SRG/ART-XC All-Sky X-ray Survey: Sensitivity Assessment Based on Aperture Photometry

A. A. Abbakumov; A. A. Lutovinov; A. D. Samorodova; E. I. Zakharov; E. V. Filippova; F. V. Korotkov; N. Y. Tyrin; R. A. Burenin; R. A. Krivonos; S. Y. Sazonov

REVIEW 2 major objections 4 minor 19 references

SRG/ART-XC All-Sky X-ray Survey: Sensitivity Assessment Based on Aperture Photometry

T0 review · 2 major / 4 minor · reviewed 2026-08-06 · deepseek-v4-flash

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗ Review JSON

Pith's one-line read The paper establishes that four SRG/ART-XC all-sky surveys can yield a reliable 4-12 keV flux upper limit at any celestial position, computed consistently by frequentist and Bayesian methods and delivered as a public web service.

desk verdict A genuinely useful all-sky upper-limit service for ART-XC, but the confidence calibration is unverified because the background is treated as a known constant. read the letter →

arxiv 2507.10060 v1 pith:CL5Y5JV3 submitted 2025-07-14 astro-ph.HE

N. Y. Tyrin , R. A. Krivonos , V. A. Arefiev , R. A. Burenin , E. I. Zakharov , A. A. Lutovinov , S. Y. Sazonov , A. D. Samorodova

show 5 more authors

E. V. Filippova A. A. Abbakumov V. V. Konoplev F. V. Korotkov V. N. Nazarov

This is my paper · ORCID

classification astro-ph.HE

keywords skysurveysX-raysourcesaperturephotometryupperlimitsPoissonstatisticsBayesianinferenceART-XCtelescopesurveysensitivity

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

This paper establishes that the summed data from the first four SRG/ART-XC all-sky surveys can be used to compute a reliable upper limit on the 4-12 keV X-ray flux for any point on the celestial sphere, not just for the positions of the 1,545 catalogued sources. The method is aperture photometry: photons are counted in a 71-arcsecond circle around the target coordinate, the background is estimated from a surrounding annulus, and the counts are converted into flux limits by inverting the Poisson likelihood. The work implements both classical frequentist confidence limits and Bayesian posterior intervals with a flat non-negative prior, and the two approaches give consistent results. This matters because many real X-ray emitters are too faint or too variable to pass the detection threshold of the published catalog; for any coordinate, an astronomer now has a quantitative statement of how bright a source there must be before ART-XC would have seen it. The capability is deployed as the public ART-XC upper-limit service at https://www.srg.cosmos.ru/uplim.

What carries the argument

The load-bearing mechanism is aperture photometry on HEALPix-tessellated count and exposure maps, coupled to two independent inversions of the same Poisson likelihood. Source counts $N$ are extracted from a $71''$ aperture, the radius that contains 90% of the ART-XC survey-mode point spread function, and the background $B$ is estimated from a $213''$-$355''$ annulus scaled to the aperture area. The likelihood is $P(N\,|\,S+B) = (S+B)^N e^{-(S+B)}/N!$; the frequentist route solves the classical Poisson confidence-limit equations for one- and two-sided limits via the regularized incomplete gamma function, and the Bayesian route integrates the posterior $f_{N,B}(S) \propto e^{-(S+B)}(S+B)^N$ with a flat prior for $S \ge 0$, choosing the two-sided interval of minimal length. The conversion chain — counts to rate by dividing by the mean exposure in the aperture and the enclosed-energy fraction $EEF = 0.96$, then rate to flux by the factor $CF = 4\times10^{-11}$ erg cm$^{-2}$ s$^{-1}$ per count rate — turns the statistical limits into physical flux limits. Regions around catalogued sources are excluded with flux-dependent radii from $3.6'$ up to $2.5^\circ$, and the same computation is offered for the combined four surveys or for each survey separately.

What would settle it

An injected-source test would settle it: take the real count and exposure maps, place simulated point sources of known 4-12 keV flux at many positions across the sky, including regions with structured background such as the Galactic plane, rerun the published pipeline, and check that the reported 95% upper limit brackets the true flux in 95% of trials with no systematic offset. If the annulus background is not representative of the aperture, or its uncertainty is not negligible, the coverage would deviate from 95% precisely in the structured-background regions.

Watch

Extended reading notes

Core claim

The central claim is that aperture photometry on the first four ART-XC all-sky surveys (December 2019 through December 2021) yields valid point-source flux confidence limits in the 4-12 keV band for arbitrary celestial coordinates, not just for catalogued sources. Counts are summed in a $71''$ aperture — the W90 radius of the survey-mode point spread function, which encloses 90% of the flux — and the background is estimated from an annulus between $213''$ and $355''$, scaled by area, with the expected count treated as Poisson-distributed and the background-rate uncertainty neglected. One-sided and two-sided limits at any requested confidence level are computed two independent ways: classical frequentist confidence limits obtained by inverting the Poisson likelihood through the regularized incomplete gamma function, and a Bayesian posterior $f_{N,B}(S) \propto e^{-(S+B)}(S+B)^N$ built on a flat non-negative prior, with two-sided intervals chosen to have minimal length; the two calculations agree. Limits are converted to count rate using the mean exposure inside the aperture and an enclosed-energy fraction of 0.96, then to flux with the factor $4\times10^{-11}$ erg cm$^{-2}$ s$^{-1}$ per unit count rate, and aperture fluxes at known source positions match the ARTSS1-5 catalog values. Over $10^5$ trial positions, the median 95% one-sided Bayesian upper limit ranges from about $2.6\times10^{-12}$ erg cm$^{-2}$ s$^{-1}$ near the ecliptic equator to lower values near the ecliptic poles, and the paper fits this latitude dependence with a quadratic $UL = a\theta^2 + b\theta + c$ in ecliptic latitude $\theta$ (degrees), with $a=-2.675\times10^{-16}$, $b=3.304\times10^{-17}$, $c=2.561\times10^{-12}$ erg s$^{-1}$ cm$^{-2}$.

Load-bearing premise

The load-bearing assumption is that the average count rate in the background annulus ($213''$ to $355''$ from the target) is exactly the background under the source aperture, with negligible uncertainty — if the true background varies on that size scale, every reported limit shifts by that variation.

Editorial extensions

If this is right

Any celestial coordinate now carries a quantitative 4-12 keV flux limit from the combined first four ART-XC surveys, turning non-detections into usable measurements for objects not in the 1,545-source catalog.
Variable and transient sources that fell below the catalog threshold can be bounded: for any known object at any position, the service gives a physical upper bound on its hard X-ray flux at the chosen confidence level.
Because limits are also computed for each individual survey, the service supports time-resolved studies across the December 2019-to-December 2021 baseline, letting limits from one epoch cross-check detections in another.
The consistency between the frequentist and Bayesian outputs serves as an internal cross-check, so a user can quote either framework and expect essentially the same answer.
Sources discovered at other wavelengths can be immediately confronted with ART-XC upper limits, making the service a practical screening tool for candidate X-ray counterparts over the whole sky.

Reading between the lines

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

Editorial extension: the fitted quadratic $UL = a\theta^2 + b\theta + c$ is effectively a closed-form sensitivity map; one could validate it by comparing its predictions against direct service queries at many latitudes, then use it to forecast the sensitivity of future scans.
Editorial extension: as the fifth and later surveys are added, background-dominated apertures imply the limiting flux should improve roughly as the inverse square root of total exposure; that scaling is a concrete prediction to test at the next data release.
Editorial extension: since exclusion zones around catalogued sources grow with source brightness, the usable sky fraction shrinks as future catalogs grow, so statistical studies using the service should track what fraction of trial coordinates fall inside excluded regions.
Editorial extension: the same aperture-photometry pipeline could be mirrored on the companion soft-X-ray survey to produce a complementary 0.2-8 keV upper-limit service, extending the cross-instrument comparison the paper already draws.

Share X Bluesky LinkedIn Reddit HN

Editorial analysis

A structured set of objections, weighed in public.

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

Desk Editor's Note

read the letter

The paper delivers exactly what it says: a public web service that returns 4–12 keV upper limits and confidence intervals for any position on the sky using ART-XC survey data. That is the genuinely new piece, and it is useful. X-ray astronomers working on transients, variables, and multiwavelength counterparts will want this tool, and the ecliptic-latitude sensitivity relation is a nice descriptive summary of the survey's depth structure. The statistical machinery is standard (Gehrels, Kraft et al.), the implementation choices are clearly described, and the Bayesian/frequentist cross-check adds confidence. I also appreciate that the paper states its main assumption out loud: the background count rate estimated from the 213–355 arcsec annulus is treated as a known constant, with uncertainty "assumed to be negligible."

That assumption is load-bearing, and the stress-test concern lands. For the faint sources where these upper limits matter most, the annulus background is a Poisson realization with relative uncertainty roughly 1/sqrt(N_ann). Ignoring that uncertainty makes the nominal 95% upper limit undercover — the limit will exclude the true flux more often than 5% of the time. The annulus can also pick up the target's own PSF wings (the 71" aperture encloses 96% of the EEF, but not 100%), which would bias B upward and the limit downward. The paper's validation against ARTSS1-5 fluxes checks central values, not coverage; a calibration offset in the intervals would not show up in that comparison. So the core promise — reliable upper limits at any given significance level — is not yet established.

This is fixable and should be fixed. The authors can marginalize over the background in the Bayesian approach, or use a profile likelihood, and they can validate with simulations: inject fake sources of known flux, run the service, and check what fraction of the time the 95% limit actually exceeds the injected flux. They should also quantify the "good agreement" with catalog fluxes — a residual plot with error bars would take one line of text. The polynomial fit in Fig. 1 is descriptive, not circular; that is fine.

The paper is well worth refereeing. The data product is valuable, the methods are standard and the flaws are specific and addressable. I would send it to a referee with a request to verify the background treatment. For my own work in X-ray transient follow-up, I would cite the service once the coverage property is demonstrated; until then, I would treat the limits as approximate.

Referee Report

2 major / 4 minor

Summary. This paper describes a method for deriving point-source flux upper limits and confidence intervals from SRG/ART-XC all-sky survey data in the 4-12 keV band. The method uses aperture photometry with a 71-arcsecond aperture and an annular background region, assumes Poisson statistics, and implements both frequentist (Gehrels) and Bayesian (Kraft et al.) confidence limits, converting counts to flux via exposure, enclosed-energy fraction, and a conversion factor. The implementation is deployed as a public web service using HEALPix maps from ARTSS1-4. The authors validate the method by comparing aperture fluxes of known catalog sources with ARTSS1-5 fluxes and illustrate the ecliptic-latitude dependence of 95% upper limits.

Significance. The paper addresses a real community need: public, coordinate-based upper limits for a hard X-ray all-sky survey. Its strengths are the use of standard, well-established statistical formulations, the explicit cross-check of frequentist and Bayesian results, a clearly described data pipeline, and the public service itself. The method is not circular, and the derived upper limits are directly useful for transient and multiwavelength follow-up. The significance of the central claim, however, is gated by two fixable validation gaps: the treatment of background uncertainty and the lack of quantitative tests of the claimed confidence-level calibration. If those are resolved, this will be a valuable and citable resource.

major comments (2)

[Methods, Eqs. (1)-(6)] The background count rate in the source aperture is treated as a known constant B, while in fact B is estimated by area-scaling counts in an annulus; the annulus counts are a Poisson realization, and the text states that the uncertainty is 'assumed to be negligible.' For faint sources, where N and B are small, this assumption is load-bearing: ignoring the background uncertainty makes nominal 95% upper limits undercover, meaning the true coverage probability falls below 0.95. In addition, the annulus can contain the target's own PSF wings because a 71-arcsecond aperture encloses only a fraction of the PSF, biasing B upward and the upper limit downward. This concern directly affects the claimed 'any given significance level' property and should be quantified, ideally with a Monte Carlo simulation that injects simulated point sources into real background maps and checks the coverage of the returned intervals.
[Implementation, Eq. (18)] The validation statement that aperture fluxes are in 'good agreement' with ARTSS1-5 catalog fluxes is not quantified: no residuals, scatter, or statistical test is provided. This comparison validates central values only and cannot detect miscalibration of the confidence-level coverage. The central claim of the paper is about the reliability of upper limits at specified significance levels, so the paper should include a coverage test, for example, simulated sources injected into the actual count maps with the fraction of trials in which the true flux lies below the returned upper limit compared with the nominal confidence level.

minor comments (4)

[Introduction] The possessive of 'it' is spelled 'its’' in the opening paragraph; it should be 'its.' Also, the reference to Voges et al. (1999) lacks a closing parenthesis.
[Methods] The text first says that the 71-arcsecond aperture corresponds to the W90 radius containing 90% of the total flux, but later sets EEF=0.96 for the same aperture; these numbers should be reconciled or the definitions clarified.
[Implementation] The statement that the frequentist and Bayesian methods give consistent results is not supported by a quantitative comparison; a figure or table showing the differences between the two sets of limits would be useful.
[Discussion and Fig. 1] The quadratic approximation to the ecliptic-latitude dependence is presented without fit uncertainties or residual statistics; reporting these would make the descriptive relation reproducible.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the upper limits are computed directly from survey count and exposure maps using standard Poisson/Bayesian statistics, with external calibration constants and no fitted parameter in the derivation chain.

full rationale

The paper's central claim is that it computes an upper flux limit in the 4-12 keV band at any significance level for any sky position, using aperture photometry. The derivation chain is: counts N in a 71" aperture, Poisson likelihood (Eq. 1), background B estimated from an annulus by area scaling, frequentist limits from Gehrels (Eqs. 2-6), Bayesian limits from Kraft et al. (Eqs. 7-12), then conversion to count rate and flux via exposure, EEF, and CF (Eqs. 13-14). None of these quantities is defined in terms of the final upper limit. The background is an independent annulus measurement, and the paper explicitly states its uncertainty is assumed negligible; that is a statistical modeling assumption and a possible correctness risk, not circularity. The EEF and CF are taken from prior ART-XC calibration/catalog work, not fitted to the quantities being predicted here. The polynomial fit in Fig. 1 is a descriptive summary of the computed median upper limits as a function of ecliptic latitude; it is not used as an input to any calculation, so it cannot be a fitted-input-called-prediction. The comparison of aperture fluxes with ARTSS1-5 catalog fluxes is a validation check of central values, not a parameter fit that forces the upper-limit result. Self-citations to Krivonos et al. (2025), Sazonov et al. (2024), and the ARTSS catalogs provide instrumental PSF parameters and conversion factors that are independently measured/calibrated, and they do not import the paper's central claim. The frequentist and Bayesian implementations agree because they solve the same Poisson problem via established statistical formulas, not because of any circular construction. Therefore, no step in the claimed derivation reduces to its own inputs, and no circularity is present.

Assumptions & free parameters 2 free parameters · 5 assumptions · 0 invented entities

The core calculation rests on standard Poisson and Bayesian statistics. The main ad hoc elements are the empirical exclusion radii and the descriptive polynomial fit. No new physical entities are introduced.

free parameters (2)

Exclusion radii for known source flux bins = 3.6 arcmin, 9 arcmin, 30 arcmin, 54 arcmin, 2.5 deg for flux bins from <1.25e-10 to >200e-10 erg/s/cm2
Empirically chosen in Table 2 to mask out cataloged sources; affect the service response near bright objects but are not part of the statistical derivation.
Polynomial coefficients a, b, c for the ecliptic latitude upper-limit relation = a=-2.675e-16 erg/s/cm2/deg2, b=3.304e-17, c=2.561e-12
Fit to the median of computed Bayesian upper limits in Fig. 1; this is a descriptive approximation of the survey sensitivity, not an input to the service.

assumptions (5)

standard math Detected counts in the aperture follow a Poisson distribution with mean (S+B).
Underlying counting statistics used in Eq. (1); standard for X-ray photon counting.
domain assumption Background count rate in the annulus is representative of the background in the source aperture and its uncertainty is negligible.
Stated in METHODS; assumed spatial uniformity of background, potentially invalid in complex regions.
domain assumption Non-negative constant prior is appropriate for Bayesian flux estimation.
Follows Kraft et al. (1991) to maintain objectivity; a flat prior is a modeling choice.
domain assumption The 71 arcsec aperture captures 90% of the point spread function flux with EEF=0.96.
Calibration from Krivonos et al. (2025); depends on the ART-XC PSF in survey mode.
domain assumption A single conversion factor CF=4e-11 erg cm^-2 s^-1 per count/s converts count rate to flux.
Taken from Sazonov et al. (2024); implies an assumed spectral shape and instrument response.

how reviews work

0 comments

Cite this review

Pith. "Pith review of SRG/ART-XC All-Sky X-ray Survey: Sensitivity Assessment Based on Aperture Photometry." pith.science (2026). https://pith.science/paper/CL5Y5JV3

@misc{pith2026250710060,
  author       = {Pith},
  title        = {Pith review of: SRG/ART-XC All-Sky X-ray Survey: Sensitivity Assessment Based on Aperture Photometry},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/CL5Y5JV3}},
  note         = {Machine review of arXiv:2507.10060}
}

read the original abstract

The Spectrum-Roentgen-Gamma (SRG) observatory continues to operate successfully in orbit at the Lagrange point L2. The Mikhail Pavlinsky ART-XC telescope has demonstrated high efficiency in conducting X-ray surveys both over large sky regions and the entire celestial sphere. A recently published source catalog, based on data from the first four and partially completed fifth sky scans, contains 1,545 objects detected in the 4-12 keV energy range. In this work, using the same sky survey data, we assess the sensitivity to point source detection across the celestial sphere based on X-ray aperture photometry - that is, we calculate the upper flux limit in the 4-12 keV band at any given significance level. The method is implemented using both Poisson statistics and Bayesian inference, with consistent results between the two approaches. This information is important for studying variable and transient X-ray sources, as well as sources that are not detected with sufficient statistical significance in the ART-XC all-sky survey. The ART-XC upper limit service is available at https://www.srg.cosmos.ru/uplim.

Figures

Figures reproduced from arXiv: 2507.10060 by the authors.

**Figure 1.** Dependence of the one-sided upper limit (in Bayesian statistics mode) on ecliptic latitude for the sum of the first four surveys, approximated by a second-order polynomial and two-sided confidence limits on the 4-12 keV flux are evaluated for any point on the celestial sphere, using data from the ART-XC telescope’s full-sky survey. The software implementation of the methods is available on the SRG project website,… view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

19 extracted references · 19 canonical work pages

[1]

Astropy Collaboration, A. M. Price-Whelan, P. L. Lim, N. Earl, N. Starkman, L. Bradley, et al., ApJ, 935, 167 (2022)

work page 2022
[2]

W. T. Eadie and L. D. Kirkpatrick, Physics Today, 26, 51 (1973) ASTRONOMY LETTERS Vol. 00 No. 0 2025 4 Tyrin et al

work page 1973
[3]

Forman, C

W. Forman, C. Jones, L. Cominsky, P. Julien, S. Murray, G. Peters, et al., ApJS,38, 357 (1978)

work page 1978
[4]

Gehrels, ApJ,303, 336 (1986)

N. Gehrels, ApJ,303, 336 (1986)

work page 1986
[5]

K.M.G´ orski,E.Hivon,A.J.Banday,B.D.Wandelt, F. K. Hansen, M. Reinecke, et al., ApJ,622, 759 (2005)

work page 2005
[6]

D. I. Karasev, A. N. Semena, I. A. Mereminskiy, A. A. Lutovinov, R. A. Burenin, R. A. Krivonos, et al., Astronomy Letters,49, 662 (2023)

work page 2023
[7]

R. P. Kraft, D. N. Burrows, and J. A. Nousek, ApJ, 374, 344 (1991)

work page 1991
[8]

Inflight calibration of SRG/ART-XC point spread function at large off-axis angles

R. Krivonos, R. Burenin, E. Filippova, I. Lapshov, A. Tkachenko, A. Semena, et al., arXiv e-prints, p. arXiv:2505.13296 (2025)

work page Pith review arXiv 2025

Show all 19 references

[9]

A. M. Levine, F. L. Lang, W. H. G. Lewin, F. A. Primini, C. A. Dobson, J. P. Doty, et al., ApJS,54, 581 (1984)

1984
[10]

Pavlinsky, A

M. Pavlinsky, A. Tkachenko, V. Levin, N. Alexandrovich, V. Arefiev, V. Babyshkin, et al., A&A, 650, A42 (2021)

2021
[11]

Pavlinsky, S

M. Pavlinsky, S. Sazonov, R. Burenin, E. Filippova, R. Krivonos, V. Arefiev, et al., A&A, 661, A38 (2022)

2022
[12]

Predehl, R

P. Predehl, R. Andritschke, V. Arefiev, V. Babyshkin, O. Batanov, W. Becker, et al., A&A, 647, A1 (2021)

2021
[13]

A. Ruiz, A. Georgakakis, S. Gerakakis, R. Saxton, P. Kretschmar, A. Akylas, et al., MNRAS,511, 4265 (2022)

2022
[14]

Sazonov, R

S. Sazonov, R. Burenin, E. Filippova, R. Krivonos, V. Arefiev, K. Borisov, et al., A&A,687, A183 (2024)

2024
[15]

Semena, I

A. Semena, I. Mereminskiy, A. Lutovinov, S. Sazonov, V. Arefiev, K. Borisov, et al., MNRAS, 529, 941 (2024)

2024
[16]

Sunyaev, V

R. Sunyaev, V. Arefiev, V. Babyshkin, A. Bogomolov, K. Borisov, M. Buntov, et al., A&A, 656, A132 (2021)

2021
[17]

Tub´ ın-Arenas, M

D. Tub´ ın-Arenas, M. Krumpe, G. Lamer, J. Haase, J. Sanders, H. Brunner, et al., A&A,682, A35 (2024)

2024
[18]

Virtanen, R

P. Virtanen, R. Gommers, T. E. Oliphant, M. Haberland, T. Reddy, D. Cournapeau, et al., Nature Methods, 17, 261 (2020)

2020
[19]

Voges, B

W. Voges, B. Aschenbach, T. Boller, H. Br¨ auninger, U. Briel, W. Burkert, et al., A&A,349, 389 (1999) ASTRONOMY LETTERS Vol. 00 No. 0 2025

1999

Pith tools

Reviewed August 6, 2026 · model on record in the stance chip above.

[1] [1]

Astropy Collaboration, A. M. Price-Whelan, P. L. Lim, N. Earl, N. Starkman, L. Bradley, et al., ApJ, 935, 167 (2022)

work page 2022

[2] [2]

W. T. Eadie and L. D. Kirkpatrick, Physics Today, 26, 51 (1973) ASTRONOMY LETTERS Vol. 00 No. 0 2025 4 Tyrin et al

work page 1973

[3] [3]

Forman, C

W. Forman, C. Jones, L. Cominsky, P. Julien, S. Murray, G. Peters, et al., ApJS,38, 357 (1978)

work page 1978

[4] [4]

Gehrels, ApJ,303, 336 (1986)

N. Gehrels, ApJ,303, 336 (1986)

work page 1986

[5] [5]

K.M.G´ orski,E.Hivon,A.J.Banday,B.D.Wandelt, F. K. Hansen, M. Reinecke, et al., ApJ,622, 759 (2005)

work page 2005

[6] [6]

D. I. Karasev, A. N. Semena, I. A. Mereminskiy, A. A. Lutovinov, R. A. Burenin, R. A. Krivonos, et al., Astronomy Letters,49, 662 (2023)

work page 2023

[7] [7]

R. P. Kraft, D. N. Burrows, and J. A. Nousek, ApJ, 374, 344 (1991)

work page 1991

[8] [8]

Inflight calibration of SRG/ART-XC point spread function at large off-axis angles

R. Krivonos, R. Burenin, E. Filippova, I. Lapshov, A. Tkachenko, A. Semena, et al., arXiv e-prints, p. arXiv:2505.13296 (2025)

work page Pith review arXiv 2025