REVIEW 3 major objections 4 minor 1 cited by
(Very) Fast astronomical photometry for meter-class telescopes
T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read Two photon-counting photometers give meter-class telescopes sub-nanosecond timing, catching a 128 mHz optical oscillation in a black hole binary.
desk verdict A modest but honest proceedings paper: the IFI instrument description is genuinely useful, while the headline optical QPO still lacks the significance analysis needed to make the case stick. 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 element is the event-list photon-counting chain. Each telescope pupil is split into four parts, each focused on a single-photon avalanche photodiode (SPAD), and every detected photon is time-tagged with about 100 ps relative and <500 ps absolute accuracy and stored in an event list. Because all analysis is done in post-processing, the same list can be rebinned from nanoseconds to minutes, which lets a small telescope produce stable pulse profiles for the Crab pulsar and compute Leahy-normalized power spectra in which a 128 mHz QPO emerges above the red-noise continuum. The fiber interface that feeds Iqueye from the telescope Nasmyth focus is what keeps the instrument stable and allows the power spectra to reach these frequencies.
What would settle it
Re-analyze the same April 2018 event list with the sky background subtracted and generate a large ensemble of noise-only light curves from the best-fitting broad-band noise model; if peaks as strong as the 128 mHz feature occur in more than a few percent of those simulations, the claimed QPO is not distinguishable from red noise.
Extended reading notes
Core claim
The paper claims to have built and operated two SPAD-based photon-counting photometers that time-tag individual photons with sub-nanosecond accuracy, and to have shown that this makes very fast photometry practical on meter-class telescopes. On the Crab pulsar, regular monitoring since 2008 finds the optical pulse leading the radio pulse by a stable 150-250 microseconds, with no significant drift over the decade. On the X-ray transient MAXI J1820+070, the power spectrum of 3600 s of optical data from April 2018 shows a significant quasi-periodic oscillation at 128 ± 2 mHz with a full width at half maximum of 24 ± 5 mHz and fractional rms variability of 3.1 ± 0.3%, sitting on top of three broad-band noise components. The same data show a weaker QPO-like feature near 71 mHz, and June 2018 observations reveal two further QPO-like features, including cases where harmonically related fits at frequency ratios 1:2, 2:3, or 3:5 are acceptable.
Load-bearing premise
The 128 mHz oscillation is called significant without a reported false-alarm probability, so the claim depends on the assumption that the fitted noise and sky-background models fully account for the continuum and the residual peak is a real signal rather than a random fluctuation of the noise.
Editorial extensions
If this is right
- Optical pulsar timing can be sustained on meter-class telescopes: the Crab pulsar's radio-optical delay has been monitored regularly since 2008 and shows no significant change, implying the geometry of the optical and radio emission regions has been stable for a decade.
- A small telescope with these instruments can detect quasi-periodic oscillations in accreting black hole binaries, as demonstrated by the 128 mHz optical QPO in MAXI J1820+070.
- The frequency and width of the optical QPO match the X-ray QPO seen with Swift, so optical monitoring alone can track the evolution of the accretion-flow modulation in such transients.
- The same fiber-fed photon-counting setup enables exploratory optical intensity interferometry on multi-kilometer baselines, giving small telescopes access to sub-milliarcsecond angular scales.
- Making Aqueye+ fiber-fed and available in target-of-opportunity mode would allow prompt fast-photometry follow-up of transients and state transitions from a meter-class telescope.
Reading between the lines
- A natural extension would be to run a formal false-alarm analysis on the 128 mHz peak, since the paper reports significance without a stated detection level; if the peak survives Monte Carlo red-noise simulations, the optical and X-ray QPOs are almost certainly the same modulation.
- The reported acceptable harmonic fits at 1:2, 2:3, or 3:5 do not single out one ratio; simultaneous optical and X-ray power spectra during a single outburst could break that degeneracy and test precession-versus-reprocessing models.
- The fiber-fed design described here could be exported to other meter-class facilities, turning existing small-telescope networks into high-time-resolution and quantum-optics instruments without new large optics.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This proceedings-style paper describes the Aqueye+ and Iqueye photon-counting instruments and their ongoing scientific programs: very fast optical timing of the Crab pulsar, with an emphasis on the radio-optical delay monitoring; optical intensity interferometry on a kilometer baseline; and fast photometry of the optical counterpart of the black hole X-ray binary MAXI J1820+070. The central new scientific claim is in Section 5.2: a 'significant' optical quasi-periodic oscillation (QPO) at 128 ± 2 mHz with FWHM 24 ± 5 mHz and fractional rms 3.1 ± 0.3%, detected in April 2018 data, together with additional lower-frequency QPO-like features in April and June 2018. The paper also discusses the consistency of this QPO with quasi-simultaneous X-ray measurements and with previously reported optical QPOs.
Significance. If the QPO detection is robust, it is scientifically valuable: optical low-frequency QPOs in black hole X-ray binaries are rare and constrain the emission geometry and the Lense-Thirring precession scenario. The paper also gives useful exposure to the capabilities of meter-class telescopes when equipped with sub-nanosecond photon-counting instruments. Strengths of the manuscript include the description of the fiber-fed Iqueye interface, the long-term regular monitoring of the Crab pulsar radio-optical delay, and the explicit cross-comparison of the optical QPO frequency with contemporaneous Swift and NuSTAR X-ray results, which provides an independent wavelength anchor. However, the paper does not report any detection significance, false-alarm probability, or null-hypothesis test for the central QPO claim, and it openly admits that harmonically related fits are equally acceptable; these omissions are load-bearing for the main scientific result. No data or code are released, so an independent check of the power-spectrum fitting is not possible from the manuscript alone.
major comments (3)
- [Section 5.2, paragraph after Figure 3] The sentence 'We detect a significant quasi-periodic oscillation (QPO) on the top of three broad-band noise components' is the central claim of the paper, yet no detection significance, confidence level, false-alarm probability, or null-hypothesis rejection threshold is reported anywhere in the manuscript. Because the power spectra are computed from non-background-subtracted 1 ms light curves and averaged over 130 s intervals, the residual peak at 128 mHz sits on a fitted red-noise-like continuum; without a statistical test against that continuum, the word 'significant' is not supported. This is a load-bearing omission that must be fixed by adding a quantitative significance estimate (e.g., sigma level from the fit, or a false-alarm probability from Monte Carlo or analytic red-noise statistics).
- [Section 5.2, June 9-10 paragraph] The text states that 'acceptable fits of the power spectrum also with two harmonically-related QPOs ... are obtained for 1:2, 2:3, or 3:5 centroid frequency ratios' and that the April observations give similar results for the same harmonic ratios. This admitted non-uniqueness directly undermines the interpretation of the 128 mHz feature as an independent oscillation: it could be a harmonic of the 71 ± 4 mHz feature rather than a separate QPO. The paper should quantify model preference among the alternative fits, for example by reporting Δχ², BIC, or likelihood ratios, and should justify why the non-harmonic model is adopted as the central result.
- [Section 5.2, Figure 3 caption and surrounding text] The fitting model is described only as 'three broad-band noise components (in part induced by the sky background)' with no specification of their functional forms, the sky-background treatment, or the fitting procedure. Since the QPO parameters (centroid, FWHM, fractional rms) are quoted with formal errors extracted from this fit, the lack of a detailed model description means the fit cannot be reproduced or independently checked, and it is unclear whether the continuum is actually fully accounted for. The authors should provide the functional form of each noise component, the background estimation method, and the fit statistic used.
minor comments (4)
- [Section 3, paragraph 2] Typo: 'indepedent' should be 'independent'.
- [Figure 3 caption] Typo: 'Lehay normalized' should be 'Leahy normalized'.
- [Section 2 and Section 4] The instrument timing accuracy is quoted as '≃ 100 ps relative time accuracy and < 500 ps absolute time accuracy' in Section 2, while Section 4 refers to 'tens of ps time resolution'; these two statements should be reconciled or clarified.
- [References] Several in-text citations refer to Astronomer's Telegrams (e.g., Zampieri et al. 2018a,b; Fiori et al. 2018; Yu et al. 2018) and the paper would benefit from a statement about whether refereed full analyses of these QPO data have been or will be published, since the present manuscript is the only citable source for the quoted parameters.
Circularity Check
No circularity: the paper reports fitted observational measurements of QPOs and compares them against independent X-ray/optical data, with no derivation reducing to its own inputs.
full rationale
This paper is an observational status report on very fast photometry instrumentation and results. The central quantitative claims are the measured QPO centroid frequencies, widths, and fractional rms amplitudes in MAXI J1820+070 (Section 5.2). These quantities are fitted parameters describing the observed power spectra; they are measurements, not predictions derived from a theory and not used as inputs to a further derivation. The paper compares the 128 mHz optical QPO with quasi-simultaneous Swift X-ray observations and with previously reported optical QPOs, which are independent external references. Many self-citations appear (Zampieri et al. 2018a,b; Fiori et al. 2018; Germanà et al. 2012; Spolon et al. 2019, etc.), but they document the same ongoing observational program and do not carry a derivation step that reduces to its own conclusion. The absence of a quoted false-alarm probability for the QPO is a legitimate statistical robustness concern, but it is not an instance of circular reasoning under the definitions used here: nothing in the paper's argument is equivalent to its inputs by construction. Therefore the appropriate finding is no significant circularity, score 0.
Assumptions & free parameters
free parameters (5)
- QPO centroid frequency (128 mHz) =
128 ± 2 mHz
- QPO full-width-half-maximum =
24 ± 5 mHz
- QPO fractional rms =
3.1 ± 0.3%
- Broad-band noise components =
not given numerically
- Lower-frequency QPO features =
71 ± 4 mHz and 151/268 mHz (June)
assumptions (2)
- domain assumption Power density spectrum follows Leahy normalization with 1 ms binning and averaging
- domain assumption The instruments' absolute timing accuracy (< 500 ps) and photon-counting linearity are as characterized in prior papers
Cite this review
Pith. "Pith review of (Very) Fast astronomical photometry for meter-class telescopes." pith.science (2026). https://pith.science/paper/HATXPNQO
@misc{pith2026190803396,
author = {Pith},
title = {Pith review of: (Very) Fast astronomical photometry for meter-class telescopes},
year = {2026},
howpublished = {\url{https://pith.science/paper/HATXPNQO}},
note = {Machine review of arXiv:1908.03396}
}
read the original abstract
Our team at the INAF-Astronomical Observatory of Padova and the University of Padova is engaged in the design, construction and operations of instruments with very high time accuracy in the optical band for applications to High Time Resolution Astrophysics and Quantum Astronomy. Two instruments were built to perform photon counting with sub-nanosecond temporal accuracy, Aqueye+ and Iqueye. Aqueye+ is regularly mounted at the 1.8m Copernicus telescope in Asiago, while Iqueye was mounted at several 4m class telescopes around the world and is now attached through the Iqueye Fiber Interface at the 1.2m Galileo telescope in Asiago. They are used to perform coordinated high time resolution optical observations and, for the first time ever, experiments of optical intensity interferometry on a baseline of a few kilometers. I will report on recent technological developments and scientific results obtained within the framework of this project.
Figures
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Forward citations
Cited by 1 Pith paper
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Reference graph
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Reviewed August 14, 2026 · model on record in the stance chip above.
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