IceCube-260217A

Jannis Necker (Leiden University), Akshay Eranhalodi (DESY), Robert Stein (JSI), and Anna Franckowiak (Ruhr University Bochum) report,

On behalf of the Zwicky Transient Facility (ZTF) and Global Relay of Observatories Watching Transients Happen (GROWTH) collaborations: 

As part of the ZTF neutrino follow up program (Stein et al. 2023), we observed the localization region of the neutrino event IceCube-260217A (Zegarelli et. al, GCN 43768
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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 2026-02-22 02:51 UTC, approximately 116.9 hours after event time due to bad weather. We covered 100.0% (0.7 sq deg) of the reported localization region. This estimate accounts for chip gaps. Each exposure was 30s with a typical depth of 20.5 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. 

ZTF and GROWTH are worldwide collaborations comprising Caltech, USA; Caltech/IPAC, USA; University of Maryland, USA; University of California, Berkeley, 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; OKC, Sweden; DZA, Germany.

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-260217A (https://gcn.nasa.gov/circulars/43768
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 ) in a time range of 1000 seconds centered on the alert event time (2026-02-17 05:47:04.510 UTC to 2026-02-17 06:03:44.510 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-260217A. We report a p-value of 1.00 in this time window.  IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, is 1.4e-01 GeV cm^-2 within the 90% spatial containment region of IceCube-260217A 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 1e+05 GeV. 

A subsequent search was performed including 2 days of data centered on the alert event time (2026-02-16 05:55:24.510 UTC to 2026-02-18 05:55:24.510 UTC). In this case, we report a p-value of 0.014, consistent with no significant excess of track events. IceCube’s sensitivity to neutrino point sources with an E^-2.5 spectrum, expressed as E^2 dN/dE evaluated at 1 TeV, is 1.6e-01 GeV cm^-2 within the 90% spatial containment region of IceCube-260217A 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) and 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 IC260217A neutrino event (GCN 43768
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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 26-02-17 at 05:55:24.52 UTC (T0) with J2000 position RA = 75.89 (+0.52, -0.56) deg, Decl. = 14.63 (+0.45, -0.43 ) deg 90% PSF containment (J2000). No cataloged gamma-ray sources are found within the 90% IC260217A 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 IC260217A 90% confidence localization. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source at the IC260217A best-fit position, the >100 MeV flux upper limit (95% confidence) is < 2.0e-10 ph cm^-2 s^-1 for ~17-years (2008-08-04 / T0), <5.5e-09 (<1.7e-07) 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 analysis, 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:

On 26-02-17 at 05:55:24.52 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 2.768 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, more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:

Date: 26-02-17
Time: 05:55:24.52 UT
RA: 75.89 (+0.52/-0.56 deg 90% PSF containment) J2000
Dec: 14.63 (+0.45/-0.43  deg 90% PSF containment) J2000

No known gamma-ray sources listed in the Fermi 4FGL-DR4 or 3FHL catalogs are located within the 90% uncertainty region of this event.

As announced in GCN Circular 43419
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 , IceCube alert notices for high-energy track alerts are now also streamed via Kafka.
IceCube Gold/Bronze track alerts are available on the Kafka topic 'gcn.notices.icecube.gold_bronze_track_alerts'.
The probability distribution of the true neutrino direction, allowing the extraction of precise 90% containment regions around the best-fit direction, is now available for revised reconstruction of high-energy track alerts.
Additional information is available at https://gcn.nasa.gov/missions/icecube.

The initial automated alert was not distributed through the classical GCN notice system, and the cause is currently under investigation.

Meanwhile, the Kafka notification was streamed with an approximate delay of 30 minutes due to a temporary technical issue. As a result, the initial GCN schema (“record_number” = 0) streamed via Kafka contains some errors (this is not the case for the GCN schema (“record_number” = 1), corresponding to the updated reconstruction, which is sent along with this circular). Specifically:
The fields “ra_dec_error” and “far” were sent as strings instead of numerical values.
record_number was sent as 1, but it should have been 0.
The field “p_astro” was sent as a percentage (30.722) instead of a decimal fraction; the correct value is 0.31.

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