TITLE: GCN CIRCULAR NUMBER: 44114 SUBJECT: EP260321a: Upper limits from a neutrino search with IceCube DATE: 26/03/25 21:30:19 GMT FROM: Sam Hori at IceCube/U Wisc-Madison <sahori@wisc.edu> The IceCube Collaboration (http://icecube.wisc.edu/) reports: IceCube has performed a search for track-like muon neutrino events arriving from the direction of the X-ray transient EP260321a which was first detected by Einstein Probe (https://gcn.nasa.gov/circulars/events/ep260321a#gcn-circular-44068) to investigate possible neutrino emission associated with the shock breakout scenario proposed for this event. The position of the updated localization (https://gcn.nasa.gov/circulars/events/ep260321a#gcn-circular-44075) was searched over a time window covering the time range between [-3 hours, +1 hours] from Einstein Probe detection (2026-03-21 09:23:07 UTC to 2026-03-21 13:23:07 UTC), during which time IceCube was recording good quality data. The chosen time window is motivated by the shock breakout scenario, to account for possible high-energy neutrino emission produced during the shock propagation within the stellar envelope and in the immediate aftermath of the breakout. In this case, we report a p-value of 1.0, consistent with background expectation. We accordingly derive a time-integrated muon-neutrino flux upper limit for this source of E^2 dN/dE = 2.8 x 10^-2 GeV cm^-2 at 90% CL, under the assumption of an E^-2 power law. 90% of events IceCube would detect from a source at this declination with an E^-2 spectrum have energies in the approximate energy range between 1 TeV and 6 PeV. 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) View this GCN Circular online at https://gcn.nasa.gov/circulars/44114. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...