TY - JOUR
T1 - Prompt Searches for Very-high-energy γ-Ray Counterparts to IceCube Astrophysical Neutrino Alerts
AU - FACT Collaboration
AU - H.E.S.S. Collaboration
AU - MAGIC Collaboration
AU - VERITAS Collaboration
AU - Fermi-LAT collaboration
AU - Icecube Collaboration
AU - Abhir, J.
AU - Biland, A.
AU - Brand, K.
AU - Bretz, T.
AU - Dorner, D.
AU - Eisenberger, L.
AU - Elsässer, D.
AU - Günther, P.
AU - Hasan, S.
AU - Hildebrand, D.
AU - Mannheim, K.
AU - Linhoff, M.
AU - Pfeifle, F.
AU - Rhode, W.
AU - Schleicher, B.
AU - Sliusar, V.
AU - Vorbrugg, M.
AU - Walter, R.
AU - Aharonian, F.
AU - Benkhali, F. Ait
AU - Aschersleben, J.
AU - Ashkar, H.
AU - Backes, M.
AU - Martins, V. Barbosa
AU - Batzofin, R.
AU - Becherini, Y.
AU - Berge, D.
AU - Böttcher, M.
AU - Boisson, C.
AU - Bolmont, J.
AU - Borowska, J.
AU - Brose, R.
AU - Brown, A.
AU - Brun, F.
AU - Bruno, B.
AU - Casanova, S.
AU - Celic, J.
AU - Cerruti, M.
AU - Chen, A.
AU - Chernyakova, M.
AU - Chibueze, J.
AU - Chibueze, O.
AU - Cornejo, B.
AU - Cotter, G.
AU - Cozzolongo, G.
AU - Damascene Mbarubucyeye, J.
AU - de Assis Scarpin, J.
AU - Delgado Giles, A.
AU - Djannati-Ataï, A.
AU - Kiryluk, J.
N1 - Publisher Copyright:
© 2026. The Author(s). Published by the American Astronomical Society.
PY - 2026/2/1
Y1 - 2026/2/1
N2 - The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multimessenger approach, which seeks to detect the γ-rays that accompany neutrinos as they are produced at their sources. Multimessenger observations have so far provided the first evidence for a neutrino source, illustrated by the joint detection of the flaring blazar TXS 0506+056 in high-energy (E > 1 GeV) and very-high-energy (VHE; E > 100 GeV) γ-rays in coincidence with the high-energy neutrino IceCube-170922A, identified by IceCube. Imaging atmospheric Cherenkov telescopes (IACTs), namely FACT, H.E.S.S., MAGIC, and VERITAS, continue to conduct extensive neutrino target-of-opportunity follow-up programs. These programs have two components: follow-up observations of single astrophysical neutrino candidate events (such as IceCube-170922A), and observation of known γ-ray sources after the identification of a cluster of neutrino events by IceCube. Here we present a comprehensive analysis of followup observations of high-energy neutrino events observed by the four IACTs between 2017 September (after the IceCube-170922A event) and 2021 January. Our study found no associations between γ-ray sources and the observed neutrino events. We provide a detailed overview of each neutrino event and its potential counterparts. Furthermore, a joint analysis of all IACT data is included, yielding combined upper limits on the VHE γ-ray flux.
AB - The search for sources of high-energy astrophysical neutrinos can be significantly advanced through a multimessenger approach, which seeks to detect the γ-rays that accompany neutrinos as they are produced at their sources. Multimessenger observations have so far provided the first evidence for a neutrino source, illustrated by the joint detection of the flaring blazar TXS 0506+056 in high-energy (E > 1 GeV) and very-high-energy (VHE; E > 100 GeV) γ-rays in coincidence with the high-energy neutrino IceCube-170922A, identified by IceCube. Imaging atmospheric Cherenkov telescopes (IACTs), namely FACT, H.E.S.S., MAGIC, and VERITAS, continue to conduct extensive neutrino target-of-opportunity follow-up programs. These programs have two components: follow-up observations of single astrophysical neutrino candidate events (such as IceCube-170922A), and observation of known γ-ray sources after the identification of a cluster of neutrino events by IceCube. Here we present a comprehensive analysis of followup observations of high-energy neutrino events observed by the four IACTs between 2017 September (after the IceCube-170922A event) and 2021 January. Our study found no associations between γ-ray sources and the observed neutrino events. We provide a detailed overview of each neutrino event and its potential counterparts. Furthermore, a joint analysis of all IACT data is included, yielding combined upper limits on the VHE γ-ray flux.
UR - https://www.scopus.com/pages/publications/105036063624
U2 - 10.3847/1538-4357/ae2c4e
DO - 10.3847/1538-4357/ae2c4e
M3 - Article
AN - SCOPUS:105036063624
SN - 0004-637X
VL - 997
JO - Astrophysical Journal
JF - Astrophysical Journal
IS - 2
M1 - 141
ER -