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REVIEW 3 major objections 6 minor 23 references

Multi-wavelength observation of MAXI J1820+070 with MAGIC, VERITAS and H.E.S.S

T0 review · 3 major / 6 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read Multi-instrument gamma-ray observations find no significant very-high-energy emission from the black-hole microquasar MAXI J1820+070 during its 2018 outburst, placing 99% confidence upper limits above 182 GeV.

desk verdict A solid VHE null result for MAXI J1820+070 from three IACT arrays, with an optical activity claim that is plausible but not yet demonstrated. read the letter →

arxiv 1908.06958 v2 pith:LG773MEU submitted 2019-08-19 astro-ph.HE

J. Hoang , E. Molina , M. López , M. Ribó , O. Blanch , J. Cortina , G. Maier , N. Park
show 9 more authors
M. de Naurois E. de Ona Wilhelmi J.P. Ernenwein D. Malyshev A. Mitchell S. Ohm R. Zanin (on behalf of the MAGIC VERITAS H.E.S.S. Collaborations)
This is my paper · ORCID
classification astro-ph.HE
keywords microquasarsMAXIJ1820+070veryhighenergygammaraysX-raybinariesblackholetransientsimagingatmosphericCherenkovtelescopesfastopticalupperlimits
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper reports a coordinated search for very-high-energy gamma rays and fast optical pulses from the newly discovered black-hole microquasar MAXI J1820+070 during its 2018 outburst. The central result is a non-detection: after more than 90 hours of observations with three imaging atmospheric Cherenkov telescope arrays (61.6 hours after quality cuts), no significant gamma-ray emission is found above 182 GeV, and 99% confidence upper limits are placed on the integral flux. The same campaign used a dedicated optical readout on one telescope to look for millisecond-scale bright flashes, finding faster transient optical activity during the hard X-ray state than in later states, although the paper notes the decline could be due to low statistics. If the non-detection is right, models in which the jet accelerates particles to very high energies must place the source's 2018 outburst below the measured limits, while the optical result, if real, points to jet-related flaring that fades in softer X-ray states.

What carries the argument

The machinery is the standard imaging atmospheric Cherenkov analysis chain applied independently by each array: shower images are reconstructed to a gamma-ray direction and energy, and the angular distance $\theta$ between the reconstructed direction and the source position separates signal from the hadronic background. MAGIC uses a random forest classifier trained on image parameters, H.E.S.S. uses a likelihood reconstruction based on a semi-analytical shower model, and VERITAS uses box cuts optimized for soft-spectrum sources; all observations use wobble mode, with the source offset from the camera center to allow simultaneous background estimation. For the optical search, the MAGIC-II central pixel samples at 10 kHz with U-band sensitivity, and the analysis averages over 1 ms windows to search for non-periodic pulses lasting 1-100 ms brighter than 13 mag. These procedures convert raw shower images and photomultiplier voltages into the reported flux upper limits and optical count rates.

What would settle it

Measure the central-pixel background rate in an off-source field during the same nights and source states; if that background is comparable to 18 count/h, the hard-state optical excess is not established. For the gamma-ray claim, a statistically significant excess above 182 GeV in an accepted re-analysis of the 2018 data, or in a future outburst observed with comparable exposure, would show that the non-detection was not a general property of this source.

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Extended reading notes

Core claim

On its own terms, the paper establishes that MAXI J1820+070 did not show statistically significant very-high-energy gamma-ray emission in any of the three datasets. Assuming a power-law spectrum with index $-2.5$ and a common energy threshold of 182 GeV, the integral flux upper limits at 99% confidence are $1.1 \times 10^{-12}$ cm$^{-2}$ s$^{-1}$ (H.E.S.S., 26.9 h), $4.7 \times 10^{-12}$ cm$^{-2}$ s$^{-1}$ (MAGIC, 22.5 h), and $2.5 \times 10^{-12}$ cm$^{-2}$ s$^{-1}$ (VERITAS, 12.2 h). Fermi-LAT's 99% upper limits above 100 MeV are also consistent with no emission. In the optical band, the central pixel detected pulses brighter than 13 mag and shorter than 100 ms at rates of 18.2 count/h in the hard state, 5 count/h in the hard-to-soft transition, and 0 count/h in the soft-to-hard transition; the paper treats the decline as suggestive but possibly a low-statistics artifact.

Load-bearing premise

The state dependence of the optical activity is the weakest link: the paper reports raw counts per hour for each epoch without background subtraction or uncertainties, so the apparent decline from 18.2 to 0 count/h rests on the unstated assumption that the meteor and other backgrounds stayed comparable across epochs.

Editorial extensions

If this is right

  • Any model predicting very-high-energy gamma rays from MAXI J1820+070's jet during the 2018 hard state must produce an integral flux below the corresponding 99% upper limit, about $10^{-12}$ cm$^{-2}$ s$^{-1}$ above 182 GeV.
  • Combined with the Fermi-LAT non-detection above 100 MeV, the upper limits say the source's high-energy output was much weaker than its X-ray output, tightening constraints on the fraction of jet power given to non-thermal particles.
  • If the optical count rates reflect source activity, fast optical flaring in this microquasar appears mainly in the hard state and is suppressed during intermediate and soft states, matching a jet-related origin.
  • The 61.6 hours of quality-selected, state-resolved observations provide a baseline against which future outbursts of this source can be compared to test whether very-high-energy emission is episodic.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • The hard-state optical rate of 18.2 count/h implies that a duty cycle for 1-100 ms pulses could in principle be estimated from the 4.5 h exposure, but the paper does not convert counts into a physical flaring rate; a dedicated per-epoch measurement of meteor background would tell whether the hard-state excess is real.
  • A simple one-zone inverse-Compton calculation using the measured X-ray luminosity and the new upper limits could turn the non-detection into a quantitative lower bound on the jet magnetic field; the paper stops at saying the limits will constrain parameters.
  • Joint VHE and optical observations of a future hard-state outburst with the same instruments would test whether the combination of optical flaring and gamma-ray quiescence is a stable property of this source or specific to the 2018 event.
  • Placing the three VHE upper limits together with the Fermi-LAT limits on a single spectral energy distribution would show whether an unbroken power law from MeV to TeV energies is ruled out; such a plot is not included in the paper.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

3 major / 6 minor

Summary. This ICRC proceedings paper reports Very High Energy (VHE) gamma-ray and fast optical observations of the low-mass microquasar MAXI J1820+070 during its 2018 outburst. The three Cherenkov arrays H.E.S.S., MAGIC, and VERITAS observed the source for a total of 61.6 h after quality cuts; using standard, independent analyses, no significant VHE emission is found. The paper quotes 99% CL integral flux upper limits above 182 GeV of 1.1e-12, 4.7e-12, and 2.5e-12 cm^-2 s^-1 for H.E.S.S., MAGIC, and VERITAS (Table 2). In addition, the MAGIC Central Pixel recorded fast optical pulses; the reported optical activity rates are 18.2 count/h during the hard state, 5 count/h during the HS-SS intermediate state, and 0 count/h during the SS-HS intermediate state (Table 3). The paper interprets these as evidence for enhanced optical activity in the hard state, likely from jet emission, suppressed in the intermediate/soft states.

Significance. The VHE null result is a solid, useful constraint: three independent collaborations, using their standard analyses, agree on a non-detection, and the quoted 99% limits are plausible given the effective exposures and energy threshold. The explicit statement of the assumed spectral index (-2.5) and common energy threshold (182 GeV) makes the limits reproducible in principle. This is a valuable contribution to the multi-wavelength picture of a well-studied transient. The optical activity claim, if quantitatively established, would add novel state-dependent information, but the current analysis is not sufficiently rigorous; the optical part of the paper is therefore a preliminary result rather than a demonstrated detection of state dependence.

major comments (3)
  1. [Section 3.2, Table 3] The optical activity rates in Table 3 are presented without uncertainties or background subtraction. The text states that the dominant background (faint meteors) has a rate of 10^-3 to 10^-5 Hz, i.e., roughly 0.04 to 3.6 per hour, and that background was estimated from observations before and after the source run, but no per-epoch background rates are given. The hard-state rate of 18.2 count/h is several times the high end of the background, so a source contribution is plausible, but the intermediate-state rate of 5 count/h is comparable to the background range. Without uncertainties and per-epoch background values, the claimed state dependence of the optical activity cannot be quantitatively assessed.
  2. [Section 4] The conclusion that the optical activity is 'enhanced in the HS' and 'likely to be suppressed in the IM and SS' rests entirely on the rates in Table 3. The paper appropriately cautions that the 0 count/h epoch could be 'due to low statistics,' but it does not apply the same caution to the positive hard-state excess. Since the Central Pixel is a single-pixel detector without spatial resolution, contamination by meteors or other transient atmospheric effects that change between epochs cannot be excluded unless per-epoch background monitoring is reported. The state-dependence interpretation is therefore not yet supported beyond a few counts.
  3. [Section 3.2] The definition of an 'optical activity' event is not given. The reader cannot tell whether the count rate corresponds to all 1-ms windows exceeding some significance threshold, to individual resolved pulses, or to another criterion. This is essential for interpreting the reported rates and for comparing them with the background rate. Please specify the event-selection criterion and the significance threshold used.
minor comments (6)
  1. [Section 3.1] The upper limits assume a power-law index of -2.5. A brief statement on the sensitivity of the limits to this assumption, or a note that they are evaluated for a fixed index, would be helpful for readers who wish to compare with model spectra.
  2. [Figure 1] The label on the Fermi-LAT panel reads '[0.1,100] GeV' while the caption says 'above 100 MeV'; these are equivalent for the energy range considered, but the inconsistency in wording may confuse readers.
  3. [Table 3] The column header 'Optical activity [count/h]' should be accompanied by a definition of the event type and by the per-epoch exposure times, which currently appear only in the 'Effective time' column without explicit per-epoch breakdown.
  4. [Section 2] The statement that 'Estimation of optical background was done using optical observations taken before and after MAXI J1820+070 optical observation' is vague. Please specify how these observations were used, whether they were matched in observing conditions, and how the background rate was derived.
  5. [Table 2] The upper limits are quoted as exact numbers with no accompanying statistical or systematic uncertainty. A sentence stating that these are 99% CL limits with the only systematic uncertainty in the assumed spectral index would be useful.
  6. [Section 4] The paper refers to 'enhanced optical activity in the HS by the Central Pixel' and points to reference [17] for details. Please clarify which results are new in this work and which are taken from previous publications.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the VHE upper limits are independent observational results, and the optical-activity interpretation is weaker than the VHE claim but not circular.

full rationale

The paper's central claim is that the separate MAGIC, VERITAS and H.E.S.S. analyses find no significant VHE emission from MAXI J1820+070, with 99% CL integral flux upper limits quoted in Table 2. These limits follow from standard, independent Cherenkov-telescope analyses of the recorded events; no parameter is fitted to the source data and then renamed a prediction. The assumed power-law index of -2.5 and the common 182 GeV threshold are stated assumptions, not quantities adjusted to force a particular limit. The optical activity rates in Table 3 are raw count rates without quoted uncertainties or background subtraction, and the paper itself cautions that the absence of events in one epoch could be 'due to low statistics'; this is a support weakness for the positive optical interpretation, not a circular step. The only self-citation, reference [11] for the Central Pixel calibration expression, is adopted as an instrumental calibration and does not define the target result or enter into any fitted quantity. No equation in the paper reduces to its own input, and no load-bearing claim depends on an unverified self-citation. The VHE result is self-contained and externally benchmarked against standard analysis procedures, so the appropriate circularity score is 0.

Assumptions & free parameters 1 free parameters · 3 assumptions · 0 invented entities

The central VHE result depends only on standard instrument calibrations and an assumed power-law index of -2.5 for converting counts to flux upper limits; the optical result depends on the [11] calibration and on background assumptions that are not quantitatively characterized. None of these are fitted to force the reported outcome.

free parameters (1)
  • Assumed photon index for VHE spectrum = -2.5
    Chosen by hand in Section 3.1 to compute integral upper limits; the quoted ULs in Table 2 would change with a different assumed index, so the result is conditional on this choice.
assumptions (3)
  • domain assumption VHE gamma-ray spectrum of MAXI J1820+070 is a power law with photon index -2.5
    Used in Section 3.1 to convert observed counts to integral flux upper limits in Table 2 and for Fermi-LAT limits; a harder or softer spectrum would change the quoted limits.
  • domain assumption Central Pixel voltage-to-optical-magnitude conversion from [11] applies to these observations
    Section 3.2 uses the empirical expression from the instrument paper to set the 13 mag brightness threshold for optical pulses.
  • domain assumption Meteor background rate is low (10^-3 to 10^-5 Hz) and does not dominate the reported optical counts
    Section 3.2 states this background rate qualitatively, but no epoch-by-epoch background subtraction is provided for Table 3.

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Cite this review

Pith. "Pith review of Multi-wavelength observation of MAXI J1820+070 with MAGIC, VERITAS and H.E.S.S." pith.science (2026). https://pith.science/paper/LG773MEU

@misc{pith2026190806958,
  author       = {Pith},
  title        = {Pith review of: Multi-wavelength observation of MAXI J1820+070 with MAGIC, VERITAS and H.E.S.S},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/LG773MEU}},
  note         = {Machine review of arXiv:1908.06958}
}
read the original abstract

MAXI J1820+070 is a new low-mass microquasar hosting a black hole recently discovered in X-rays by the MAXI instrument. It is the counterpart of ASASSN-18ey, discovered in optical a few days before by ASAS-SN. This source underwent a major outburst in 2018, during which it completed the typical "q-shaped" path in the hardness-intensity diagram. MAGIC, VERITAS and H.E.S.S. gamma-ray telescopes observed the sky position of MAXI J1820+070 for a total of more than 90 hours in 2018. In addition, some observations were carried out using MAGIC Central Pixel - a dedicated central pixel capable of detecting fast optical signals (10 kHz sampling rate, peak sensitivity in the U-band). This contribution presents the methods used to search for transient optical and very high energy gamma-ray emission from MAXI J1820+070, as well as the latest results in these energy ranges.

Figures

Figures reproduced from arXiv: 1908.06958 by the authors.

Figure 1
Figure 1. MAXI J1820+070 data at different energy ranges. From top to bottom: MAXI/GSC hardness ratio of the 4–10 to 2–4 keV fluxes1on which the states after [17] are superimposed (named here Hard State, Intermediate, and Soft State); Fermi-LAT2 integral flux upper limits above 100 MeV with a 99% confidence level and a power-law index of -2.5; Swift/BAT3 light curve in the 15–50 keV range showing the nights in which the sourc… view at source ↗
Figure 2
Figure 2. Hardness-intensity diagram obtained from MAXI/GSC X-ray observations in the 2–4 and 4– 10 keV bands. The nights in which H.E.S.S., MAGIC or VERITAS observations were carried out have a triangle, a circle, and a square, respectively, around the MAXI point [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗

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Works this paper leans on

23 extracted references · 9 canonical work pages

  1. [17]

    Shidatsu, S

    M. Shidatsu, S. Nakahira, K. L. Murata, R. Adachi, N. Kawai, Y . Ueda et al., X-Ray and Optical Monitoring of State Transitions in MAXI J1820+070, ApJ 874 (2019) 183 [1903.01686]

    arXiv 2019

  2. [1]

    I. F. Mirabel and L. F. Rodríguez, Sources of Relativistic Jets in the Galaxy, ARA&A 37 (1999) 409 [astro-ph/9902062]

    arXiv 1999

  3. [2]

    Fender and T

    R. Fender and T. Muñoz-Darias, The Balance of Power: Accretion and Feedback in Stellar Mass Black Holes, vol. 905, p. 65. Springer International Publishing Switzerland, 2016. 10.1007/978-3-319-19416-5_3

    doi:10.1007/978-3-319-19416-5_3 2016

  4. [3]

    R. A. Remillard and J. E. McClintock, X-Ray Properties of Black-Hole Binaries, ARA&A 44 (2006) 49 [astro-ph/0606352]

    arXiv 2006

  5. [4]

    Matsuoka, K

    M. Matsuoka, K. Kawasaki, S. Ueno, H. Tomida, M. Kohama, M. Suzuki et al., The MAXI Mission on the ISS: Science and Instruments for Monitoring All-Sky X-Ray Images, PASJ 61 (2009) 999 [0906.0631]

    arXiv 2009

  6. [5]

    M. A. Tucker, B. J. Shappee, T. W.-S. Holoien, K. Auchettl, J. Strader, K. Z. Stanek et al., ASASSN-18ey: The Rise of a New Black Hole X-Ray Binary, ApJL 867 (2018) L9 [1808.07875]. 6 MWL observation of MAXI J1820+070 J. Hoang

    arXiv 2018

  7. [6]

    K. C. Gendreau et al., The Neutron star Interior Composition Explorer (NICER): design and development, vol. 9905 of Proc. SPIE, p. 99051H, July, 2016, DOI

    work page 2016

  8. [7]

    Gaia DR2 Distances and Peculiar Velocities for Galactic Black Hole Transients

    P. Gandhi, A. Rao, M. A. C. Johnson, J. A. Paice and T. J. Maccarone, Gaia Data Release 2 distances and peculiar velocities for Galactic black hole transients, MNRAS 485 (2019) 2642 [1804.11349]

    work page Pith review arXiv 2019

Show all 23 references
  1. [8]

    C. A. L. Bailer-Jones, J. Rybizki, M. Fouesneau, G. Mantelet and R. Andrae, Estimating Distance from Parallaxes. IV . Distances to 1.33 Billion Stars in Gaia Data Release 2, AJ 156 (2018) 58 [1804.10121]

    2018 arXiv

  2. [9]

    M. A. P. Torres, J. Casares, F. Jiménez-Ibarra, T. Muñoz-Darias, M. Armas Padilla, P. G. Jonker et al., Dynamical confirmation of a black hole in MAXI J1820+070, arXiv e-prints (2019) arXiv:1907.00938 [1907.00938]

    2019 arXiv

  3. [10]

    Aleksi ´c, S

    J. Aleksi ´c, S. Ansoldi, L. A. Antonelli, P. Antoranz, A. Babic, P. Bangale et al.,The major upgrade of the MAGIC telescopes, Part II: A performance study using observations of the Crab Nebula, Astroparticle Physics 72 (2016) 76 [1409.5594]

    2016 arXiv

  4. [11]

    Hassan, J

    T. Hassan, J. Hoang, M. López, J. A. Barrio, J. Cortina, D. Fidalgo et al., MAGIC sensitivity to millisecond-duration optical pulses, ArXiv e-prints (2017) [ 1708.07698]

    2017 arXiv

  5. [12]

    Park, Performance of the VERITAS experiment, ArXiv e-prints (2015) [ 1508.07070]

    N. Park, Performance of the VERITAS experiment, ArXiv e-prints (2015) [ 1508.07070]

    2015 arXiv

  6. [13]

    H. A. Krimm, S. T. Holland, R. H. D. Corbet, A. B. Pearlman, P. Romano, J. A. Kennea et al., The Swift/BAT Hard X-Ray Transient Monitor, ApJS 209 (2013) 14 [1309.0755]

    2013 arXiv

  7. [14]

    Homan, P

    J. Homan, P. Uttley, K. Gendreau, Z. Arzoumanian, M. Saylor, J. F. Steiner et al., A rapid state transition in MAXI J1820+070, The Astronomer’s Telegram11820 (2018)

    2018

  8. [15]

    S. E. Motta, J. Bright and R. Fender, Swift observes MAXI J1820+070 in transition from the soft to the hard-intermediate state, The Astronomer’s Telegram12064 (2018)

    2018

  9. [16]

    V . P. Fomin, A. A. Stepanian, R. C. Lamb, D. A. Lewis, M. Punch and T. C. Weekes,New methods of atmospheric Cherenkov imaging for gamma-ray astronomy. I. The false source method, Astroparticle Physics 2 (1994) 137

    1994

  10. [18]

    Albert, E

    J. Albert, E. Aliu, H. Anderhub, P. Antoranz, A. Armada, M. Asensio et al., Implementation of the Random Forest method for the Imaging Atmospheric Cherenkov Telescope MAGIC, Nuclear Instruments and Methods in Physics Research A 588 (2008) 424 [0709.3719]

    2008 arXiv

  11. [19]

    de Naurois and L

    M. de Naurois and L. Rolland, A high performance likelihood reconstruction of γ-rays for imaging atmospheric Cherenkov telescopes, Astroparticle Physics 32 (2009) 231 [0907.2610]

    2009 arXiv

  12. [20]

    Holler, A

    M. Holler, A. Balzer, R. Chalmé-Calvet, M. de Naurois and D. Zaborov for the H. E. S. S. collaboration , Photon Reconstruction for H.E.S.S. Using a Semi-Analytical Shower Model, arXiv e-prints (2015) arXiv:1509.02896 [1509.02896]

    2015 arXiv

  13. [21]

    M. K. Daniel, The VERITAS standard data analysis, International Cosmic Ray Conference 3 (2008) 1325 [0709.4006]

    2008 arXiv

  14. [22]

    Fender, Microquasars: Hard X-ray/γ-ray emission, in American Institute of Physics Conference Series (F

    R. Fender, Microquasars: Hard X-ray/γ-ray emission, in American Institute of Physics Conference Series (F. A. Aharonian and H. J. Völk, eds.), vol. 558 of American Institute of Physics Conference Series, pp. 221–233, Apr, 2001, astro-ph/0101233, DOI

    2001 arXiv

  15. [23]

    Bosch-Ramon and D

    V . Bosch-Ramon and D. Khangulyan, Understanding the Very-High Emission from Microquasars, Int. J. Mod. Phys. D 18 (2009) 347 [0805.4123]. 7

    2009 arXiv

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