT
RA: 240.91 (+2.62/-4.44 deg 90% PSF containment) J2000
Dec: 28.67 (+1.70/-1.70 deg 90% PSF containment) J2000

This neutrino candidate event traversed through the corner of the instrumented IceCube volume, yielding an event direction reconstruction with larger than average uncertainties.   We encourage follow-up by ground and space-based instruments to help identify a possible astrophysical source for the candidate neutrino.

There are four Fermi 4FGL-DR4 or 3FHL catalog sources in the 90% uncertainty region: 4FGL J1606.5+2717 at RA: 241.64 deg, Dec: 27.30 deg J2000 (1.51 deg away from the best-fit event position); 4FGL J1556.1+2812 at RA: 239.05 deg, Dec: 28.21 deg J2000 (1.70 deg away from the best-fit event position); and 4FGL J1555.3+2903 at RA: 238.83 deg, Dec: 29.06 deg J2000 (1.86 deg away from the best-fit event position); and 4FGL J1612.2+2828 at RA: 243.06 deg, Dec: 28.47 deg J2000 (1.90 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

Sahil Atri (U Roma), Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Eleonora Troja (U Roma), Camila Angulo Valdez (UNAM), Nat Butler (ASU), Simone Dichiara (Penn State University), Tsvetelina Dimitrova (ASU), Alexander Kutyrev (GSFC/UMD), William H. Lee (UNAM), Océlotl López (UNAM) and Margarita Pereyra (UNAM) report:

We observe the field of the IceCube-250421A (bronze) event with the DDOTI/OAN wide-field imager at the Observatorio Astronómico Nacional on the Sierra of San Pedro Martir (http://ddoti.astroscu.unam.mx) on the night of 2025-04-22 UTC.

DDOTI automatically pointed to the region centred in RA, Dec (J2000)= 16:03:46.49 +28:43:50.8, covering about 96% of the IceCube error region. DDOTI observed from 04:50 UTC to 11:47 UTC (from T+ 11.74 h to T+ 18.69 h after the trigger) with a total exposure of 3.5 hours, alternating with other scientific programs.

Comparing our observations to the USNO-B1 and PanSTARRS PS1 DR2 catalogues, we detect the candidate reported by SN 2025cbj at w=18.53+/0.04, consistent with the observations from the Large Array Survey Telescope (Garrappa et al., GCN Circ. 40208
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 ). 

No other uncatalogued sources within the observed field to an average 10-sigma limiting AB magnitude of:

 w > 20.7

This value is not corrected for the Galactic extinction.

We thank the staff of the Observatorio Astronómico Nacional on the Sierra of San Pedro Mártir.

L. Pfeiffer (Univ. of Wuerzburg), S. Buson (DESY, Univ. of Wuerzburg), S. Garrappa (Weizmann Institute of Science) on behalf of the Fermi-LAT collaboration:

We report an analysis of observations of the vicinity of the IC250421A high-energy neutrino event (GCN 40195
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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 25-04-21 at 17:06:08.07 UT (T0) with J2000 position RA = 240.91 (+2.62, -4.44) deg, Decl. = +28.67 (+1.70, -1.70) deg (90% PSF containment). There are five catalogued gamma-ray sources (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546
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 ) located within the 90% IC250421A localization region. These are:

4FGL J1606.5+2717, associated with B2 1604+27;

4FGL J1556.1+2812, associated with NVSS J155611+281134;

4FGL J1612.2+2828, associated with TXS 1610+285;

4FGL J1545.5+2839, associated with WISEA J154520.76+283508.6;

4FGL J1555.3+2903 (unassociated).

Based on a preliminary analysis of the LAT data over the timescale of one day and one month prior T0, these objects are not significantly detected at gamma-rays.

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 IC250421A bestfit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC250421A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 1.21 e-10 ph cm^-2 s^-1 for ~16-years (2008-08-04 / T0), < 4.61e-09 (<1.01e-07) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

Within the error circle for the direction of the neutrino, 1.39 deg offset from the best-fitIC250421 position, a ~14 sigma excess of gamma rays, Fermi J1605.3+2959, was detected in an analysis of the integrated LAT data (0.1 - 300 GeV) between 2008-08-04 and T0. Assuming a power-law spectrum, the candidate gamma-ray source has best-fit localization of RA = 241.39 deg, Dec = +30.00 deg (0.16 deg 99% containment, 0.13 deg 95% containment, 0.08 deg 68% containment) with best fit spectral parameters flux = (1.20± 0.12) × 10^-8 ph cm^-2 s^-1, index = 3.18 ± 0.23. 
A possible counterpart for Fermi J1605.3+2959 is the FSRQ CGRaBS J1605+3001 (a.k.a. BZQ J1605+3001, Stephen E. & Healey et al 2007, ApJS, 171, 61) with a redshift of 2.41 (Sloan Digital Sky Survey 2017, ApJS, 233, 25), located at 0.03 deg from the Fermi J1605.3+2959 best-fit position, within the 68% localization error.

A preliminary analysis of the temporal variability at the position of Fermi J1605.3+2959 reveals that the source underwent an approximately six-months long period of enhanced activity between late 2023 and early 2024.

In addition, in a preliminary analysis of the LAT data over 1-month timescale, we test the presence of an additional point source at the position of the optical transient SN2025cbj, a ~60-days old SN of Type-IIn which was reported in spatial coincidence with IC250421A in GCN#40208
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 . No significant gamma-ray emission was detected. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the SN2025cbj best-fit position, the >100 MeV flux upper limit (95% confidence) is  < 1.13e-08 ph cm^-2 s^-1 for a 1-month integration time before T0.

Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this region will continue. For these observations the Fermi-LAT contact person is L. Pfeiffer (leonard.pfeiffer 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-250421A (https://gcn.nasa.gov/circulars/40195
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 ) in a time range of 1000 seconds centered on the alert event time (2025-04-21 16:57:48.070 UTC to 2025-04-21 17:14:28.070 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-250421A. 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-250421A is 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 2e+02 GeV and 7e+04 GeV. 

A subsequent search was performed including 2 days of data centered on the alert event time (2025-04-20 17:06:08.070 UTC to 2025-04-22 17:06:08.070 UTC). In this case, we report a p-value of 0.26, 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-250421A ranges from 1.6e-01 to 1.7e-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.

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