REVIEW 3 major objections 4 minor 21 references
Searching for GRBs at VHE with MAGIC: the status before CTA
T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read MAGIC has followed up 103 gamma-ray bursts at very high energies; the scientific payoff will be upper limits.
desk verdict A candid status report from MAGIC: the compiled 103-burst follow-up sample is useful for CTA planning, but the abstract overpromises upper limits the paper defers, and the sample's targeting biases are unquantified. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The load-bearing mechanism is the automatic GRB follow-up procedure: on a Gamma-ray Coordinate Network alert, the telescopes reposition at up to $7^\circ$/s while the data acquisition is re-initialized and the mirrors are focused, so observation can begin almost immediately; a Burst Advocate evaluates the alert and can prolong or stop the run. This is what lets an imaging atmospheric Cherenkov telescope with a $3.5^\circ$ field of view and a trigger threshold near 50 GeV respond to satellite-discovered transients. The companion piece is the upper-limit framework, which must fold in extragalactic background light attenuation: at redshift $z=1$ the intrinsic flux is reduced by a factor of about 2.3 at 100 GeV and roughly $1.5\times10^6$ at 1 TeV, so the low energy threshold is what keeps distant GRBs within reach.
What would settle it
Count, from the GCN alert stream, every GRB between 2013 and 2018 that was visible from the MAGIC site under good or decent weather, and check whether all of them appear in the 103-GRB list; any systematic gap would show the sample is not the unbiased record the paper implies. Alternatively, obtain the upgrade's claimed hardware-failure reduction by comparing failure counts per fast repositioning before and after 2013, which the paper asserts but does not quantify.
Extended reading notes
Core claim
The paper's central claim is that MAGIC has built a substantial and growing sample of GRB observations at very high energies: 103 bursts observed with good or decent weather and no technical obstacles up to December 2018, many within hours of the trigger, including Fermi-LAT-detected bursts and late-time afterglow observations. It argues that the upgraded automatic procedure, which keeps the data acquisition running while the telescopes slew, is what made prompt follow-up routine and reduced hardware failures. With no significant detection in the sample, the paper asserts the value of the program lies in upper limits: each non-detection, properly accounting for extragalactic background light absorption, can be compared with synchrotron and synchrotron-self-Compton model predictions to reduce parameter degeneracies. The paper also presents the near-term hardware path, lowering the energy threshold toward about 30 GeV, as the way to extend these searches to more distant bursts and to overlap with Fermi-LAT.
Load-bearing premise
The record of 103 GRBs is treated as a complete, unbiased list of all MAGIC follow-ups with favorable conditions in the period, but the paper does not compare it with the full stream of alerts; if weather, visibility, or human decisions silently selected which bursts were observed, any later stacking or upper-limit interpretation would inherit that bias.
Editorial extensions
If this is right
- Once the analysis of the 103 bursts is published, the flux upper limits will provide a direct test of synchrotron and synchrotron-self-Compton emission models for GRB afterglows.
- The higher rate of Fermi-LAT-triggered follow-ups and late-time observations means the sample covers phases of GRB evolution that earlier VHE searches rarely reached.
- If the energy threshold is lowered to about 30 GeV with Sum-Trigger II, MAGIC will be able to observe more distant GRBs and to compare signals directly with Fermi-LAT.
- The 103-GRB program and its upper limits establish the observing pattern and expectations that CTA will build on for GRB science.
Reading between the lines
- A stacked analysis of the full 103-GRB sample could produce a meaningful ensemble constraint on VHE emission, but only if the selection function of which alerts were followed is published; without it, the stacking result would be hard to interpret.
- If the upper limits exclude standard SSC parameters for the nearest bursts, that would strengthen the case for hadronic or external-shock contributions to GRB high-energy emission.
- The paper's claim of reduced hardware failures is testable: comparing failure rates per fast repositioning before and after 2013 would quantify the upgrade's benefit.
- The same upper-limit catalog could be used to bound intergalactic magnetic fields through VHE time delays, a connection the paper does not pursue.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This ICRC 2019 proceedings paper reports the status of the MAGIC very-high-energy (VHE) gamma-ray burst (GRB) follow-up program in advance of the Cherenkov Telescope Array. It describes the MAGIC telescopes' capabilities, the automatic alert and fast-repositioning procedure, the upgrade implemented in 2013 that reduced hardware failures, and the follow-up strategy including the role of the Burst Advocate and late-time observations of Fermi-LAT bursts. The paper claims that up to December 2018 MAGIC followed 103 GRBs under favorable conditions, presents a skymap of those events, and lists 25 GRBs observed between May 2015 and December 2018 in Table 1, with columns for trigger time, T90, redshift, zenith angle, and delay. The paper reviews VHE emission models as context, and concludes by discussing planned hardware and analysis improvements. Crucially, the abstract states that the paper focuses on upper limits and results from the 2013-2018 sample, but the body explicitly states that the analysis is still ongoing and will appear in a forthcoming publication; no upper limits are presented.
Significance. If the reported sample count and procedure description are accurate, this paper provides a useful community status report and a preliminary catalog that could serve as a precursor for CTA-era GRB follow-up. It documents a concrete operational improvement: the 2013 upgrade reduced hardware failures during fast repositioning. The paper is honest in its body about the analysis being incomplete, and the references to MAGIC performance papers are standard instrument citations. However, the paper contains no new measurements, no upper limits, and no quantitative validation of the sample's completeness or selection. Its value as a catalog for future population-level constraints is therefore currently limited by the absence of a documented selection function.
major comments (3)
- [Abstract and Section 3.3] The abstract states that the paper 'focuses on the ULs and results obtained from a sample of GRBs observed between 2013 and 2018', but Section 3.3 explicitly says that 'The analysis of the GRBs listed in Table 1 is currently ongoing' and that these GRBs 'will be the topic of a forthcoming publication.' No upper limit or result appears anywhere in the paper. This is a direct contradiction between the abstract and the body. The authors should either present the promised upper limits or revise the abstract to describe the paper as a status report and catalog without results.
- [Section 3.3 and Table 1] The 103-GRB sample is defined only by 'favorable conditions (good or decent weather and without technical problems preventing the analysis of the data)', with no selection function, no comparison against all received GCN alerts, and no trigger log. Given that the paper motivates future flux upper limits from this sample and presents it as a precursor for CTA, the lack of a quantitative selection function, including observability thresholds, the Burst Advocate's decision criteria described in Section 3.2, and the deliberate preferential follow-up of Fermi-LAT bursts at late times, prevents the catalog from being interpreted as a population-level draw. Please add a selection flowchart or at least a numerical breakdown of excluded alerts by reason such as weather, visibility, technical failure, or human decision.
- [Section 3.3 and Reference [21]] The total of 103 GRBs includes events observed between 2013 and April 2015 that are not listed in this paper but are delegated to reference [21]. As a result, the paper's headline count is not independently verifiable from the presented material. The authors should either reproduce the pre-2015 list with the same parameters as in Table 1 or provide a machine-readable cross-check against GCN alerts for the full period, so that readers can trace the construction of the 103-GRB sample.
minor comments (4)
- [Abstract and Section 1] The sentence 'Gamma-Ray Bursts (GRBs) are one of the most elusive and enigmatic class of sources' has a subject-verb agreement error; it should be 'classes of sources' or 'one class of the most elusive...'.
- [Section 3.1] In the sentence 'making it suitable to observe and possibly detect far-away sources, like most GRBs, for which the the effect of the gamma rays absorption on the Extragalactic Background Light (EBL) is relevant', there is a duplicated 'the' and the phrase 'gamma rays absorption' should be 'gamma-ray absorption'.
- [Section 3.2] The description of the Burst Advocate's role in 'evaluating the relevance of the alert (if it is real or not)' is vague. Please specify what criteria are used to judge whether an alert is real, since this is a part of the human decision layer that affects the sample selection.
- [Table 1] In the Notes column, the entries 'LAT' and 'late' conflate two different properties: whether the GRB had a Fermi-LAT detection and whether a late-time observation was performed. Consider splitting these into two separate columns or using distinct markers so the sample's LAT bias is readily apparent for each event.
Circularity Check
No circularity: this is a status report with no fitted parameters, predictions, or derivation chain that reduces to its inputs.
full rationale
This paper is a status report on MAGIC's GRB follow-up program (2013–2018), not a derivation. There is no fitted parameter, no predicted observable, and no model output: the abstract's mention of upper limits explicitly refers to a forthcoming publication ('will be the topic of a forthcoming publication'), and no ULs are computed here. The catalog of 103 GRBs in Section 3.3 is presented as a list of observed events, with selection described qualitatively as 'favorable conditions (good or decent weather and without technical problems preventing the analysis of the data)' and, for the pre-May-2015 subset, delegated to reference [21]; however, an incomplete selection function is an observational-bias or correctness concern, not a circularity, because the paper does not claim that the sample was derived from a model or that its properties follow from assumptions. Self-citations (e.g., [18], [19] for MAGIC hardware and performance, [21] for earlier GRB follow-ups) are standard instrument and collaboration references; they are not invoked to justify a result, and no 'uniqueness theorem' or ansatz is imported. Thus the derivation chain—if any—is empty, and there is no step where an output equals an input by construction.
Assumptions & free parameters
assumptions (3)
- domain assumption GCN alerts provide accurate and timely GRB positions and trigger times.
- domain assumption The MAGIC performance parameters cited from [19] apply to GRB observations.
- domain assumption The Dominguez et al. EBL model [20] provides a valid estimate of attenuation.
Cite this review
Pith. "Pith review of Searching for GRBs at VHE with MAGIC: the status before CTA." pith.science (2026). https://pith.science/paper/RJ3LORN5
@misc{pith2026190902802,
author = {Pith},
title = {Pith review of: Searching for GRBs at VHE with MAGIC: the status before CTA},
year = {2026},
howpublished = {\url{https://pith.science/paper/RJ3LORN5}},
note = {Machine review of arXiv:1909.02802}
}
read the original abstract
Gamma-Ray Bursts (GRBs) are one of the main targets for current and next generation Imaging Atmospheric Cherenkov Telescopes (IACTs). Given their transient behavior, especially in the case of their prompt emission phase, performing fast follow-up observations is challenging for IACTs, which have a narrow field of view and limited duty cycle. Despite this, MAGIC plays a major role in the search for Very High Energy (VHE, E>100 GeV) gamma-ray emission from GRBs: this is possible thanks to its fast repositioning speed, low energy threshold and high sensitivity at the lowest energies. In 2013 the MAGIC GRB automatic procedure was upgraded, increasing the number of GRBs followed in the prompt and early afterglow phases and decreasing dramatically hardware failures during fast repositioning. Currently, only GRB 190114C was firmly detected in the VHE band, while for other GRBs no significant detection was achieved. In such a case, upper limits (ULs) can give insight into the physics driving such eluding sources, especially on their emission mechanisms. In this contribution we report on the status of the GRB follow-up with MAGIC and focus on the ULs and results obtained from a sample of GRBs observed between 2013 and 2018. This GRB catalog is the result of the MAGIC well-designed and tested follow-up procedure, and it serves as a precursor of GRBs observation with the next generation IACT system, the Cherenkov Telescope Array (CTA).
Figures
Reference graph
Works this paper leans on
-
[21]
Carosi, A. et al., Recent follow-up observations of GRBs in the very high energy band with the MAGIC telescopes, in proceedings of ICRC2015, PoS(ICRC2015)809 6
-
[1]
et al., MAGIC detects the GRB 190114C in the TeV energy domain., GCN 23701 (2019)
Mirzoyan, R. et al., MAGIC detects the GRB 190114C in the TeV energy domain., GCN 23701 (2019)
work page 2019
-
[2]
Razmik Mirzoyan, First time detection of a GRB at sub-TeV energies; MAGIC detects the GRB 190114C, The Astronomer’s Telegram 12390 (2019)
work page 2019
-
[3]
Ajello, M. et al., A Decade of Gamma-Ray Bursts Observed by Fermi-LAT: The Second GRB Catalog, ApJ 878 (2019) [astro-ph.HE/1906.11403]
arXiv 2019
-
[4]
External forward shock origin of high energy emission for three GRBs detected by Fermi
Kumar, P. and Duran, R. Barniol, External forward shock origin of high-energy emission for three gamma-ray bursts detected by Fermi, MNRAS 409 (2010) 226 [astro-ph.HE/0910.5726]
work page Pith review arXiv 2010
-
[5]
Zhang, B. and Meszaros, P., High-Energy Spectral Components in Gamma-Ray Burst Afterglows, ApJ 559 (2001) 110 [astro-ph/0103229] 5 Searching for GRBs at VHE with MAGIC: the status before CTA Alessio Berti
arXiv 2001
-
[6]
Böttcher, M. and Dermer, C. D., High-energy Gamma Rays from Ultra-high-energy Cosmic-Ray Protons in Gamma-Ray Bursts, ApJ 499 (1998) L131 [astro-ph/9801027]
arXiv 1998
-
[7]
The high energy tail of gamma-ray burst 941017: Comptonization of synchrotron self absorbed photons
Pe’er, A. and Waxman, E., The High-Energy Tail of GRB 941017: Comptonization of Synchrotron Self-absorbed Photons, ApJ 603 (2004) L1 [astro-ph/0310836]
work page Pith review arXiv 2004
Show all 21 references
-
[8]
and Esin, Ann A., On the Synchrotron Self-Compton Emission from Relativistic Shocks and Its Implications for Gamma-Ray Burst Afterglows, ApJ 548 (2001) 787 [astro-ph/0005253]
Sari, R. and Esin, Ann A., On the Synchrotron Self-Compton Emission from Relativistic Shocks and Its Implications for Gamma-Ray Burst Afterglows, ApJ 548 (2001) 787 [astro-ph/0005253]
2001 arXiv
-
[9]
et al., MAGIC observation of the GRB 080430 afterglow, A&A 517 (2010) A5 [astro-ph.HE/1004.3665]
Aleksi ´c, J. et al., MAGIC observation of the GRB 080430 afterglow, A&A 517 (2010) A5 [astro-ph.HE/1004.3665]
2010
-
[10]
et al., MAGIC upper limits on the GRB 090102 afterglow, MNRAS 437 (2014) 3103 [astro-ph.HE/1311.3637]
Aleksi ´c, J. et al., MAGIC upper limits on the GRB 090102 afterglow, MNRAS 437 (2014) 3103 [astro-ph.HE/1311.3637]
2014
-
[11]
and Rees, M
Meszaros, P. and Rees, M. J., Delayed GeV Emission from Cosmological Gamma-Ray Bursts - Impact of a Relativistic Wind on External Matter, MNRAS 269 (1994) L41 [astro-ph/9404056]
1994 arXiv
-
[12]
Beloborodov, Andrei M., Optical and GeV-TeV Flashes from Gamma-Ray Bursts, ApJ 618 (2005) L13 [astro-ph/0410050]
2005 arXiv
-
[13]
Fan, Y . Z. and Wei, D. M.,Late internal-shock model for bright X-ray flares in gamma-ray burst afterglows and GRB 011121, MNRAS 364 (2005) L42 [astro-ph/0506155]
2005 arXiv
-
[14]
Plaga, R., Detecting intergalactic magnetic fields using time delays in pulses of γ-rays, Nature 374 (1995) 430
1995
-
[15]
et al., Photosphere-internal shock model of gamma-ray bursts: case studies of Fermi/LAT bursts, MNRAS 415 (2011) 1663 [astro-ph.HE/1002.2634]
Toma, K. et al., Photosphere-internal shock model of gamma-ray bursts: case studies of Fermi/LAT bursts, MNRAS 415 (2011) 1663 [astro-ph.HE/1002.2634]
2011 arXiv
-
[16]
Waxman, E., Cosmological Gamma-Ray Bursts and the Highest Energy Cosmic Rays, Phys. Rev. Lett. 75 (1995) 386 [astro-ph/9505082]
1995 arXiv
-
[17]
and Xu, G., Neutrino Bursts from Gamma-Ray Bursts, ApJ 427 (1994) 708
Paczynski, B. and Xu, G., Neutrino Bursts from Gamma-Ray Bursts, ApJ 427 (1994) 708
1994
-
[18]
et al., The major upgrade of the MAGIC telescopes, Part I: The hardware improvements and the commissioning of the system, Astropart
Aleksi ´c, J. et al., The major upgrade of the MAGIC telescopes, Part I: The hardware improvements and the commissioning of the system, Astropart. Phys. 72 (2016) 61 [astro-ph.IM/1409.6073]
2016 arXiv
-
[19]
et al., The major upgrade of the MAGIC telescopes, Part II: A performance study using observations of the Crab Nebula, Astropart
Aleksi ´c, J. et al., The major upgrade of the MAGIC telescopes, Part II: A performance study using observations of the Crab Nebula, Astropart. Phys. 72 (2016) 76 [astro-ph.IM/1409.5594]
2016 arXiv
-
[20]
et al., Extragalactic background light inferred from AEGIS galaxy-SED-type fractions, Mon
Domínguez, A. et al., Extragalactic background light inferred from AEGIS galaxy-SED-type fractions, Mon. Not. R. Astr. Soc. 410 (2011) 2556 [astro-ph.CO/1007.1459]
2011 arXiv
Reviewed August 14, 2026 · model on record in the stance chip above.
Discussion (0). Continue with ORCID to comment.