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The second H.E.S.S. gamma-ray burst catalogue: 15 years of observations with the H.E.S.S. telescopes

This paper presents the second H.E.S.S. gamma-ray burst catalogue, summarizing 15 years of observations (2004–2019) that yielded only two VHE detections (GRB 180720B and GRB 190829A) and established the largest set of VHE upper limits, concluding that detected bursts are not a distinct population but rather those with luminous X-ray emission and favorable conditions, thereby underscoring the future potential of next-generation telescopes like CTAO.

Original authors: A. Acharyya, F. Aharonian, C. Arcaro, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, J. Borowska, F. Brun, B. Bruno, C

Published 2026-03-24

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CC BY 4.0

Original authors: A. Acharyya, F. Aharonian, C. Arcaro, H. Ashkar, M. Backes, V. Barbosa Martins, R. Batzofin, Y. Becherini, D. Berge, K. Bernlöhr, M. Böttcher, C. Boisson, J. Bolmont, J. Borowska, F. Brun, B. Bruno, C. Burger-Scheidlin, S. Casanova, J. Celic, M. Cerruti, S. Chandra, A. Chen, M. Chernyakova, J. O. Chibueze, O. Chibueze, T. Collins, B. Cornejo, G. Cotter, J. Damascene Mbarubucyeye, I. D. Davids, J. de Assis Scarpin, M. de Bony de Lavergne, M. de Naurois, E. de Oña Wilhelmi, A. G. Delgado Giler, J. Devin, A. Djannati-Ataï, J. Djuvsland, A. Dmytriiev, K. Egberts, K. Egg, J. -P. Ernenwein, C. Escañuela Nieves, M. D. Filipovic, G. Fontaine, S. Funk, S. Gabici, Y. A. Gallant, M. Genaro, J. F. Glicenstein, J. Glombitza, M. -H. Grondin, L. Heckmann, B. Heß, J. A. Hinton, W. Hofmann, T. L. Holch, M. Holler, D. Horns, Z. Huang, M. Jamrozy, F. Jankowsky, I. Jaroschewski, D. Jimeno Sanchez, I. Jung-Richardt, E. Kasai, K. Kasprzak, K. Katarzyński, D. Kerszberg, B. Khélifi, W. Kluzniak, N. Komin, K. Kosack, D. Kostunin, R. G. Lang, S. Lazarević, M. Lemoine-Goumard, J. -P. Lenain, P. Liniewicz, A. Luashvili, J. Mackey, D. Malyshev, D. Malyshev, V. Marandon, M. Mayer, A. Mehta, A. Mikhno, A. M. W. Mitchell, R. Moderski, M. O. Moghadam, L. Mohrmann, A. Montanari, E. Moulin, J. Niemiec, P. O'Brien, L. Olivera-Nieto, S. Panny, M. Panter, R. D. Parsons, U. Pensec, P. Pichard, S. Pita, G. Pühlhofer, M. Punch, A. Quirrenbach, M. Regeard, A. Reimer, O. Reimer, I. Reis, H. Ren, B. Reville, F. Rieger, G. Rowell, B. Rudak, E. Ruiz-Velasco, K. Sabri, V. Sahakian, H. Salzmann, D. A. Sanchez, A. Santangelo, M. Sasaki, F. Schüssler, M. Senniappan, J. N. S. Shapopi, W. Si Said, H. Sol, S. Spencer, Ł. Stawarz, S. Steinmassl, T. Tanaka, A. M. Taylor, G. L. Taylor, R. Terrier, M. Tsirou, T. Unbehaun, C. van Eldik, M. Vecchi, C. Venter, J. Vink, T. Wach, S. J. Wagner, A. Wierzcholska, M. Zacharias, A. A. Zdziarski, W. Zhong, S. J. Zhu, A. Zech

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

Imagine the universe is a vast, dark ocean, and Gamma-Ray Bursts (GRBs) are like massive, sudden underwater volcanic eruptions. For a split second, they blast out more energy than our entire Sun will produce in its whole lifetime. Scientists have known about these "cosmic fireworks" for decades, but they've always been tricky to study because they happen so fast and so far away.

This paper is a report card from a team of astronomers using a giant, high-tech "eye" in the sky called H.E.S.S. (High Energy Stereoscopic System). Located in the highlands of Namibia, H.E.S.S. isn't a normal telescope; it's a camera that doesn't look at light directly. Instead, it looks for the faint, blue flashes of light created when high-energy gamma rays crash into Earth's atmosphere, creating a shower of particles.

Here is the story of their 15-year hunt, explained simply:

1. The Great Cosmic Hunt (The Setup)

Think of H.E.S.S. as a very fast, very sensitive security camera. When a satellite (like Swift or Fermi) spots a GRB, it sends out a "SOS" alert. The H.E.S.S. team has to react instantly. They have to swing their massive telescopes around (like a giraffe turning its neck) and start watching the spot within minutes.

The goal? To catch the afterglow of the explosion in "Very High Energy" (VHE) light. This is the most energetic light in the universe, far beyond what our eyes or normal telescopes can see.

2. The Results: Mostly Silence, Two Big Wins

Over 15 years (from 2004 to 2019), the team watched 89 different gamma-ray bursts.

The Bad News: For 87 of them, the telescopes saw nothing. The sky was quiet.
The Good News: They successfully caught the afterglow of two specific bursts: GRB 180720B and GRB 190829A.

Because they didn't find a signal for the other 87, they didn't just give up. Instead, they calculated the "maximum possible brightness" those bursts could have had without the telescopes seeing them. This created a massive library of "upper limits"—a record of how faint these cosmic explosions can be before we lose track of them. This is the largest collection of such data ever made.

3. The Detective Work: Why did we miss them?

The scientists asked: "Are the bursts we missed just normal, or are they a different kind of monster?"

They compared the "missed" bursts to the "caught" ones and the general population of bursts. They found that the ones H.E.S.S. did catch (and the ones caught by other telescopes) shared a few special traits:

They were closer: Like a lighthouse, the closer the light source, the easier it is to see.
They were brighter: They had a very bright "X-ray glow" (the afterglow).
The timing was right: The telescopes got there just as the light was still strong.

The Analogy: Imagine trying to hear a whisper in a noisy room. If the person whispering is right next to you (close), speaking loudly (bright), and you are listening at the exact moment they speak (timing), you hear them. If they are far away, whispering softly, or you arrive late, you hear nothing. The H.E.S.S. team realized they were mostly missing the "whispers" because their "ears" (telescopes) weren't sensitive enough or fast enough to catch the distant, faint ones.

4. The Physics Puzzle: The "Burn-Out" Limit

The paper also tried to solve a physics mystery. When a GRB happens, particles are accelerated to near the speed of light. Scientists have a theory called Synchrotron Self-Compton (SSC).

The Theory: Imagine electrons as tiny balls spinning in a magnetic field. They spin so fast they shoot out light (X-rays). Then, those X-rays hit other electrons and get boosted up to super-high energies (Gamma rays).
The Problem: There's a speed limit. If the electrons spin too fast, they "burn out" and can't reach the highest energies.
The Conclusion: For the bursts H.E.S.S. watched, the math showed that the "burn-out" limit wasn't broken. The bursts simply weren't bright enough in gamma rays to be seen by H.E.S.S. The models fit perfectly with the "silence" they observed.

5. The Future: Bigger Eyes, Faster Reflexes

The paper ends on a hopeful note. The current telescopes (H.E.S.S.) are like a good pair of binoculars, but the universe is huge and dark.

The Next Generation: A new observatory called CTAO (Cherenkov Telescope Array) is being built. It will have telescopes that are much more sensitive (like upgrading from binoculars to a super-powerful microscope) and can react even faster.
The Promise: With these new tools, scientists hope to catch the "whispers" from the farthest corners of the universe, even those that are currently invisible to us.

Summary

This paper is a testament to the power of "negative results." Even though H.E.S.S. didn't find many new gamma-ray bursts, the data they collected tells us exactly how bright these explosions need to be for us to see them. It proves that we are currently only seeing the "superstars" of the cosmic explosion world, and we need bigger, faster telescopes to hear the rest of the orchestra.

1. Problem and Scientific Context

Gamma-ray bursts (GRBs) are transient events releasing immense energy, followed by broadband afterglows. While prompt emission is well-studied in sub-MeV ranges, detecting Very-High-Energy (VHE; $E > 100$ GeV) emission from GRBs has historically been a major challenge for Imaging Atmospheric Cherenkov Telescopes (IACTs) due to:

Unpredictability: GRBs occur randomly, making rapid repointing difficult.
Atmospheric Absorption: VHE photons are attenuated by the Extragalactic Background Light (EBL), limiting the observable horizon, especially for high-redshift sources.
Sensitivity Limits: Current IACTs often lack the sensitivity to detect faint VHE signals from distant or less luminous bursts.

Although recent detections (e.g., GRB 180720B, GRB 190829A by H.E.S.S.; GRB 190114C by MAGIC) have confirmed VHE emission exists, it remains unclear whether these are distinct populations or if current non-detections are due to observational biases (e.g., redshift, flux, delay time). This paper aims to address these gaps by presenting a comprehensive 15-year catalogue of H.E.S.S. GRB observations to constrain emission models and characterize the VHE GRB population.

2. Methodology

Observation Strategy and Sample Selection

Instrument: The High Energy Stereoscopic System (H.E.S.S.) in Namibia, comprising four 12m telescopes (CT1-4) and one 28m central telescope (CT5). CT5 offers a lower energy threshold ( $\lesssim 100$ GeV) and faster slewing capabilities.
Timeframe: Observations from 2004 to 2019.
Alert System: Utilized the General Coordinates Network (GCN) to receive alerts from Swift/BAT, Fermi/GBM, INTEGRAL, MAXI, and HETE-2.
Selection Criteria:
- Well-localized (loc): Swift/BAT alerts with position uncertainty $\lesssim 3'$ (smaller than H.E.S.S. PSF).
- Poorly-localized (un-loc): Fermi/GBM alerts with uncertainty $> 0.2^\circ$ .
- Delay Constraints: Observations were prioritized based on redshift (if known) and zenith angle. For $z \le 0.1$ , delays up to 24h were allowed; for unknown redshifts, the limit was 4h.
Sample Size: Initially 107 potential matches were found. After data quality checks (removing runs with hardware issues, bad weather, or bright stars), 89 GRBs remained for analysis (66 loc, 23 un-loc).

Data Analysis

Reconstruction: Used two independent analysis chains (Model++ with ParisAnalysis and ImPACT with HAP) to cross-check results.
Configuration:
- Mono: CT5 only (lowest energy threshold, used for well-localized sources).
- Hybrid: All 5 telescopes (better off-axis performance).
- Stereo: CT1-4 only.
Background Estimation: Ring background method for significance maps; reflected background method for integral flux upper limits (ULs) of well-localized sources.
Statistical Approach:
- Significance calculated via Li & Ma method.
- Upper limits (95% CL) derived using the Rolke method.
- Stacked Analysis: Combined $\theta^2$ distributions (squared angular distance) from well-localized GRBs to search for a cumulative signal.

Modeling and Population Studies

Emission Modeling: A single-zone Synchrotron Self-Compton (SSC) model was applied to three specific GRBs (GRB 100621A, GRB 131030A, GRB 161001A) selected for their bright X-ray flux, low redshift, and short observation delays. The model assumed a relativistic shock propagating into either a constant ISM or a wind-like medium.
Population Comparison: Kolmogorov–Smirnov (KS) tests were performed to compare the distributions of prompt and afterglow parameters (flux, fluence, spectral index, $T_{90}$ $T_{90}$ , redshift) between:
1. The H.E.S.S. observed sample.
2. The VHE-detected sample (including GRB 180720B and GRB 190829A).
3. The general Swift/Fermi population.

3. Key Contributions

Comprehensive Catalogue: Presented the largest set of VHE upper limits for GRBs to date, covering 15 years of H.E.S.S. operations.
Re-analysis: Re-analyzed early observations (2004–2007) using modern calibration and reconstruction algorithms, improving sensitivity compared to the first H.E.S.S. GRB catalogue.
Stacked Search: Performed the first stacked analysis of H.E.S.S. GRB data to search for a faint cumulative VHE signal, finding no significant detection.
Bias Quantification: Systematically quantified selection biases, demonstrating that H.E.S.S. observations are statistically representative of the general GRB population in terms of duration and redshift, but biased toward brighter fluxes.
Theoretical Constraints: Provided robust constraints on SSC models for three specific bursts, showing that non-detections are consistent with standard forward-shock scenarios given current instrument thresholds.

4. Results

Detection Status

No New Detections: Aside from the previously published detections of GRB 180720B and GRB 190829A, no significant VHE signals were detected in the 89 analyzed GRBs.
Significance Distribution: The significance distribution for well-localized GRBs is consistent with a normal distribution centered at zero ( $\mu = -0.003 \pm 0.126$ ), indicating no excess gamma-ray emission.
Stacked Analysis: The combined analysis of 12 mono clusters (11.1 hours) and 47 stereo clusters (60.2 hours) yielded statistical significances of 0.27 and 0.63, respectively, confirming no cumulative VHE signal.

Population Characteristics

Prompt Phase: VHE-detected GRBs exhibit significantly higher fluence and peak flux compared to the general Swift/Fermi population (deviations $> 3\sigma$ to $4.7\sigma$ ).
Afterglow Phase: VHE-detected GRBs show significantly brighter X-ray flux and fluence in the Swift/XRT band compared to the general population.
Redshift: VHE-detected GRBs tend to be at lower redshifts ( $z < 1$ ), though the deviation from the general population is only $\sim 2\sigma$ .
Conclusion on Population: VHE-detected GRBs are not a distinct physical population but rather a subset of the general population characterized by luminous X-ray emission, low redshift, and favorable observing conditions (low zenith angle, short delay).

Modeling Results

For the three selected GRBs (100621A, 131030A, 161001A), the standard SSC model successfully explained the non-detections.
The predicted SSC flux lies well below the H.E.S.S. upper limits once Klein-Nishina effects and EBL attenuation are included.
To reach the H.E.S.S. upper limits, the models would require explosion energies ( $E_{sh}$ ) of $\sim 10^{55}$ erg, which are physically unrealistic. This confirms that current non-detections are due to sensitivity limits rather than a failure of the standard shock model.

5. Significance and Outlook

Current Limitations: The study highlights that current-generation IACTs (like H.E.S.S.) are primarily sensitive to the most luminous, nearby GRBs. The combination of EBL absorption and instrumental energy thresholds ( $\sim 100$ GeV) severely limits the detection horizon to $z \lesssim 0.3$ for typical fluxes.
Future Prospects (CTAO): The results underscore the critical need for next-generation instruments like the Cherenkov Telescope Array Observatory (CTAO).
- Lower Threshold: CTAO's Large-Sized Telescopes (LSTs) aim for thresholds of $\sim 30$ GeV, which would extend the detectable redshift range to $z \sim 0.8$ (assuming 90% EBL absorption).
- Sensitivity: A tenfold improvement in sensitivity is expected to detect VHE emission from GRBs with X-ray fluxes as low as $10^{-12}$ erg cm $^{-2}$ s $^{-1}$ .
Synergy: The paper emphasizes the importance of wide-field monitors (like SVOM, SWGO, LHAASO) providing rapid alerts and redshift estimates to enable deep, targeted follow-up by IACTs.

In summary, this paper establishes that while VHE GRBs are rare with current technology, they are likely a bright subset of the general population. The lack of detections is an instrumental and environmental limitation rather than a fundamental absence of VHE emission, setting the stage for transformative discoveries with future observatories.