XMM-Newton follow-up of a sample of apparent low surface brightness galaxy groups detected in the ROSAT All-Sky Survey

Angie Veronica; Claudia Spinelli; Florian Pacaud; Konstantinos Migkas; Miriam E. Ramos-Ceja; Thomas H. Reiprich; Weiwei Xu

REVIEW 2 major objections 4 minor 49 references

XMM-Newton follow-up of a sample of apparent low surface brightness galaxy groups detected in the ROSAT All-Sky Survey

T0 review · 2 major / 4 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read XMM-Newton follow-up shows that two galaxy groups have unusually flat X-ray surface brightness profiles, with slope parameter below 0.4.

desk verdict Careful, honestly caveated follow-up that resolves three RASS groups into nine components and reports two strikingly flat beta profiles, but the flat-beta claim needs a systematic-error test before it moves into cosmology. read the letter →

arxiv 2506.19718 v1 pith:5GKYKFUU submitted 2025-06-24 astro-ph.CO

Claudia Spinelli , Angie Veronica , Florian Pacaud , Thomas H. Reiprich , Konstantinos Migkas , Weiwei Xu , Miriam E. Ramos-Ceja This is my paper · ORCID

classification astro-ph.CO

keywords galaxygroupsX-raysurfacebrightnessprofilesbetamodelclustercosmologyROSATAll-SkySurveyXMM-Newtonfollow-upprojectioneffectsselectionfunctions

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 tests whether a newly cataloged population of apparent low-surface-brightness galaxy groups is real by following up three of them with XMM-Newton. Each pointing turns out to contain several distinct groups, nine in total, some at very different redshifts along the same line of sight. The two central groups of two pointings have very flat X-ray surface brightness profiles, with $\beta$ parameters of $0.36\pm0.02$ and $0.39\pm0.01$, well below the canonical $2/3$. Once the original ROSAT fluxes are split among the individual groups, none exceeds the flux limit of earlier RASS cluster catalogs. The authors conclude that flat-profile groups exist, while whether they form a separate population requires a larger sample; if such groups are common, survey selection functions and cluster counts would need correcting.

What carries the argument

The load-bearing tool is the single $\beta$-model surface brightness profile, $S(r) \propto [1+(r/r_c)^2]^{-3\beta+1/2}$, which assigns each group a slope parameter $\beta$ measuring how flat its X-ray emission is; the canonical comparison point is $\beta = 2/3$. Around it, the analysis stacks XMM-Newton imaging and spectroscopy: wavelet-filtered point-source masking, background-subtracted images, spectral fits with a cosmic X-ray background model anchored to ROSAT sky-background annuli, and core-excised temperatures converted to $R_{500}$ through the Lovisari et al. (2015) mass-temperature relation. The $\beta$ parameter carries the central claim, because the two central groups violate the canonical value by wide margins.

What would settle it

Observe AS0924 and Group 4 deeply enough to fit their surface brightness profiles without masking the western half and with a blank-sky background rather than ROSAT annuli; if the recovered $\beta$ rises to $2/3$, the flat-profile claim is an artifact. A complementary check is to inject a simulated $\beta = 2/3$ group into the same masking and fitting pipeline and see whether it returns $\beta < 0.4$.

Watch

Extended reading notes

Core claim

On its own terms, the paper establishes that the apparent new class of low-surface-brightness groups found in a reanalysis of the ROSAT All-Sky Survey is not a single-object illusion, but its members are more complex than single objects. In the three usable XMM-Newton pointings, each RASS detection is resolved into two to four X-ray-emitting groups, and nine groups are characterized in total, with redshifts from optical catalogs or X-ray spectral fits. The two central groups, AS0924 and Group 4, show very flat surface brightness profiles with $\beta = 0.36 \pm 0.02$ and $\beta = 0.39 \pm 0.01$, respectively, while the other groups have $\beta < 0.6$. After the original RASS fluxes are divided among the resolved components, every individual group falls below the $\approx 3\times10^{-12}\,\mathrm{erg\,s^{-1}\,cm^{-2}}$ flux limit of earlier RASS-based catalogs. The paper concludes that flat-profile groups exist, and that the apparent new population is partly explained by blended groups at the same or different redshifts.

Load-bearing premise

The load-bearing premise is that the fitted $\beta$ values are the intrinsic gas slopes rather than artifacts of the analysis: the fits use masked apertures (including exclusion of the entire western half of AS0924), a ROSAT-based sky background, and a restricted radial range, on only three usable pointings.

Editorial extensions

If this is right

Flat-profile groups with $\beta < 0.4$ exist in X-ray observations, so survey selection modeling must include them.
The original RASS detections were blends of two to four groups, so projection and blending, not only low surface brightness, explain part of the apparent new population.
Since every resolved group lies below the RASS flux limit, earlier catalogs were not missing these systems by flux alone; source detection and deblending are the relevant limitations.
Cluster count analyses that ignore such systems would bias number densities downward and mass and redshift estimates upward.
A representative follow-up of about 25 systems, roughly 500 ks of XMM-Newton time, is the stated next step to decide whether a separate population exists.

Reading between the lines

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

If flat $\beta$ values are common among low-mass groups, the X-ray-selected cluster mass function may need a profile-dependent completeness correction that current surveys do not apply.
The fraction of blends among the 303 new detections could be estimated statistically by comparing follow-up resolution rates with line-of-sight projection expectations from simulations.
A direct extension would be to measure $\beta$ for a volume-limited X-ray group sample, not just apparently flat ones, to see whether the $\beta$ distribution is continuous or genuinely bimodal.

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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 thing to know: this is a solid, workmanlike paper that gives the first good X-ray look at three RASS-selected low-surface-brightness candidates, and what it finds is messier than the parent sample assumed. Each pointing breaks up into multiple groups—nine total, with temperatures, masses, and beta slopes. Two central groups (AS0924 and Group 4) come out with beta ~0.36–0.39, well below the canonical 2/3. That is a genuine new measurement, and the optical/NED matching to sort out redshifts is a real service.

What the paper does well: the data reduction is careful and transparent. They drop one flaring observation, exclude MOS1 after background-template trouble, treat Group 3 separately, and say so. They show spectra and cumulative profiles. They don't oversell the sample size—they explicitly call for ~25 more systems. The flux-split argument is logically sound: if three RASS sources are actually nine groups, then the individual fluxes are expected to fall below the old survey limits. That part is partly by construction, but they admit it and frame it as a selection-function issue, not a discovery.

The soft spot is the one the stress-test flags: the two beta<0.4 values carry no systematic error budget. The background comes from ROSAT annuli, not local source-free XMM regions; half of some apertures are masked; the entire western half of AS0924 is excluded; and for AS0924 part of the fitted region falls outside the usable XMM FoV. All of those choices can flatten a cumulative beta-model profile in the same direction. I don't think the effect can easily turn 0.36 into a canonical 2/3—that would be a huge background error—but the quoted ±0.01–0.02 errors are only statistical, and the paper never tries the obvious robustness checks. Given that the existence claim rests on just these two values, a referee should ask for those tests.

The circularity concern in the reader's take is real but minor: the 'not missing' conclusion is essentially how they define the split, and they don't pretend otherwise. The larger issue is small-N and the unquantified systematics on the headline claim, not the logic.

Who this is for: people working on X-ray group selection functions and RASS-based catalogs. It deserves a serious referee. My recommendation: send it out, and require a systematic-error assessment for the beta slopes—vary the background, mask, and radial range—or at minimum an explicit paragraph on how much each choice could bias beta. With that, the paper is a useful, citable data point.

Referee Report

2 major / 4 minor

Summary. The paper presents XMM-Newton follow-up observations of three low-surface-brightness galaxy groups selected from the ROSAT All-Sky Survey reanalysis of Xu et al. (2018, 2022). After rejecting one flared pointing, the authors perform imaging, surface brightness, and spectral analysis and find that each XMM pointing contains multiple extended X-ray sources, nine in total. They fit single beta-model surface brightness profiles and report flat slopes, with beta < 0.4 for the central groups AS0924 and Group 4. They also decompose the RASS flux among the nine groups and find that all individual fluxes lie below the approximately 3e-12 erg/s/cm2 flux limit of earlier RASS cluster catalogs. The paper concludes that groups with flat X-ray surface brightness profiles exist, while carefully noting that the sample is small and that further follow-up is needed to establish whether they form a separate population.

Significance. If the measured beta values are robust, the paper provides direct evidence for galaxy groups with unusually flat X-ray surface brightness profiles, with consequences for the selection functions of X-ray cluster surveys and for cosmological applications. The analysis is careful and honestly caveated: the flared observation is dropped, MOS1 is excluded from spectral fitting, Group 3 is treated as a special low-count case, and the R500 errors are stated to neglect scaling-relation scatter. The paper also demonstrates that a single RASS detection can be a blend of several groups at different redshifts, a useful cautionary result. However, the headline beta < 0.4 claim depends on single-beta-model fits to cumulative profiles without an explicit systematic error budget, so the significance is conditional on additional robustness tests being supplied.

major comments (2)

[Section 4, Table 3 (beta column), Fig. A.1] The central claim that flat profiles exist relies on beta values whose quoted uncertainties are statistical only; no systematic error budget is presented for the beta fits. The sky background is estimated from ROSAT annuli rather than from source-free regions in the XMM data, the analysis masks sectors facing bright neighbors (Group 1 and Group 2), and for AS0924 the R500/temperature loop excludes the entire western half with 0.5 R500 partly outside the usable XMM-Newton FoV. Each of these choices can bias beta low, and the reported errors (0.36 +/- 0.02 for AS0924, 0.39 +/- 0.01 for Group 4) are not tested against alternative background normalizations, mask geometries, or radial fitting ranges. I request robustness tests that vary these choices and a systematic error term added to the beta values, since the existence claim is load-bearing for the paper's conclusions.
[Section 4, Fig. 3 and Fig. A.1] The surface brightness fits are presented as fits to cumulative flux profiles. Because cumulative profiles have strongly correlated data points between adjacent radii, fitting them as if the points were independent can underestimate the parameter uncertainties and can bias the best-fit beta depending on the chosen integration limit. The paper does not state whether the covariance matrix is included in the lmfit minimization. I recommend fitting the differential surface brightness profile with Poisson errors, or explicitly propagating the cumulative-profile covariance, and checking the stability of beta against the outer radius used in the fit.

minor comments (4)

[Section 3.2] The text refers to 'A3472' and 'A3475' in the paragraph on Observation 2; these appear to be typos for 'A3742' and 'A3745' and should be corrected.
[Table 3] The flux unit header is incomplete: '10^-13 erg s cm' should read '10^-13 erg s^-1 cm^-2'.
[Section 5 and Table 3] The X-ray redshift quoted for Group 2 in the text (0.107 +0.014/-0.008) differs from the value in Table 3 (0.111 +0.013/-0.008); please reconcile the two values.
[Section 5, flux-limit discussion] The statement that all nine individual fluxes fall below the RASS flux limit should be explicitly framed as a consistency check rather than an independent test, because each component flux is derived by decomposing the same RASS-detected total flux; the paper's own wording 'this is not surprising' already gestures at this, but the later bullet conclusion that the systems are 'strictly speaking, not missing' would benefit from the same qualification.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the central beta, temperature, and redshift measurements are independent XMM-Newton fits, and the sub-limit flux conclusion is explicitly presented as a consequence of splitting the measured RASS flux.

full rationale

The paper's central claims—flat surface brightness profiles with beta < 0.4 for AS0924 and Group 4—are obtained by fitting single beta models to background-subtracted, exposure-corrected XMM-Newton surface brightness profiles (Sect. 4, Fig. A.1). These fits are not defined in terms of the conclusions they support; the beta parameter is a standard model parameter from Cavaliere & Fusco-Femiano, and the fit is to independent XMM-Newton data. Temperatures, X-ray redshifts, and fluxes come from spectral fitting with a stated foreground/background model, with scaling relations adopted from external work (Lovisari et al. 2015). The statement that the individual fluxes of the nine identified groups fall below the previous RASS flux limit is an arithmetic consequence of splitting the measured total RASS flux among multiple components; the paper itself acknowledges this by calling it 'not surprising' and by saying 'strictly speaking, they are not missing from previous RASS-based cluster catalogs.' Thus it is not presented as an independent prediction. Self-citations to Xu et al. (2018, 2022) supply the parent sample and detection method, but the XMM-Newton follow-up analysis is new, externally falsifiable data reduction, not a restatement of those papers' inputs. No uniqueness theorem or ansatz is imported from the authors' prior work to force the beta values. Concerns about masking choices, ROSAT-based sky background, and statistical-only error bars are legitimate robustness issues, but they are matters of systematic uncertainty, not circularity.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

The paper's claims rest on standard X-ray analysis assumptions: beta-model shape, background modeling from ROSAT annuli and published CXB templates, and scaling relations from Lovisari et al. 2015. The beta slopes and temperatures are fitted to data, so the flat-profile result is only as robust as those fits. No entities are invented.

free parameters (4)

beta (surface brightness slope, per group) = 0.36 to 0.58
Fitted slope of the single beta-model; the paper's flat-profile claim rests on these values.
M500 / R500 = R500 from 2.15 to 25.15 arcmin depending on group
Derived from fitted temperatures via the M500-T relation of Lovisari et al. 2015; used for flux integration and masses.
adopted redshifts (AS0924, A3742, Group 4) = 0.016, 0.018, 0.036 respectively
Choice of zassoc over zopt or zX-ray affects distances, luminosities, and masses; e.g., AS0924 has zopt = 0.032 +/- 0.018 versus zassoc = 0.016.
CXB model normalizations = variable in spectral fits
Local Hot Bubble, Milky Way halo, and power-law AGN normalizations fitted in the spectral model, anchored to fixed temperatures and slopes from the literature.

assumptions (5)

domain assumption Single beta-model describes group surface brightness profiles
Used to extract the slope parameter and fluxes; a different profile shape would change beta and the integrated fluxes.
domain assumption Sky background from ROSAT annuli is representative
Background spectra extracted from the same annular regions as X18; errors in this background propagate into beta and flux.
domain assumption M500-T relation of Lovisari et al. 2015 holds for these low-mass, low-redshift groups
Used to convert core-excised temperatures to R500 and M500; intrinsic scatter is neglected in error bars.
domain assumption NED galaxy redshift catalog is sufficiently complete for group membership assignment
Optical redshifts from median of NED galaxies per source; sparse or contaminated membership affects adopted redshifts.
domain assumption CXB spectral parameters from McCammon et al. 2002 and De Luca & Molendi 2004 apply to these fields
Fixed temperatures and photon index for the background model; inaccurate background modeling would bias faint-source spectra.

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Cite this review

Pith. "Pith review of XMM-Newton follow-up of a sample of apparent low surface brightness galaxy groups detected in the ROSAT All-Sky Survey." pith.science (2026). https://pith.science/paper/5GKYKFUU

@misc{pith2026250619718,
  author       = {Pith},
  title        = {Pith review of: XMM-Newton follow-up of a sample of apparent low surface brightness galaxy groups detected in the ROSAT All-Sky Survey},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/5GKYKFUU}},
  note         = {Machine review of arXiv:2506.19718}
}

abstract

Galaxy cluster cosmology relies on complete and pure samples spanning a large range of masses and redshifts. In Xu et al. (2018) and Xu et al. (2022), we discovered an apparently new population of galaxy groups and clusters with, on average, flatter X-ray surface brightness profiles than known clusters; this cluster population was missed in previous cluster surveys. The discovery of such a new class of objects could have a significant impact on cosmological applications of galaxy clusters. In this work we use a subsample of these systems to assess whether they belong to a new population. We follow up three of these galaxy groups and clusters with high-quality XMM-Newton observations. We produce clean images and spectra and use them for model fitting. We also identify known galaxies, groups, or clusters in the field. The observations reveal that all three systems are composed of multiple groups each, either at the same or at different redshifts. In total, we characterize nine groups. We measure flat surface brightness profiles with slope parameter $\beta < 0.6$; i.e, less than the canonical $\beta = 2/3$. For the two main central groups, we even measure $\beta < 0.4$. When the fluxes for the three observations are split up across the nine identified groups, none of them exceeds the typical flux limit adopted in previous RASS cluster catalogs, $\approx 3 \times 10^{-12}\,\mathrm{erg s^{-1}cm^{-2}}$ in the 0.1$-$2.4 keV energy band. The observations reveal that groups with flat surface brightness profiles exist. Whether they form a new, separate population requires additional follow-up observations of further systems from the Xu et al. sample, given the complexity we have discovered. Such extended low surface brightness systems, as well as multiple systems and projection effects, need to be taken into account when determining selection functions of group and cluster samples.

Figures

Figures reproduced from arXiv: 2506.19718 by the authors.

**Figure 2.** Flux comparison performed between ROSAT and XMM [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗

**Figure 3.** Cumulative sky background subtracted flux in the 0.5-2 keV energy band and best-fit [PITH_FULL_IMAGE:figures/full_fig_p008_3.png] view at source ↗

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Pith tools

Reviewed August 15, 2026 · model on record in the stance chip above.

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