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

Using archival data from the HAWC observatory, this study performed a coincidence search for gamma rays and neutrinos across 368 IceCube alerts up to July 8, 2025, finding a background-consistent detection rate of approximately 5% with no significant evidence of a common astrophysical origin for the observed neutrino-gamma ray pairs.

Original authors: 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. Caballer

Published 2026-02-20

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: 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

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: A Cosmic Detective Story

Imagine the universe is a giant, dark ocean. For a long time, we've only been able to see the surface waves (light, radio waves, X-rays). But recently, we've started hearing "splashes" from deep underwater—these are neutrinos, ghostly particles that rarely interact with anything.

The IceCube observatory (a giant detector buried in the Antarctic ice) acts like a hydrophone, listening for these splashes. When it hears a loud one, it sends out a "SOS" alert to the rest of the astronomy world, saying, "Something big just happened over there!"

However, IceCube is like a hydrophone in a stormy ocean; it can tell you where the splash happened roughly, but it's often fuzzy. It's like hearing a splash and guessing, "It's probably somewhere in this whole bay," rather than "It's right at this specific dock."

The HAWC Observatory (High Altitude Water Cherenkov) is the other detective. Located on a volcano in Mexico, it's a giant camera that takes pictures of the sky in high-energy gamma rays (the most energetic form of light).

The Mission: This paper is about a team of detectives trying to match the "SOS" from IceCube with a "photo" from HAWC. They wanted to see if, when IceCube heard a neutrino splash, HAWC could snap a picture of a gamma-ray flare at the exact same time and place. If they match, it's the "smoking gun" proof that we've found a cosmic particle accelerator.

The Investigation: How They Did It

1. The Search Area (The "Region of Interest")

IceCube sends alerts with a "95% containment radius." Think of this like a bullseye on a dartboard. The center is the most likely spot, but the splash could have happened anywhere inside the circle. HAWC looked inside these circles for every single alert they received.

2. The Time Machine (The "Daily Maps")

HAWC has been taking pictures of the sky every day since 2015. The team didn't just look at the "average" picture of the sky (which shows steady, boring stars). They looked at the daily snapshots to see if anything suddenly got brighter—like a lightbulb flicking on for a few days.

3. The Algorithm (The "Smart Filter")

Looking at thousands of daily pictures is boring and prone to human error. So, they used a computer program called the Bayesian Block Algorithm.

Analogy: Imagine you are watching a stock market ticker. Most of the time, the price wiggles a little bit up and down (noise). But sometimes, it suddenly jumps up and stays high (a signal).
The algorithm is like a super-smart accountant that ignores the tiny wiggles and only highlights the days where the price really changed. It asks: "Is this jump real, or just random noise?"

The Results: What Did They Find?

They checked 368 alerts from IceCube. Here is what happened:

The "False Alarms": About 5% of the time, the algorithm thought it saw a gamma-ray flare. But when they looked closer, these were just random fluctuations (noise). This is exactly what you'd expect by chance. It's like flipping a coin 100 times and getting heads 5 times in a row just by luck.
The "Real" Matches: Two of those 5% "matches" turned out to be famous cosmic monsters called Active Galactic Nuclei (AGN): Markarian 421 and Markarian 501.
- These are supermassive black holes at the centers of distant galaxies, shooting out jets of energy.
- IceCube had sent an alert pointing near them, and HAWC saw them glowing brightly.

The Catch: Even though they found these two famous black holes, the team concluded that they probably didn't cause the neutrino alerts.

Why? The neutrino alerts were "fuzzy" (large circles), and the black holes were just inside those circles by chance.
Also, the black holes were actually "quiet" (not flaring) at the exact moment the neutrino was detected. It's like hearing a car crash and seeing a parked car nearby; the parked car is there, but it didn't cause the crash.

The Sensitivity Test: How Good is HAWC?

To make sure they weren't missing anything, they did a "simulation test." They took a blank spot in the sky and digitally "injected" a fake flare (like a fake firework) to see if HAWC could spot it.

The Finding: HAWC is great at spotting big, bright fireworks that last a few days. But if the firework is dim or lasts only for a few hours, HAWC might miss it.
The Analogy: If you are trying to hear a whisper in a noisy room, you need the whisper to be very loud or last a long time to be sure you heard it. If it's a quiet whisper for a split second, you'll think it was just the wind.

The Conclusion: What Does It All Mean?

No Smoking Gun Yet: They didn't find a definitive link where a specific neutrino came from a specific gamma-ray flare. The "coincidences" they found were likely just bad luck (false positives).
The Tools are Getting Better: The fact that they found the famous Markarian black holes proves their method works. They are getting better at filtering the noise.
The Future: The universe is full of these high-energy events, but they are rare and hard to catch. The paper suggests that future, bigger telescopes (like LHAASO in China or SWGO in the Southern Hemisphere) will have "sharper eyes" and "bigger ears" to finally catch the real connection between neutrinos and gamma rays.

In a nutshell: The HAWC team acted as a cosmic matchmaker, trying to introduce IceCube's neutrinos to HAWC's gamma rays. They found a few potential dates, but the couples didn't seem to be a perfect match. However, the experiment proved that their "dating app" (the algorithm) is working, and with better technology, they hope to find the perfect cosmic couple soon.

1. Problem Statement

The origin of ultra-high-energy cosmic rays (UHECRs) remains one of the most significant open questions in high-energy astrophysics. While neutrinos are considered a definitive signature of hadronic acceleration (pion decay), only a few astrophysical neutrino sources (e.g., TXS 0506+056, NGC 1068) have been confirmed. The IceCube Neutrino Observatory has released a catalog of high-probability astrophysical neutrino alerts (IceCat-1 and subsequent public alerts), but a definitive multi-messenger counterpart for many of these events has not been established.

The specific problem addressed in this work is the lack of temporal and spatial correlation between IceCube neutrino alerts and transient or steady-state $\gamma$ -ray emission in the TeV-PeV energy range. The authors aim to determine if any of the 368 IceCube alerts (up to July 2025) coincident with the High Altitude Water Cherenkov (HAWC) observatory's field of view correspond to $\gamma$ -ray flares or known steady sources, which would provide conclusive evidence of hadronic processes.

2. Methodology

Data Sources:

IceCube: 368 public alerts (Bronze: $P_{astro} \geq 30\%$ ; Gold: $P_{astro} \geq 50\%$ ) from 2011 to July 2025. Alerts include location (Right Ascension/Declination), 95% containment radius, energy, and astrophysical probability.
HAWC: Two datasets were utilized:
1. Pass 5 Time-Integrated Data: 2,565 days of reconstructed data for steady-state source analysis.
2. Daily Maps: Individual daily flux maps from March 20, 2015, to January 15, 2024, used to search for transient flares.

Analysis Pipeline:

Region of Interest (ROI) Construction: For each IceCube alert, an ROI was defined based on the alert's 95% containment radius.
Steady-State Search: The Pass 5 dataset was scanned within each ROI to identify known $\gamma$ -ray sources.
Transient Search (Daily Maps):
- For each day within the alert's visibility window, the pixel with the maximum flux within the ROI was identified.
- A light curve of maximum flux vs. time was constructed for each alert.
- Bayesian Block Algorithm (BBA): The light curves were analyzed using the BBA to detect statistically significant deviations (flares) from the background.
  - Two spectral assumptions were tested to cover different energy ranges: a hard spectrum (spectral index $\Gamma = 2.0$ ) and a soft spectrum ( $\Gamma = 3.0$ ).
  - Calibration: The BBA's "ncp prior" (penalty factor for adding blocks) was calibrated via Monte Carlo simulations to achieve a False Positive Rate (FPR) of 5%. The optimal priors were found to be 3.6 for the 2.0 maps and 5.9 for the 3.0 maps.

Sensitivity Studies:

Injection tests were performed using the AERIE framework to simulate Crab Nebula-like flares of varying durations (1–10 days) and fluxes (0.5–2.0 Crab units) to determine HAWC's detection threshold.

3. Key Contributions

Systematic Search: This is a comprehensive search for $\gamma$ -ray counterparts to a large sample of IceCube alerts (368 events) using HAWC's unique wide-field, high-duty-cycle capabilities.
Algorithm Calibration: The authors provide a rigorous calibration of the Bayesian Block Algorithm specifically for HAWC daily maps, establishing a 5% FPR threshold to distinguish real flares from noise.
Spectral Sensitivity Analysis: By comparing $\Gamma=2.0$ and $\Gamma=3.0$ spectral assumptions, the study highlights how energy estimation biases affect transient detection sensitivity.
Multi-Messenger Modeling: The paper attempts to model the hadronic emission (pion decay) for the two most promising candidates (Mrk 421 and Mrk 501) to see if they can explain the observed neutrino energies.

4. Results

Detection Statistics:

Coincidence Rate: The analysis yielded a coincident detection rate of approximately 4.3% (16 alerts) for the 2.0 maps and 4.6% (17 alerts) for the 3.0 maps.
Background Consistency: These rates are consistent with the expected background (5% FPR), suggesting that most detections are likely statistical fluctuations or steady sources rather than transient flares triggered by the neutrino alerts.
Temporal Correlation: No temporal coincidences were found where a $\gamma$ -ray flare occurred at the exact time of an IceCube alert. All "detections" were spatial coincidences with known sources or statistical noise.

Specific Candidates:

Markarian 421 (Mrk 421) & Markarian 501 (Mrk 501): Two alerts (IC111208B and IC240307G) contained these Active Galactic Nuclei (AGN) within their ROIs.
- Mrk 421: Alert occurred before HAWC operations began; no temporal data available.
- Mrk 501: Alert occurred just after the daily map dataset cutoff.
- Steady-State Analysis: Using the 2,565-day Pass 5 data, both sources were detected with high significance ( $121\sigma$ for Mrk 421, $26\sigma$ for Mrk 501).
- Hadronic Modeling: Fits assuming pion decay (required to produce the reported neutrinos of 123 TeV and 191 TeV) yielded inconclusive results. The derived spectral indices were unphysical (e.g., $\Gamma \approx -0.25$ for Mrk 501), and the reduced $\chi^2$ values were poor. This suggests the data does not support a simple pion-decay model for these specific alerts, or the $\gamma$ -rays are absorbed in the source environment.

Sensitivity Limits:

HAWC can detect Crab-like ( $1 \times$ Crab) flares in 1 day for hard spectra (2.0 maps) but requires 1.25 $\times$ Crab flux for soft spectra (3.0 maps).
Dimmer events require multi-day durations to be detected. Events below $0.75 \times$ Crab flux are generally undetectable at $3\sigma$ significance.

5. Significance and Conclusion

Null Result for Transients: The study concludes that there is currently no evidence of transient $\gamma$ -ray flares coincident with the analyzed IceCube neutrino alerts. The observed coincidences are consistent with background expectations and known steady-state sources.
Limitations: The lack of temporal correlation is attributed to:
1. The low probability that many alerts are truly astrophysical.
2. The large angular uncertainty (PSF) of IceCube alerts (up to 15 degrees), which dilutes the signal.
3. The possibility that the neutrino emission is not accompanied by observable TeV $\gamma$ -rays (e.g., due to absorption or different emission mechanisms).
Future Outlook: The authors emphasize that future observatories with higher sensitivity and better angular resolution, such as LHAASO and the planned Southern Wide-field Gamma-ray Observatory (SWGO), along with next-generation neutrino detectors (KM3NeT, P-ONE), are necessary to constrain these transient events and definitively identify the sources of high-energy neutrinos.

In summary, while the HAWC observatory successfully monitored the sky for these alerts, the current data does not support a direct temporal link between the specific IceCube alerts analyzed and TeV $\gamma$ -ray flares, reinforcing the need for more sensitive instruments and larger datasets to solve the cosmic ray origin puzzle.