Prompt Searches for Very-high-energy γ-Ray Counterparts to IceCube Astrophysical Neutrino Alerts

FACT Collaboration; H.E.S.S. Collaboration; MAGIC Collaboration; VERITAS Collaboration; Fermi-LAT Collaboration; IceCube Collaboration

doi:10.3847/1538-4357/ae2c4e

Prompt Searches for Very-high-energy γ-Ray Counterparts to IceCube Astrophysical Neutrino Alerts

FACT Collaboration
, H.E.S.S. Collaboration
, MAGIC Collaboration
, VERITAS Collaboration
, Fermi-LAT Collaboration
, IceCube Collaboration

School of Science

Swiss Federal Institute of Technology Zurich
University of Würzburg
GSI Helmholtz Centre for Heavy Ion Research
TU Dortmund University
University of Geneva
Dublin Institute for Advanced Studies
Max Planck Institute for Nuclear Physics
Yerevan State University
Heidelberg University
University of Groningen
Laboratoire Leprince-Ringuet
University of Namibia
North West University
German Electron Synchrotron
University of Potsdam
Université Paris Cité
Linnaeus University
Humboldt University of Berlin
Observatoire de Paris
Sorbonne Université
University of Oxford
Université Paris-Saclay
Friedrich-Alexander University Erlangen-Nürnberg
Polish Academy of Sciences
University of the Witwatersrand
University of Adelaide
CPPM
University of Tübingen
University of Innsbruck
Jagiellonian University in Kraków
Nicolaus Copernicus University in Toruń
Institute for High Energy Physics
Nicolaus Copernicus Astronomical Center of the Polish Academy of Sciences
Laboratoire Univers et Particules de Montpellier
Université de Bordeaux
A. Alikhanian Yerevan Institute of Physics
The University of Tokyo
Konan University
University of Amsterdam
University of Siena
University of Barcelona
CSIC - Institute of Astrophysics of Andalusia
National Institute for Astrophysics
University of Udine
University of Rome La Sapienza
Max Planck Institute for Physics (Werner Heisenberg Institute)
University of Padua
University of Zagreb
Homi Bhabha National Institute
Centro Brasileiro de Pesquisas Físicas
Complutense University
University of La Laguna
University of Lodz
National Institute for Nuclear Physics
CIEMAT
Autonomous University of Barcelona
University of Pisa
Polytechnic University of Bari
University of Insubria
University of Bergen
Port d’Informació Cientà-fica (PIC)
University of Turin
University of Rijeka
Hiroshima University
ICRANet-Armenia
University of Split
Josip Juraj Strossmayer University of Osijek
University of Turku
Tokai University
University of Trieste
Bulgarian Academy of Sciences
University of Catania
Yamagata University
University of Oulu
Chiba University
Nagoya University
Kyoto University
University of Southern Denmark
DePauw University
Temple University
University of Utah
Harvard-Smithsonian Center for Astrophysics
Washington University St. Louis
University of California at Los Angeles
California Polytechnic State University, San Luis Obispo
University of Minnesota Twin Cities
University of California at Santa Cruz
Dartmouth College
University of Delaware
University of Maryland, College Park
NASA Goddard Space Flight Center
University of Iowa
Ruhr University Bochum
University of Galway
McGill University
Ball State University
Anderson University
Columbia University
University College Dublin
Munster Technological University
University of Colorado Denver
University of Alabama
Purdue University
Seoul National University
University of Indianapolis
University of Maryland, Baltimore County
Marquette University
Loyola University Chicago
University of Canterbury
University of Wisconsin-Madison
Institute of Physics Bhubaneswar
Université libre de Bruxelles
University of Copenhagen
University of Kansas
Harvard University
Michigan State University
South Dakota School of Mines & Technology
University of California at Irvine
Technical University of Munich
University of California at Berkeley
Ohio State University
Chalmers University of Technology
RWTH Aachen University
Uppsala University
University of Rochester
Karlsruhe Institute of Technology
Johannes Gutenberg University Mainz
Georgia Institute of Technology
Queen's University Kingston
Adelaide University
Drexel University
University of Nevada, Las Vegas
Stony Brook University
Massachusetts Institute of Technology
Pennsylvania State University
Vrije Universiteit Brussel
Université catholique de Louvain
University of Münster
University of Alberta
University of Wuppertal
Southern University and A&M College
Academia Sinica - Institute of Physics
Stockholm University
Lawrence Berkeley National Laboratory
Simon Fraser University
Chung-Ang University
Yale University
Mercer University
University of Alaska Anchorage
Sungkyunkwan University
Ghent University
University of Wisconsin-River Falls
Istituto di Astrofisica Spaziale e Fisica Cosmica di Bologna

Research output: Contribution to journal › Article › peer-review

Abstract

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.

Original language	English
Article number	141
Journal	Astrophysical Journal
Volume	997
Issue number	2
DOIs	https://doi.org/10.3847/1538-4357/ae2c4e
Publication status	Published - 1 Feb 2026

Bibliographical note

Publisher Copyright:
© 2026. The Author(s). Published by the American Astronomical Society.

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10.3847/1538-4357/ae2c4e

Cite this

@article{1be916009ef343dc883f500c33a7d023,

title = "Prompt Searches for Very-high-energy γ-Ray Counterparts to IceCube Astrophysical Neutrino Alerts",

abstract = "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.",

author = "\{FACT Collaboration\} and \{H.E.S.S. Collaboration\} and \{MAGIC Collaboration\} and \{VERITAS Collaboration\} and \{Fermi-LAT Collaboration\} and \{IceCube Collaboration\} and J. Abhir and A. Biland and K. Brand and T. Bretz and D. Dorner and L. Eisenberger and D. Els{\"a}sser and P. G{\"u}nther and S. Hasan and D. Hildebrand and K. Mannheim and M. Linhoff and F. Pfeifle and W. Rhode and B. Schleicher and V. Sliusar and M. Vorbrugg and R. Walter and F. Aharonian and Benkhali, \{F. Ait\} and J. Aschersleben and H. Ashkar and M. Backes and Martins, \{V. Barbosa\} and R. Batzofin and Y. Becherini and D. Berge and M. B{\"o}ttcher and C. Boisson and J. Bolmont and J. Borowska and R. Brose and A. Brown and F. Brun and B. Bruno and S. Casanova and J. Celic and M. Cerruti and A. Chen and M. Chernyakova and J. Chibueze and O. Chibueze and B. Cornejo and G. Cotter and G. Cozzolongo and \{Damascene Mbarubucyeye\}, J. and \{de Assis Scarpin\}, J. and \{Delgado Giles\}, A. and A. Djannati-Ata{\"i} and M. Filipovic",

note = "Publisher Copyright: {\textcopyright} 2026. The Author(s). Published by the American Astronomical Society.",

year = "2026",

month = feb,

day = "1",

doi = "10.3847/1538-4357/ae2c4e",

language = "English",

volume = "997",

journal = "Astrophysical Journal",

issn = "0004-637X",

publisher = "American Astronomical Society",

number = "2",

}

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 - Filipovic, M.

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 -

Prompt Searches for Very-high-energy γ-Ray Counterparts to IceCube Astrophysical Neutrino Alerts

Abstract

Bibliographical note

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