How does the identification of an unidentified ultra‑high‑energy gamma‑ray source reshape multi‑messenger astronomy priorities and international scientific collaboration models?
The identification of unidentified ultra-high-energy (UHE) gamma-ray sources has fundamentally transformed the landscape of multi-messenger astronomy, driving both strategic research priorities and novel international collaboration frameworks. The Large High Altitude Air Shower Observatory (LHAASO) reported the detection of 12 UHE gamma-ray sources reaching energies up to 1.4 PeV, with subsequent expansions revealing approximately 90 sources, 43 of which emit above 100 TeV—a tenfold increase from the four sources previously known at such energiesMultiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157 | Astronomy & Astrophysics (A&A)aanda +1. Among these catalogued sources, more than half remain unidentified, requiring coordinated follow-up across multiple wavelengths and messenger types to determine their natureDetection prospects of Very and Ultra High-Energy gamma rays from extended sources with ASTRI, CTA, and LHAASOarxiv .
The discovery of PeVatron candidates—cosmic accelerators capable of energizing particles to 1015 electron volts—has elevated the search for cosmic-ray origins to a central position in astrophysics research agendas. These UHE sources represent the most extreme particle accelerators in the Galaxy, and their identification has intensified efforts to distinguish between leptonic (electron-driven) and hadronic (proton-driven) emission mechanismsMulti-messenger Probe of Galactic PeVatrons - NASA ADSharvard +1. The detection of gamma rays with energies approaching 1 PeV from the Crab Nebula provided the first model-independent evidence of a leptonic PeVatron, while other sources remain candidates for hadronic acceleration that would produce accompanying neutrino signalsHAWC, VERITAS, Fermi-LAT, and XMM-Newton Follow-up Observations of the Unidentified Ultra-high-energy Gamma-Ray Source LHAASO J2108+5157 - IOPscienceiop .
LHAASO's discovery of a giant UHE gamma-ray bubble structure in the Cygnus star-forming region marked the first identification of cosmic-ray origins with energies exceeding 10 PeVLHAASO discovers giant ultra-high-energy gamma-ray bubble ...phys . This finding demonstrated that super cosmic-ray accelerators significantly increase cosmic-ray density in surrounding interstellar space, far exceeding the Milky Way's average levels and providing a possible explanation for the excess diffuse gamma-ray emission from the Galactic PlaneLHAASO discovers giant ultra-high-energy gamma-ray bubble ...phys .
The identification of UHE gamma-ray sources has made neutrino correlation searches a high-priority endeavor. The IceCube Neutrino Observatory conducted systematic searches using 11 years of track event data on the 12 original LHAASO UHE sources, performing both individual source and stacking analyses across source classes including supernova remnants and pulsar wind nebulaeSearches for Neutrinos from Large High Altitude Air Shower ...iop +1. Although no significant neutrino emission was detected, the resulting flux upper limits constrained the hadronic contribution to gamma-ray emission for sources including the Crab Nebula and LHAASO J2226+6057Searches for Neutrinos from Large High Altitude Air Shower ...iop .
Extended searches examining LHAASO-identified microquasars using ten years of IceCube data similarly found no significant neutrino signal, establishing that this population can account for only a small fraction of the diffuse neutrino flux along the Galactic Plane[PDF] Search for neutrino emission from LHAASO observed Microquasar ...arxiv . These null results carry substantial scientific value, enabling theorists to exclude certain physics models of acceleration mechanisms while guiding future observation strategiesExploring the LHAASO ultra-high-energy gamma-ray sources for neutrinos – IceCubewisc .
The identification of PeVatrons has directly influenced the scientific priorities and technical specifications of upcoming observatories. IceCube-Gen2 is explicitly designed to resolve the high-energy neutrino sky from TeV to EeV energies and investigate cosmic particle acceleration through multi-messenger observationsIceCube-Gen2: the window to the extreme Universemit . The observatory will increase annual cosmic neutrino detection rates by a factor of ten compared to IceCube and detect sources five times fainter, with planned completion by 2033IceCube-Gen2: the window to the extreme Universemit .
The synergy between IceCube-Gen2 and the Cherenkov Telescope Array (CTA) represents a deliberate design strategy to correlate neutrino detections above a few TeV with contemporaneous very-high-energy gamma-ray emission, providing crucial simultaneous coverage to pinpoint PeVatrons and determine whether particle acceleration is hadronic or leptonic in originL: Report of the Panel on Particle Astrophysics and Gravitationnationalacademies . Near-future kilometer-scale detectors under construction in the northern hemisphere, including KM3NeT and Baikal-GVD, will further enhance sensitivity, while IceCube-Gen2's effective area will increase by approximately a factor of fiveSearches for Galactic Neutrinos with the IceCube Neutrino observatoryarxiv .
The Astrophysical Multimessenger Observatory Network (AMON) exemplifies how UHE source discoveries have necessitated new collaborative structures. AMON provides a framework for correlating high-energy astrophysical signals across all astronomical messengers: photons, neutrinos, cosmic rays, and gravitational wavesMoU | AMONpsu . Its primary objectives include enabling participating observatories to share data with strict anonymity and confidentiality, enhancing combined sensitivity through sub-threshold data coincidence searches, and enabling follow-up imaging with minimal latencyMoU | AMONpsu .
The AMON Memorandum of Understanding establishes detailed protocols governing participation. Triggering observatories (wide field-of-view instruments) feed sub-threshold event streams into the framework, which processes them to search for temporal and spatial correlations and generate alerts. Follow-up observatories then search for electromagnetic counterparts with high-throughput instrumentsMoU | AMONpsu . New triggering observatories must undertake studies with simulated or archival data and complete trial periods demonstrating positive impact on coincidence searches before full participationMoU | AMONpsu .
The data release procedures specified in the AMON MoU represent sophisticated intellectual property protections while enabling collaborative science. All data sent to AMON remains the intellectual property of contributing observatories and cannot be released without approvalMoU | AMONpsu . High-significance alerts are initially distributed only to participating observatories via private Gamma-ray Burst Coordinates Network channels, with information about which observatories triggered withheld by default until participants conferMoU | AMONpsu . The framework includes phased implementation: archival analysis, real-time in-network distribution, and finally open public distributionMoU | AMONpsu .
The GCN/TAN network (Gamma-Ray Burst Coordinates Network and Transient Astronomy Network) coordinates multi-messenger alerts, initially created for gamma-ray burst follow-up by spaceborne detectors but now expanded to include many instruments tracking non-GRB transients such as active galaxy flaringReal-time Alerts - IceCube Neutrino Observatorywisc . IceCube's alert categories include EHE (extremely high-energy) alerts for single neutrinos from the Northern Hemisphere with typical pointing errors less than 0.5 degrees, occurring four to six times annuallyReal-time Alerts - IceCube Neutrino Observatorywisc .
The VODF (Very-high-energy Open Data Format) initiative, formed in early 2023, represents a convergence of eleven VHE experiments—ASTRI, CTAO, FACT, Fermi-LAT, HAWC, H.E.S.S., IceCube, KM3NeT, MAGIC, SWGO, and VERITAS—to specify common high-level data formats from gamma-ray facilities and neutrino detectorsPoS(ICRC2023)1510sissa . This initiative aims to better respect FAIR principles and IVOA recommendations while enabling joint multi-wavelength and multi-messenger analysesPoS(ICRC2023)1510sissa .
LHAASO has established memoranda of understanding with multiple gamma-ray and neutrino experiments worldwide, including VERITAS, ANTARES, H.E.S.S., and Baikal-GVD, with additional agreements under discussion with CTA, MAGIC, and IceCubeZhen Chao: LHAASO and it's First Few Discoveriesyoutube . This network enables coordinated observations across different wavelength and messenger types, with LHAASO's source catalog becoming an integral component of global telescope coordinationZhen Chao: LHAASO and it's First Few Discoveriesyoutube .
The availability of LHAASO's unidentified source catalog, combined with real-time alerts, is expected to greatly enhance global telescope network coordination and joint detection prospects for astrophysical neutrinos and gamma-ray signals[PDF] Chapter 6 Multimessenger Physics* - INSPIREinspirehep . Since 2019, more than 40 neutrino singlet alerts located within LHAASO's field of view have been reported, creating opportunities for cross-correlation searches. LHAASO can also provide public alerts for follow-up neutrino detections by searching for hotspots with event clusters exceeding 2.75σ significance above estimated cosmic-ray background levels[PDF] Chapter 6 Multimessenger Physics* - INSPIREinspirehep .
The Southern Wide-field Gamma-ray Observatory (SWGO) exemplifies how UHE source discoveries have driven international facility planning. In July 2019, 39 research institutions from nine countries—Argentina, Brazil, Czech Republic, Germany, Italy, Mexico, Portugal, the United Kingdom, and the United States—signed an agreement creating this R&D collaborationInternational collaboration for a new gamma-ray observatory launchedswgo . The observatory is explicitly designed as a time-variability explorer filling an empty space in the global multi-messenger network of gravitational, electromagnetic, and neutrino observatoriesInternational collaboration for a new gamma-ray observatory launchedswgo .
Site selection criteria directly reflect multi-messenger science requirements. The observatory must be located at altitudes of at least 4,400 meters in the Andean region—the only Southern Hemisphere location meeting this criterionSILAFAE XII¾ - Ulisses Barres: The Southern Wide Field Gamma Ray Observatory (SWGO)youtube . The latitude range of 15-30 degrees south ensures the Galactic center remains at low zenith distance, a primary science target, while maintaining overlap regions for joint studies with LHAASOSILAFAE XII¾ - Ulisses Barres: The Southern Wide Field Gamma Ray Observatory (SWGO)youtube . Four South American countries proposed sites meeting these requirements: Argentina with two locations in Salta province, Bolivia at Chacaltaya, Chile with two locations in the Atacama Astronomical Park, and Peru with ground-based locations in Arequipa and lakes in CuzcoAn update on site search activities for SWGOarxiv .
SWGO's Southern Hemisphere sky coverage will complement LHAASO's northern coverage, providing daily monitoring of the full sky at very high energiesL: Report of the Panel on Particle Astrophysics and Gravitationnationalacademies . The combination of CTA and LHAASO/SWGO creates an integrated observational capability maximizing scientific opportunities for all-sky multi-messenger astronomyL: Report of the Panel on Particle Astrophysics and Gravitationnationalacademies .
CTA's Key Science Projects have incorporated specific provisions for PeVatron and unidentified LHAASO source follow-up. The Galactic Plane Survey will provide approximately 10-hour point-source equivalent exposure per sky-tile, representative of nominal observation time devoted to the PeVatron key science project in survey modeSensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnantsunisi . Deep observations demonstrate that PeVatrons can be confirmed with typical exposures of 250 hours, while 50-hour observations yield approximately 50% confirmation probability for hard-spectrum (Γp≤2.3) sourcesSensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnantsunisi .
The planned follow-up strategy for unidentified LHAASO sources involves allocating substantial deep exposure time—50-250 hours—with the full CTA South array under nominal conditions, or equivalent exposure combining 10 hours nominal and 90 hours high night-sky-background observations on the SST sub-arraySensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnantsunisi . Moonlight observations of PeVatrons using only the SST array achieve similar sensitivities to the full array above energies of several tens of TeV, enabling cost-effective follow-upSensitivity of the Cherenkov Telescope Array to spectral signatures of hadronic PeVatrons with application to Galactic Supernova Remnantsunisi .
Simulations testing classification methods on unidentified extended sources from the first LHAASO catalog demonstrate that with 50 hours exposure, 68.2% of sources can be correctly classified with CTA, increasing to 74.2% with 300 hoursClassification of Unidentified Extended LHAASO Sources based on their Gamma-Ray Morphology: Prospects for Future IACTs.arxiv . The ASTRI Mini-Array achieves comparable classification rates of 65.2% at 50 hours and 69.7% at 300 hoursClassification of Unidentified Extended LHAASO Sources based on their Gamma-Ray Morphology: Prospects for Future IACTs.arxiv .
The U.S. National Science Foundation has established the Multi-Messenger Coordination for Windows on the Universe (MMC-WoU) program specifically to support framework development for optimizing electromagnetic follow-up observations of gravitational wave events or high-energy particle detectionsNSF 24-542: Multi-Messenger Coordination for Windows on the ...nsf . The program aims to reduce operational redundancy across ground and space-based observatory networks, foster open collaboration in MMA follow-up data collection, and broaden access to the multi-messenger enterpriseNSF 24-542: Multi-Messenger Coordination for Windows on the ...nsf .
The 2020 Decadal Survey prioritized multi-messenger astrophysics as a frontier scientific pursuit through its "New Windows on the Dynamic Universe" themeWindows on the Universe: Establishing the Infrastructure for a Collaborative Multi-messenger Ecosystemarxiv . NSF's strategic plan for fiscal years 2022-2026 incorporates the Windows on the Universe program goals to build capabilities and accelerate synergy between observations and theory for integrated multi-messenger astrophysical explorationsNSF 24-542: Multi-Messenger Coordination for Windows on the ...nsf .
The Decadal Survey identified NASA's highest-priority sustaining activity as a space-based time-domain and multi-messenger program comprising small- and medium-scale missionsNew Report Charts Path for Next Decade of Astronomy and ...nationalacademies . NASA should establish a time-domain program to sustain space-based electromagnetic capabilities required to study transient phenomena and follow up multi-messenger eventsInteractive Overview: Pathways to Discovery in Astronomy and Astrophysics for the 2020snationalacademies .
The first probe-class missions should be a Far-Infrared probe and an X-ray probe to complement the Athena mission, with a recommended cadence of one probe mission per decade at a $1.5 billion cost capNew Report Charts Path for Next Decade of Astronomy and ...nationalacademies +1. The Decadal Survey also endorsed IceCube-Gen2 and technology development for future gravitational wave observatories as important for meeting scientific objectives, despite falling under NSF's Physics division purviewInteractive Overview: Pathways to Discovery in Astronomy and Astrophysics for the 2020snationalacademies .
The shift from isolated, single-messenger collaborations to consolidated multi-messenger consortia represents a structural transformation driven by epistemic requirements. Theoretical models of transient events like supernovae and neutron star mergers predict nearly simultaneous emission of different messengers, each governed by distinct physical processes requiring specialized detection technologiesSpecial issue spotlight: Shaping a multi-messenger universeaip . No single instrument or institution can capture all relevant signals—the observational requirements themselves demand coordination across distributed, heterogeneous facilitiesSpecial issue spotlight: Shaping a multi-messenger universeaip .
Socio-epistemic network analysis of scientific literature from 1997-2023 reveals that institutional and conceptual foundations were established years before gravitational waves were detected, with "future-oriented imaginaries" profoundly shaping research practices and prioritiesSpecial issue spotlight: Shaping a multi-messenger universeaip . The analysis demonstrates a clear shift from isolated collaborations to consolidated multi-messenger consortia that rapidly expanded following the first gravitational-wave detectionSpecial issue spotlight: Shaping a multi-messenger universeaip .
Multi-messenger astronomy increasingly relies on large collaborations—IceCube comprises 300 physicists, computer scientists, and engineers, while more than 1,000 scientists contribute to LIGO What is multi-messenger astronomy? | | UW–Madison wisc . Astronomers seeking to understand extreme and mysterious objects must become comfortable operating in large groups, as discovery of small, difficult phenomena requires big teams What is multi-messenger astronomy? | | UW–Madison wisc .
Major optical observatories have adapted their policies to accommodate multi-messenger alerts. The European Southern Observatory recognizes unpredictable Targets of Opportunity requiring immediate observations of astronomical events that cannot be anticipated on sufficient timeframes for regular proposal submissionTarget of Opportunity Proposalseso . ESO's Rapid Response Mode enables automated triggering of observations within 4 hours of an event, with the telescope reaching target locations within approximately 6 minutes (or 12 minutes if focus changes are required)Target of Opportunity Proposalseso .
RRM observations receive overriding priority over other observations unless the latter are strictly time-critical, reflecting the premium placed on rapid multi-messenger follow-upTarget of Opportunity Proposalseso . Director's Discretionary Time provisions allow up to 5% of available science time for compelling questions involving time-critical observations of unexpected events or rapid follow-up that may lead to breakthrough resultsMicrosoft Word - Cou_1847_rev_Science_Policies_050520.docxeso .
The prototype transient handler system installed on the Large-Sized Telescope-1 receives real-time science alerts from the GCN network, enabling gamma-ray burst observations within 15 minutes of alert receiptCTA – the World's largest ground-based gamma-ray observatorycnrs . This infrastructure demonstrates the technical implementation of multi-messenger response capabilities at individual facilities.
The identification of UHE sources has profound implications for understanding cosmic-ray origins—a mystery persisting for over a century since cosmic-ray discoveryMulti-messenger Probe of Galactic PeVatrons - NASA ADSharvard . LHAASO observations have prompted rethinking of mechanisms by which high-energy particles are generated and propagated in the Milky Way, encouraging deeper exploration of violent celestial phenomena and testing of basic physical laws under extreme conditionsLHAASO Discovers a Dozen PeVatrons and Photons Exceeding 1 PeV and Launches Ultra-High-Energy Gamma Astronomy Era - Faculty of Science - UNIGEunige .
These observations indicate that proton PeVatrons most likely arise from young stellar clusters rather than supernova remnants, though this remains an active area of investigationPeVatron physics in the context of LHAASO resultsyoutube . The detection of 1.4 PeV photons from the Cygnus region suggests acceleration to energies corresponding to cosmic-ray energies of 2.3 PeV, implying acceleration efficiencies of approximately 15%—a factor of several times stronger than normal supernova remnant shock wavesZhen Chao: LHAASO and it's First Few Discoveriesyoutube .
Future studies combining IceCube-Gen2 and CTA observations may test scenarios for PeV neutrino and gamma-ray emissions from the Galactic center, potentially confirming Sagittarius A* as a source of particles with energies exceeding 100 PeVSagittarius $\mathrm{A}^{\star}$ as a plausible source candidate for ...arxiv . The neutrino flux predicted from LHAASO sources is comparable with gamma-ray flux in the TeV range, making these objects interesting candidates for future neutrino experiments of sufficient sensitivity to confirm or reject hadronic emission scenariosMultiwavelength study of the galactic PeVatron candidate LHAASO J2108+5157 | Astronomy & Astrophysics (A&A)aanda .
The discovery of dozens of UHE sources has launched an era for UHE gamma-ray observation, with LHAASO's findings suggesting that spectra of most sources are not truncated at previously assumed limitsLHAASO Discovers a Dozen PeVatrons and Photons Exceeding 1 PeV and Launches Ultra-High-Energy Gamma Astronomy Era - Faculty of Science - UNIGEunige . Through continued UHE gamma astronomy and multi-messenger coordination, the century-old mystery of cosmic-ray origins may soon be resolvedLHAASO Discovers a Dozen PeVatrons and Photons Exceeding 1 PeV and Launches Ultra-High-Energy Gamma Astronomy Era - Faculty of Science - UNIGEunige .