IceCube-260916A
GCN Circular 45705
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 high-energy IC260916A neutrino event (GCN 45598) 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 26-09-16 at 17:33:51.90 UTC (T0) with J2000 position RA = 156.18 (+0.51, -0.44) deg, Decl. = 4.22 (+0.49, -0.50) deg 90% PSF containment (J2000).
No cataloged >100 MeV gamma-ray source is located within the 90% IC260916A localization error (FL16Y, The Fermi-LAT 16 Year Source List, The Fermi-LAT collaboration 2026, arXiv:2602.22148). Preliminary analysis indicates no significant (>5 sigma) new excess emission (> 100 MeV) at the IC260916A best-fit position. Assuming a power-law spectrum (photon index = 2.0 fixed) for a point source, the >100 MeV flux upper limit (95% confidence) is <4.5e-10 ph cm^-2 s^-1 for ~18-years (2008-08-04 / T0), <9.3e-08(<2.4e-07) ph cm^-2 s^-1 for a 1-month (1-day) integration time before T0.
In the analysis of the ~18-years integrated LAT data (100 MeV - 1 TeV), a new 5.0 sigma excess emission, Fermi J1026+0359, was detected at a separation of 0.47 deg from the neutrino best-fit position and within the IC260916A 90% confidence localization. The statistical significance is calculated following the prescription adopted in the Fourth Fermi-LAT catalog (The Fermi-LAT collaboration 2022, ApJS, 260, 53).
Assuming a power-law spectrum, the best-fit localization is (J2000) RA: 156.59 deg, Dec: 3.99 deg (0.17 deg 99% containment, 0.08 deg 68% containment). The gamma-ray best-fit spectral parameters are flux = (1.4 +/- 0.8)e-9 ph cm^-2 s^-1 and index = 2.17 +/- 0.21 (statistical uncertainty only). The most likely counterpart to Fermi J1026+0359 is the blazar 3HSP J102523.0+040229 (The 3HSP catalogue of extreme and high-synchrotron peaked blazars, Chang Y.-L. et al., 2019), separated 0.25 deg away from the best-fit position of the gamma-ray excess, and within its 99% containment. In a preliminary analysis of the LAT data over 1-day and 1-month prior T0, Fermi J1026+0359 is not significantly detected in the LAT data.
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.
GCN Circular 45615
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-260916A (https://gcn.nasa.gov/circulars/45598) in a time range of 1000 seconds centered on the alert event time (2026-09-16 17:25:31.930 UTC to 2026-09-16 17:42:11.930 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-260916A. 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-260916A 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 (2026-09-15 17:33:51.930 UTC to 2026-09-17 17:33:51.930 UTC). In this case, we report a p-value of 0.05, 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-260916A 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)
GCN Circular 45598
The IceCube Collaboration (http://icecube.wisc.edu/) reports:
On 26-09-16 at 17:33:51.90 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.
Documentation regarding the alert streams and their astrophysical purity can be found here: https://gcn.nasa.gov/missions/icecube.
This alert has an estimated false alarm rate of 3.8028 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/143132_29436198.amon), more sophisticated reconstruction algorithms have been applied offline, with the direction refined to:
Date: 26-09-16
Time: 17:33:51.90 UT
RA: 156.18 (+0.51/-0.44 deg 90% PSF containment) J2000
Dec: 4.22 (+0.49/-0.5 deg 90% PSF containment) J2000
As announced in GCN Circular 43419 (https://gcn.nasa.gov/circulars/43419), 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.
The corresponding sky map is distributed as a FITS file and follows the explicit naming convention IceCube-YYMMDDX, where YYMMDD indicates the date of the event and X is a letter distinguishing multiple alerts on the same day. The download link is provided through the GCN schema distributed via Kafka.
Detailed documentation describing the alert distribution, schemas, and probability maps is available at: https://gcn.nasa.gov/missions/icecube.
No known gamma-ray sources listed in the Fermi LAT 16-year Source List (FL16Y) are located within the 90% uncertainty region of this event.
We report here sources within the 90% confidence contour available in the multi-order probability map, rather than those selected using the simplified rectangular error-box reported in this Circular.
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