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Investigating IceCube Neutrino Alerts with the HAWC $γ$-Ray Observatory

The HAWC Collaboration, R. Alfaro, C. Alvarez, A. Andrés, E. Anita-Rangel, M. Araya, J. C. Arteaga-Velázquez, D. Avila Rojas, H. A. Ayala Solares, R. Babu, E. Belmont-Moreno, A. Bernal, K. S. Caballero-Mora, T. Capistrán, F. Carreón, S. Casanova, J. Cotzomi, S. Coutiño de León, C. de León, E. De la Fuente, P. Desiati, N. Di Lalla, R. Diaz Hernandez, M. A. DuVernois, J. C. Díaz-Vélez, K. Engel, C. Espinoza, N. Fraija, S. Fraija, A. Galván-Gámez, J. A. García-González, F. Garfias, N. Ghosh, A. Gonzalez Muñoz, M. M. González, J. A. González, J. A. Goodman, D. Guevel, J. Gyeong, J. P. Harding, S. Hernández-Cadena, I. Herzog, J. Hinton, D. Huang, F. Hueyotl-Zahuantitla, P. Hüntemeyer, A. Iriarte, S. Kaufmann, D. Kieda, K. Leavitt, J. Lee, W. H. Lee, H. León Vargas, J. T. Linnemann, A. L. Longinotti, G. Luis-Raya, K. Malone, O. Martinez, J. Martínez-Castro, J. A. Matthews, P. Miranda-Romagnoli, P. E. Mirón-Enriquez, E. Moreno, M. Mostafá, M. Najafi, A. Nayerhoda, L. Nellen, M. U. Nisa, R. Noriega-Papaqui, N. Omodei, M. Osorio-Archila, E. Ponce, Y. Pérez Araujo, E. G. Pérez-Pérez, C. D. Rho, D. Rosa-González, M. Roth, H. Salazar, D. Salazar-Gallegos, A. Sandoval, M. Schneider, J. Serna-Franco, M. Shin, A. J. Smith, Y. Son, R. W. Springer, O. Tibolla, K. Tollefson, I. Torres, R. Torres-Escobedo, E. Varela, L. Villaseñor, X. Wang, Z. Wang, I. J. Watson, H. Wu, S. Yu, H. Zhou

TL;DR

This study conducts a targeted, Bayesian-Block–based search for coincident TeV γ-ray activity in archival HAWC data at IceCube alert positions, using IceCat-1 as the alert catalog. By calibrating the BBA with MC simulations and analyzing both the Pass 5 dataset and daily maps, the authors report a background-consistent coincidence rate of about $4\%$–$5\%$ and identify two spatial coincidences with AGN Markarian 421 and 501, though no temporal coincidences with IceCube events are observed. Multi-messenger modeling of these Markarian coincidences with HAWC data shows that simple pion-decay–driven scenarios are not decisively supported, highlighting degeneracies with leptonic SSC models and environmental gamma-ray absorption; the work emphasizes the need for next-generation wide-field observatories and complementary neutrino detectors to robustly pinpoint neutrino emitters. The results illustrate the challenges of correlating TeV γ-rays with TeV–PeV neutrinos and set practical sensitivity benchmarks for joint searches in current data.

Abstract

Neutrino emission from astrophysical sources has long been considered a signature of cosmic-ray acceleration. The IceCube neutrino observatory has observed a diffuse flux of TeV-PeV neutrinos, but very few confirmed sources have emerged. With the recent publication of IceCube Event Catalog (IceCat-1), IceCube has released a list of the most promising astrophysical neutrino events since May 2011. Using the archival data from the High Altitude Water Cherenkov (HAWC) $γ$-ray observatory, we perform a coincidence search for gamma rays and neutrinos using a Bayesian Block algorithm with the public IceCube alerts from IceCat-1, along with additional alerts issued later. In this work, we consider 368 alerts, up to July 8, 2025, that are within HAWC's field of view. We observe approximately a 5\% coincident detection rate, which is consistent with expectations from background. Two of these detections contain the Active Galactic Nuclei (AGN) Markarian 421 and Markarian 501. We discuss the likelihood that the neutrino/$γ$-ray coincidences are false positives and a brief overview of the results.

Investigating IceCube Neutrino Alerts with the HAWC $γ$-Ray Observatory

TL;DR

This study conducts a targeted, Bayesian-Block–based search for coincident TeV γ-ray activity in archival HAWC data at IceCube alert positions, using IceCat-1 as the alert catalog. By calibrating the BBA with MC simulations and analyzing both the Pass 5 dataset and daily maps, the authors report a background-consistent coincidence rate of about – and identify two spatial coincidences with AGN Markarian 421 and 501, though no temporal coincidences with IceCube events are observed. Multi-messenger modeling of these Markarian coincidences with HAWC data shows that simple pion-decay–driven scenarios are not decisively supported, highlighting degeneracies with leptonic SSC models and environmental gamma-ray absorption; the work emphasizes the need for next-generation wide-field observatories and complementary neutrino detectors to robustly pinpoint neutrino emitters. The results illustrate the challenges of correlating TeV γ-rays with TeV–PeV neutrinos and set practical sensitivity benchmarks for joint searches in current data.

Abstract

Neutrino emission from astrophysical sources has long been considered a signature of cosmic-ray acceleration. The IceCube neutrino observatory has observed a diffuse flux of TeV-PeV neutrinos, but very few confirmed sources have emerged. With the recent publication of IceCube Event Catalog (IceCat-1), IceCube has released a list of the most promising astrophysical neutrino events since May 2011. Using the archival data from the High Altitude Water Cherenkov (HAWC) -ray observatory, we perform a coincidence search for gamma rays and neutrinos using a Bayesian Block algorithm with the public IceCube alerts from IceCat-1, along with additional alerts issued later. In this work, we consider 368 alerts, up to July 8, 2025, that are within HAWC's field of view. We observe approximately a 5\% coincident detection rate, which is consistent with expectations from background. Two of these detections contain the Active Galactic Nuclei (AGN) Markarian 421 and Markarian 501. We discuss the likelihood that the neutrino/-ray coincidences are false positives and a brief overview of the results.
Paper Structure (14 sections, 7 equations, 7 figures, 4 tables)

This paper contains 14 sections, 7 equations, 7 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Observatories
  3. HAWC
  4. IceCube
  5. Data
  6. Methodology
  7. Data Collection
  8. Bayesian Block Algorithm (BBA)
  9. Calibrating the BBA
  10. Results
  11. Sensitivity Studies
  12. Multi-messenger Analysis with IceCube Alerts
  13. Systematic Uncertainties
  14. Conclusion

Figures (7)

  • Figure 1: HAWC's full sky data map with the 368 visible IceCube alerts. The white dots show the alerts that lie within the declination interval $-20^\circ$ to $60^\circ$. This range is selected to constrain large IceCube alerts and prevent the ROI from extending beyond HAWC's FOV. The plot is drawn in celestial coordinates with an index of 2.7 and a pivot energy ($E_p$) of 7 TeV.
  • Figure 2: The top two plots indicate two daily maps of Mrk 421, one where it is quiet (LEFT) and when it is flaring (RIGHT). The red circle indicates the IceCube alert's ROI. HAWC's PSF is indicated in black. The bottom plot shows the frequency that a given RA, Dec has the max flux in the daily map. For a recurrent flaring source like Mrk 421. HAWC sees Mrk 421 to $121\sigma$ with the Pass 5 time-integrated data.
  • Figure 3: The results for the 10 calibrations runs for 2.0 (upper) and 3.0 (lower). For low ncp_prior values, both index fits return over a 100% FPR, which is floored to 1. As the ncp_prior increases, the FPR decreases until it crosses the 0.05 threshold, indicated by the dashed horizontal line. The value where this happens is indicated by the vertical dashed line and is 3.6 for the 2.0 maps and 5.9 for the 3.0 maps. If the FPR gets below 0.001, it is set to 0.001 for readability.
  • Figure 4: The light curves and BBA fits to the HAWC daily light curves for the neutrino alerts containing Mrks 421 (upper plot) and 501 (lower plot). The alerts that captured the two Markarians are 11-12-08B and 24-03-07G, respectively. Mrk 421's alert occurred before HAWC started operations, and Mrk 501's alert occurred just after the cut-off for the daily maps. Neither IceCube alert occurred within the data set available for this analysis.
  • Figure 5: Examples of false positives for the 2.0 (upper plot) and 3.0 (lower plot) maps. Aside from the two Markarians, the 2.0 maps all share this slight excess in the first year of data. For the 3.0 maps, there's generally a low flux value around MJD 59400, though it moves slightly, depending on the map.
  • ...and 2 more figures