IceCube-250406A

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

On 2025-04-06 at 22:50:35.34 UT IceCube detected a track-like event with a high probability of being of astrophysical origin. The event was selected by the ICECUBE_Astrotrack_GOLD alert stream. The average astrophysical neutrino purity for bronze alerts is 50%. This alert has an estimated false alarm rate of 0.9358 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/140752_31006975.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 2025-04-06
Time:   22:50:35.34 UT
RA: 81.47 (+0.54, -0.53 deg 90% PSF containment) J2000
Dec: 16.96 (+0.42, -0.50 deg 90% PSF containment) J2000

No gamma-ray sources from 4FGL or 3FHL catalogs are in the 90% uncertainty region. We encourage further follow-up by ground and space-based instruments to help identify a possible astrophysical source for the candidate neutrino.

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

Rosa L. Becerra (U Roma), Alan M. Watson (UNAM), Eleonora Troja (U Roma), Camila Angulo Valdez (UNAM), Sahil Atri (U Roma), 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 observed the field of the IceCube 250406A (goldtrack) event (IceCube Collaboration, GCN Circ. 40103
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 ) 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 2025-04-07 UTC from 03:12 UTC to 05:19 UTC (from T+4.4 h to T+6.5 h after the trigger) and obtained a total exposure of 1.2 hours.

In Becerra et al. (GCN Circ. 40101
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 ), we reported a 5-sigma upper limits in the original uncertainty region distributed as a GCN notice (https://gcn.gsfc.nasa.gov/notices_amon_g_b/140752_31006975.amon). 

Here, we report observations of the updated uncertainty region (IceCube Collaboration, GCN Circ. 40103
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 ). In this region, we detect no uncataloged source to a 5-sigma AB limiting magnitude of:

w > 20.0

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.

Jannis Necker (DESY), Akshay Eranhalodi (DESY), Robert Stein (JSI), Anna Franckowiak (Ruhr University Bochum) and Jesper Sollerman (Stockholm) report:

As part of the ZTF neutrino follow up program (Stein et al. 2023), we observed the localization region of the neutrino event IceCube-250406A (Zegarelli et. al, GCN 40103
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 ) with the Palomar 48-inch telescope, equipped with the 47 square degree ZTF camera (Bellm et al. 2019, Graham et al. 2019). We started observations in the g- and r-band beginning at 2025-04-07 03:54 UTC, approximately 5.1 hours after event time. We covered 69.0% (0.6 sq deg) of the reported localization region. This includes 10.1% (0.1 sq deg) at galactic latitude < 10 deg. This estimate accounts for chip gaps. Each exposure was 300s with a typical depth of 21.0 mag.
 
The images were processed in real-time through the ZTF reduction and image subtraction pipelines at IPAC to search for potential counterparts (Masci et al. 2019). AMPEL (Nordin et al. 2019, Stein et al. 2021) was used to search the alerts database for candidates. We reject stellar sources (Tachibana and Miller 2018) and moving objects, and apply machine learning algorithms (Mahabal et al. 2019). 

No candidate counterparts were detected. This is consistent with non-detections reported by other optical telescopes (Becerra et al, GCN 40101
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 , 40102
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 , 40108
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 ).

Observations of this field will continue as part of our standard ToO cadence for high-energy neutrinos (Stein et al. 2023).

Based on observations obtained with the Samuel Oschin Telescope 48-inch and the 60-inch Telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. ZTF is supported by the National Science Foundation under Award #2407588 and a partnership including Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, USA; University of Wisconsin at Milwaukee, USA; Cornell University, USA; Drexel University, USA; University of North Carolina at Chapel Hill, USA; Institute of Science and Technology, Austria; National Central University, Taiwan; Operations are conducted by Caltech's Optical Observatory (COO), Caltech/IPAC, and the University of Washington at Seattle, USA.

GROWTH acknowledges generous support of the NSF under PIRE Grant No 1545949.
Alert distribution service provided by DIRAC@UW (Patterson et al. 2019).
Alert database searches are done by AMPEL (Nordin et al. 2019).
Alert filtering is performed with the nuztf (Stein et al. 2021, https://github.com/desy-multimessenger/nuztf ).

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-250406A (https://gcn.nasa.gov/circulars/40103
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 ) in a time range of 1000 seconds centered on the alert event time (2025-04-06 22:42:15.340 UTC to 2025-04-06 22:58:55.340 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-250406A. 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-250406A 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 9e+04 GeV. 

A subsequent search was performed including 2 days of data centered on the alert event time (2025-04-05 22:50:35.340 UTC to 2025-04-07 22:50:35.340 UTC). In this case, we report a p-value of 1.00, 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-250406A is 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.

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

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

We report an analysis of observations of the vicinity of the high-energy IC250406A neutrino event (GCN 40103
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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 2025-04-06 at 22:50:35.34 UTC (T0) with J2000 position RA = 81.47 (+0.54, -0.53) deg, Decl. = 16.96 (+0.42, -0.50) deg 90% PSF containment (J2000). No cataloged gamma-ray sources are found within the 90% IC250406A localization error (4FGL-DR4; The Fourth Fermi-LAT catalog Data Release 4, The Fermi-LAT collaboration 2023, arXiv:2307.12546
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 ).

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) within the IC250406A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC250406A best-fit position, the >100 MeV flux upper limit (95% confidence) is <1.72e-10 ph cm^-2 s^-1 for ~16-years (2008-08-04 / T0), <2.13e-08 (<1.58e-07) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.

The closest catalogued gamma-ray source to the IC250406A best-fit position is 4FGL J0521.2+1637 (The Fermi-LAT collaboration 2020, ApJS, 247, 33), associated with the compact steep spectrum quasar 3C 138, located approximately 1.15° away from the IceCube event, outside the 90% neutrino localization region. Since October 2024, the source has exhibited strong and prolonged gamma-ray flaring activity (see ATel #16845, ATel #17107). A preliminary analysis confirms that this enhanced activity is ongoing, with a daily averaged gamma-ray flux (E > 100 MeV) of (0.39± 0.29) × 10^-6 photons cm^-2 s^-1 (statistical uncertainty only), as observed on April 6, 2025. This represents an increase in flux by a factor of approximately 93 compared to the average flux reported in the 4FGL-DR4.

Since Fermi normally operates in an all-sky scanning mode, regular monitoring of this source will continue. For this source, 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.

P.A. Evans (U. Leicester) and J.A. Kennea (PSU) report on behalf of the Swift-XRT team,

Swift-XRT observed the field of IceCube-250406A (IceCube Collaboration, GCN 40103
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 ) between 22:21 UT on 2025 April 7 and 03:36 UT on 2025 April 8 (85 to 104 ks after the trigger), gathering 4.1 ks over 7 pointings centred on the position from the refined position notice.

Three X-ray sources are detected, all of which have counterparts in the 1RXS Rosat catalogue (Voges et al., 1999) and show no sign of elecated flux compared to those data. The sources are:

Source 1: Swift J052456.1+170357 at RA, Dec = 81.23375, +17.0660 which is equivalent to: 

RA (J2000):   05h 24m 56.10s
Dec (J2000): +17d 03' 57.8" 

with an uncertainty of 4.7 arcsec (radius, 90% confidence), and consistent with the position of 1RXS J052456.3+170400. The mean 0.3-10 keV count-rate is 0.030 (+/-0.007)  ct/sec, which corresponds to 1.3 (+/-0.3) x 10^-12 erg/cm^2/s.


Source 2: Swift J052427.1+172323 at RA, Dec = 81.11301, +17.3898 which is equivalent to: 

RA (J2000):   05h 24m 27.12s
Dec (J2000): +17d 23' 23.2" 

with an uncertainty of 5.7 arcsec (radius, 90% confidence), and consistent with the position of 1RXS J052425.7+172301. The mean 0.3-10 keV count-rate is 0.051 (+0.016, -0.013) ct/sec, which corresponds to 2.2 (+0.7, -0.6) x 10^-12 erg/cm^2/s.

Source 3: Swift J052425.1+172240 at RA, Dec = 81.10483, +17.3780 which is equivalent to: 

RA (J2000):   05h 24m 25.16s
Dec (J2000): +17d 22' 40.9" 

with an uncertainty of 8.0 arcsec (radius, 90% confidence), and consistent with the position of 1RXS J052425.7+172301 . The mean 0.3-10 keV count-rate is 0.042 (+0.015, -0.012) ct/sec, which corresponds to 1.8 (+0.6, -0.5) x 10^-12 erg/cm^2/s.


Note that sources 2 and 3 correspond to the same Rosat source (due to the comparitively large Rosat position errors). In fact,  neither source looks particularly point-like and both are within 30" of * 111 Tau -- a fifth magnitude BY Dra star. At this brightness the star will cause optical loading -- the detection of spurious 'X-rays' which are actually an accumulation of optical photons. We therefore view both of these sources with scepticism.

A fourth source is also detected, Swift J052440.8+163236 at RA, Dec = 81.17009, 16.5435 degrees and an uncertainty of 8.4 arcsec (radius, 90% confidence). However, this is classified "poor" which corresponds approximately to a 1-sigma detection and is thus likely spurious.

The full XRT analysis can be viewed online at https://www.swift.ac.uk/neutrino/NEUTRINO_FIELD00043/.