TITLE: GCN CIRCULAR NUMBER: 30876 SUBJECT: IceCube-210922A: No significant detection in HAWC DATE: 21/09/24 17:51:30 GMT FROM: Hugo Ayala at Pennsylvania State University <hgayala@psu.edu> Hugo Ayala (Penn State) reports on behalf of the HAWC collaboration (http://www.hawc-observatory.org/collaboration): On 2021/09/22 18:17:20 UTC, the IceCube collaboration reported a track-like very-high-energy event that has a high probability of being an astrophysical neutrino, IceCube-210922A. Location is at RA: 60.73 (+0.96/-0.66 deg 90% PSF containment) J2000 Dec: -4.18 (+0.42/-0.55 deg 90% PSF containment) J2000 (GCN circular 30862). We performed two types of analyses for the follow-up. The first is for a steady source in archival data and the second is a search for a transient source. We assume a power-law spectrum with an index of -2.3 for both analyses. Search for a steady source in archival data: The archival data spans from November 2014 to June 2019. We searched inside the reported IceCube error region. The most significant location, with p-value 3.72e-2 (0.26 post-trials), is at RA 60.03 deg, Dec -3.84 deg (±0.30 deg 68% containment) J2000. We set a time-integrated 95% CL upper limit on gamma rays at the maximum position of: E^2 dN/dE = 3.42e-13 (E/TeV)^-0.3 TeV.cm^-2.s^-1 Search for a transient source. Since the event was not in our field of view at the time reported, we report the combined result for the transits before and after the IceCube event. Data acquisition started on 2021/09/21 12:58:23 UTC and ended 2021/09/23 13:05:05 UTC. The most significant location, with p-value 0.12 (0.64 post-trials), is at RA 60.12 deg, Dec -4.74 deg (±0.30 deg 68% containment) J2000. We set a time-integrated 95% CL upper limit at the position of maximum significance of: E^2 dN/dE = 8.8e-12 (E/TeV)^-0.3 TeV.cm^-2.s^-1 HAWC is a very-high-energy gamma-ray observatory operating in Central Mexico at latitude 19 deg. north. Operating day and night with over 95% duty cycle, HAWC has an instantaneous field of view of 2 sr and surveys 2/3 of the sky every day. It is sensitive to gamma rays from 300 GeV to 100 TeV.