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Searching for supermassive black holes binaries within SRG/eROSITA-De I: Properties of the X-ray selected candidates

T0 review · 2 major / 4 minor · reviewed 2026-08-16 · deepseek-v4-flash

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Pith's one-line read Four eROSITA X-ray scans alternating bright and faint identify 16 plausible supermassive black hole binaries, one with an apparent one-year period.

desk verdict An honest, well-scoped candidate paper whose selection is explicitly uncalibrated against red noise; worth refereeing now, with the false-positive analysis being the load-bearing next step. read the letter →

arxiv 2505.02708 v1 pith:EV4ZSKVO submitted 2025-05-05 astro-ph.HE astro-ph.GA

D. Tubín-Arenas , M. Krumpe , D. Homan , A. Markowitz , M. Powell , G. Lamer , T. Urrutia , A. Schwope
show 15 more authors
H. Winkler S. Bahic J. Buchner C. Andonie M. Salvato A. Merloni J. Kurpas S. Ciroi F. di Mille A. Chaturvedi A. Rau Z. Igo I. Grotova Z. Liu K. Nandra
This is my paper · ORCID
classification astro-ph.HEastro-ph.GA
keywords supermassiveblackholebinarieseROSITAall-skysurveysquasi-periodicX-rayvariabilityAGNred-noisemonitoringSMBHBcandidatessoftselectiongalacticnuclei
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 starts the first systematic X-ray search for supermassive black hole binaries (SMBHBs) using the repeated all-sky scans of eROSITA, which sampled the sky roughly every six months over two and a half years. The authors argue that galaxies whose soft X-ray flux alternates between bright and faint states on consecutive scans ('up-down-up-down') are plausible SMBHB candidates, and they present 16 such objects with optical confirmation of their extragalactic, nuclear origins. The best candidate, eRASSt J0530-4125, shows a quasi-periodic bright-faint cycle of about one year in the observed frame, which, if it is the binary orbital period, would place the two black holes about 1.2 milli-parsecs apart. From the rarity of the selected pattern, they estimate an optimistic upper limit of about 0.05 SMBHB candidates per X-ray-emitting galaxy, consistent with theoretical predictions. The paper stresses that single-black-hole red-noise variability can mimic a few cycles of periodicity, so the candidate status depends on longer monitoring and a false-positive analysis deferred to a companion paper.

What carries the argument

The load-bearing object is the eROSITA multi-epoch light curve: four all-sky scans spaced six months apart (plus a partial fifth) create a repeating sample of the sky at half-year intervals. The selection machinery is the 'up-down-up-down' pattern: a source must alternate between bright and faint flux levels from scan to scan, with adjacent epochs differing by more than $3\sigma$, an average bright-to-faint factor $F>3$, and a normalized significance $S>3$; the monitored primary sample tightens this to $F>5$ and a bright-state flux above roughly $5\times10^{-13}\,\mathrm{erg\,s^{-1}\,cm^{-2}}$. The pattern is the observable proxy for the predicted periodic feeding of mini-disks in a binary surrounded by a circumbinary disk. The rest of the machinery—Gaia astrometry to remove stars, optical spectroscopy to confirm nuclei and measure black-hole masses, and Swift/NICER/XMM follow-up to fill the six-month eROSITA gaps—exists to test whether the alternation survives denser sampling and to exclude non-binary interlopers such as flares or off-nuclear ultraluminous sources.

What would settle it

Simulate millions of AGN red-noise light curves with power-spectral slopes 1–2, sample each at the eROSITA cadence (one point every six months, four or five epochs), and count how many pass the up-down-up-down, $F>3$, $S>3$ criteria; if the false-positive rate predicts a sample comparable to 16, the candidate list and the ~0.05 upper limit lose their force. On the observational side, additional cycles of NICER/Swift monitoring that show eRASSt J0530-4125's ~1-year phase and amplitude drifting or disappearing would falsify the periodic interpretation.

Watch

Extended reading notes

Core claim

The discovery claim is that the eROSITA 'up-down-up-down' pattern is a workable X-ray selection for sub-parsec supermassive black hole binaries. Selecting sources whose consecutive 0.2–2.3 keV flux measurements differ by more than $3\sigma$, with a bright-to-faint flux ratio above 3 and a normalized bright-faint separation above 3, yields 16 monitored candidates; optical spectroscopy confirms 15 of them as nuclear extragalactic sources, with single-epoch black hole masses near $10^7\,M_\odot$. The most promising object, eRASSt J0530-4125, displays a tentative period of $\sim$1 year in its combined eROSITA, Swift-XRT, and NICER light curves, with no change in spectral shape, and, under the binary interpretation, would have an orbital separation of 1.2 milli-pc and an orbital velocity of roughly $8000\,\mathrm{km\,s^{-1}}$. Stacked spectra show power-law photon indices $\Gamma\sim1.8$–$2.8$ for 14 of 15 sources, consistent with AGN-like emission. One of the 16, eRASSt J1003-2607, was reclassified as a likely off-nuclear ultra-luminous X-ray source and removed. The paper's headline demographic result is an optimistic upper limit of $\sim0.05$ SMBHB candidates per X-ray-emitting galaxy, and it explicitly leaves confirmation to future multi-cycle monitoring and to the red-noise false-positive analysis of the companion paper.

Load-bearing premise

The load-bearing premise is that a four-epoch alternating X-ray pattern signals genuine quasi-periodicity, rather than the random flickering ('red noise') that a single accreting black hole can produce.

Editorial extensions

If this is right

  • eROSITA-style all-sky monitoring becomes a discovery channel for sub-parsec SMBHBs: 16 candidates from the first four scans, with one showing a one-year X-ray cycle.
  • The one-year periodicity of eRASSt J0530-4125, if binary in origin, fixes the orbital separation at about 1.2 milli-pc and an orbital speed near 8000 km/s, giving a predicted Fe Kα energy shift of 0.17 keV that XMM-Newton can barely resolve.
  • The pattern's rarity translates into an optimistic upper limit of ~0.05 SMBHB candidates per X-ray-emitting galaxy, consistent with theoretical expectations below 1–10%.
  • Most candidates show AGN-like X-ray spectra (photon index 1.8–2.8) and low black-hole masses near $10^7$ solar masses, making them plausible targets for high-resolution Fe Kα spectroscopy with future missions.
  • The candidate list is explicitly provisional: confirmation requires the periodic signal to persist across several cycles in denser monitoring, and red-noise false positives are deferred to the companion paper.

Reading between the lines

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

  • If the eROSITA pattern survives red-noise testing, the ~0.05 per-galaxy upper limit would be a floor for X-ray-selected SMBHBs, since eROSITA is only sensitive to periods near 4 and 12 months; longer-period binaries would be invisible to this search and would raise the true fraction.
  • The quick disappearance of eRASSt J1141+0635's apparent 120-day period under denser NICER/Swift cadence suggests that future surveys should treat low-cadence 'up-down' patterns as triage filters whose candidates need immediate higher-cadence observations before any demographic claim.
  • The one reclassified off-nuclear source, likely an ultra-luminous X-ray source, shows that X-ray-selected SMBHB samples require positional and spectroscopic vetting; similar contamination could affect purely photometric searches in other bands.
  • If the one-year period of eRASSt J0530-4125 is confirmed, it becomes one of the few SMBHB candidates where X-ray monitoring and high-resolution Fe Kα spectroscopy could be combined to test the binary interpretation directly.
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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

2 major / 4 minor

Summary. This paper presents the first in a series of searches for supermassive black hole binary (SMBHB) candidates in SRG/eROSITA data. The authors select extragalactic X-ray sources showing alternating 'up-down-up-down' flux patterns across the four (or five) eROSITA all-sky surveys, requiring adjacent epoch flux differences >3σ, a bright-to-faint flux factor F>5, and a significance S>3 for the primary sample of 16 sources. Optical spectroscopy confirms 15/16 as extragalactic nuclear sources; one is classified as an off-nuclear ULX-like source. X-ray follow-up with Swift, NICER, and XMM-Newton is presented, with eRASSt J0530-4125 showing tentative ~1-year quasi-periodic variability. Stacked X-ray spectra are mostly power laws with Γ~1.8–2.8. Using the selected candidates and an assumed eROSITA sensitivity window, the authors estimate an optimistic upper limit of ~0.05 SMBHB candidates per X-ray-emitting galaxy. The paper repeatedly emphasizes that red-noise AGN variability can mimic the selection pattern and defers a quantitative false-positive analysis to Paper II.

Significance. If the selection is robust, this is the first systematic X-ray-selected SMBHB candidate sample from an all-sky survey, providing a valuable target list for multi-wavelength follow-up and a demographic constraint in a regime where most searches are optical. The strengths of the paper are the careful optical spectroscopic confirmation, the multi-wavelength characterization of the candidates, the transparent treatment of alternative scenarios (TDEs, QPEs, HLXs), and the explicit acknowledgment of the red-noise caveat. The ~0.05 fraction, once properly calibrated against a red-noise null, would be an interesting observational constraint. However, the main quantitative claims currently rest on an unquantified selection completeness and false-positive rate, so the significance is conditional.

major comments (2)
  1. [§6.1 and §5.2] The selection's false-positive rate under red-noise AGN variability is not quantified. The paper states in §6.1 that 'We cannot rule out the presence of red-noise variability in our sample until further follow-up campaigns provide strong evidence of either periodic or stochastic variability,' and §5.2 notes that the Lomb-Scargle periodogram is evaluated only against a white-noise null. Since the candidate list of 16 sources and the fraction estimate in §6.3 are derived from the F>5, S>3 selection over only 4–5 epochs spaced six months apart, a red-noise false-positive control is load-bearing. Red-noise PSDs with slopes 1–2 are documented to produce few-cycle alternating patterns (Krishnan et al. 2021; Witt et al. 2022), so the authors should provide at least a preliminary simulation-based false-positive rate, or explicitly reframe the candidate list and the fraction as raw selection rates pending Paper II.
  2. [§6.3 and §7] The conversion from the raw selection rate to the ~0.05 fraction relies on two ad hoc inputs: a 'sensitive time window' of ~3 months and a uniform period distribution over 30 years. Neither is derived from a simulation of the selection function, and the fraction is directly proportional to both. Furthermore, the numerator is not corrected for red-noise false positives. The conclusion's phrasing '~0.05 SMBHB per X-ray emitting galaxy' overstates the support, since the calculation is an upper limit on the number of selected candidates, not on the confirmed SMBHB population. The authors should either recalibrate these inputs or restrict the claims to selection rates only.
minor comments (4)
  1. [§2.2.1] The significance criterion is written as F_i - F_{i+1} over the error sum, without absolute value; as written it cannot select 'down-up-down-up' profiles. Presumably the absolute value is intended; please clarify.
  2. [Figures 7 and 8] The repeated 'Obs dates [years]' ticks on the sub-panels are visually noisy; the axis label should be placed once per column to improve readability.
  3. [Introduction] The sentence 'Most of these candidates are have timescales in the regime...' contains a typo ('are have'); please correct.
  4. [§5.2.1] The phrase 'We do not discard the possibility that the variability pattern is the result of stochastic processes' is awkward; 'We do not rule out' would be clearer and consistent with the rest of the paper's language.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the candidate selection, follow-up analysis, and ~0.05 fraction estimate are based on observational criteria and literature relations, not on recycled fitted values.

full rationale

The paper's central outputs are an observationally selected sample of X-ray variable candidates and an estimated upper limit on the SMBHB fraction. The selection criteria (up-down-up-down patterns, F > 3 or 5, S > 3) are defined directly from eROSITA flux measurements and uncertainties, with no parameter fitted to the later claims. The ~0.05 fraction in Sect. 6.3 is computed by combining the observed number of candidates with a literature-based BLR size (Greene et al. 2010), an assumed uniform period distribution, and an explicitly stated eROSITA sensitivity window; it is an estimate with stated assumptions, not a prediction derived from a fitted value. The use of Tubín-Arenas et al. (2024) for 3-sigma upper limits in the selection is self-citation, but it is a calibration/data product with independent content (aperture photometry on standard eROSITA products) and does not itself assert the SMBHB nature of any source. The paper explicitly defers the red-noise false-positive rate to Paper II and repeatedly cautions that quasi-periodic signals could be stochastic AGN variability; this is an acknowledged limitation and missing control, not a circular step. No equation or claim in the paper reduces to its own inputs by construction, so no circularity step meets the evidentiary bar.

Assumptions & free parameters 1 free parameters · 5 assumptions · 0 invented entities

The central result is a candidate list and an upper-limit estimate. The fraction estimate rests on one hand-chosen sensitivity window and several literature/assumption-based inputs (BLR size, uniform period distribution), but no parameters are fit to the target data in a way that would make the result circular.

free parameters (1)
  • eROSITA sensitive time window = ~3 months
    Sect. 6.3 assumes ~2 months of the ~12-month periods and ~1 month of the ~4-month periods are detectable; this hand-chosen window directly sets the 8e-3 factor in the ~0.05 upper limit.
assumptions (5)
  • domain assumption The 4-5 epoch eROSITA light-curve pattern (up-down-up-down) selects quasi-periodic candidates before the red-noise false-positive rate is quantified.
    Invoked in Sect. 2.2.1 selection criteria; Sect. 6.1 states 'We cannot rule out the presence of red-noise variability in our sample' and defers testing to Paper II.
  • domain assumption eROSITA's six-month cadence provides sufficient sampling to identify quasi-periods of ~4 and ~12 months.
    Sect. 2.2.1 and Sect. 6.3; no independent validation of detection completeness is provided in this paper.
  • domain assumption The BLR size-luminosity relation from Greene et al. (2010) applies to the average sample properties, giving a BLR radius of ~0.01 pc.
    Sect. 6.3 uses this to estimate the binary separation and the sensitive period window for the fraction estimate.
  • ad hoc to paper SMBHB orbital periods are uniformly distributed over the assumed 30-year range.
    Sect. 6.3 explicitly assumes uniform distribution to convert the sensitive time window into an 8e-3 duty-cycle factor for the ~0.05 upper limit.
  • standard math Keplerian orbital dynamics (d = [GMP^2/4pi^2]^(1/3)) apply to the binary separation estimate for eRASSt J0530-4125.
    Sect. 5.2.1 uses Kepler's third law; this is standard mechanics, though its applicability to an unconfirmed binary is an assumption.

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Cite this review

Pith. "Pith review of Searching for supermassive black holes binaries within SRG/eROSITA-De I: Properties of the X-ray selected candidates." pith.science (2026). https://pith.science/paper/EV4ZSKVO

@misc{pith2026250502708,
  author       = {Pith},
  title        = {Pith review of: Searching for supermassive black holes binaries within SRG/eROSITA-De I: Properties of the X-ray selected candidates},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/EV4ZSKVO}},
  note         = {Machine review of arXiv:2505.02708}
}
abstract

Abridged: Supermassive black hole binaries (SMBHBs) separated by (sub)-pc scales represent one of the latest stages of hierarchical galaxy assembly. However, many of these objects are hidden behind large columns of gas and dust at the center of galaxies and are difficult to detect. The X-ray and UV emission in these systems are predicted to vary regularly on timescales comparable to that of the orbital period of the binary. This is the first of a series of papers where we aim to find SMBHB candidates based on quasi-periodic light curves from the soft X-ray instrument eROSITA on board the Spectrum-Roentgen-Gamma (SRG) observatory and X-ray follow-up. We searched the multi-epoch SRG/eROSITA all-sky surveys for extragalactic sources that show an `up-down-up-down' or `down-up-down-up' profile (from scan to scan) in their 0.2--2.3 keV flux light curves. We compiled a sample of 16 sources that are suitable for X-ray follow-up campaigns given their brightness and significant variability between bright and faint SRG/eROSITA flux levels. We triggered extensive Swift-XRT and NICER monitoring campaigns on the best SMBHB candidates to confirm or discard their tentative periodicities. Optical spectroscopic observations confirmed the nuclear and extragalactic nature of 15/16 objects and enabled single-epoch SMBH mass measurements and BPT classifications. Our most promising candidate, eRASSt J0530-4125, shows X-ray quasi-periodic variability with a typical time scale of one year in the observed frame. By stacking the X-ray observations of each source in our sample, we find that 14/15 sources can be modeled by a power law with a photon index ranging from $\Gamma\sim1.8-2.8$. Based on our selection, we estimate an optimistic upper limit on the fraction of SMBHB candidates to be $\sim 0.05$ per galaxy. We emphasize that further observational evidence is needed to confirm the SMBHB nature of our sources.

Figures

Figures reproduced from arXiv: 2505.02708 by the authors.

Figure 1
Figure 1. Visualization of the selection for SMBHB candidates based on their [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Distribution of the sample based on the factor ( [PITH_FULL_IMAGE:figures/full_fig_p005_2.png] view at source ↗
Figure 3
Figure 3. Optical images of the SMBHB candidates. The cutout images were retrieved from the Legacy Survey Viewer with a pixel [PITH_FULL_IMAGE:figures/full_fig_p007_3.png] view at source ↗
Figures from the paper (8 more)
Figure 4
Figure 4. Figure 4: Optical spectra compilation of the best SMBHB candidates in their rest frame. The spectra are sorted from top to bottom [PITH_FULL_IMAGE:figures/full_fig_p010_4.png]
Figure 5
Figure 5. Figure 5: Visualization of the full spectral fit of eRASSt J1906- [PITH_FULL_IMAGE:figures/full_fig_p011_5.png]
Figure 6
Figure 6. Figure 6: [N ii]-BPT ([O iii]λ5007/Hβ versus [N ii]λ6583/Hα) di￾agnostic diagram for the SMBHB candidates. The black curve marks the separation proposed by Kewley et al. (2001, denoted as Ke01 in the diagrams), between the theoretical maximum ion￾ization driven by pure star-form…
Figure 7
Figure 7. Figure 7: X-ray light curves of the X-ray monitored SMBHB candidates. SRG/ [PITH_FULL_IMAGE:figures/full_fig_p013_7.png]
Figure 8
Figure 8. Figure 8: Continuation of Fig. 7. X-ray light curves of the second half of the X-ray monitored SMBHB candidates. [PITH_FULL_IMAGE:figures/full_fig_p014_8.png]
Figure 9
Figure 9. Figure 9: X-ray properties of eRASSt J0530-4125 observed with eROSITA, [PITH_FULL_IMAGE:figures/full_fig_p015_9.png]
Figure 10
Figure 10. Figure 10: X-ray properties of eRASSt J1141+0635 observed with eROSITA, NICER, Swift-XRT, and XMM-Newton. The panels are similar to those shown in [PITH_FULL_IMAGE:figures/full_fig_p016_10.png]
Figure 7
Figure 7. Figure 7: We note that the Swift-XRT upper limits of 2022 are still consistent with the periodic prediction. However, deeper obser￾vations are needed to confirm or rule out any possible rebright￾ening expected from our SMBHB predictions. NICER observed the galaxy between July an…

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Forward citations

Cited by 1 Pith paper

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Reference graph

Works this paper leans on

13 extracted references · 13 canonical work pages · cited by 1 Pith paper

  1. [348]

    Data and markers similar as in Fig. A.5. eRASSt J1124-0348 (or 6dFGS gJ112456.3-034840) is a low luminosity Seyfert II Galaxy with a redshift of z = 0.021. It only has a ROSAT all-sky survey upper limit of ∼ 4× 10−13 erg s−1 cm−2 on the 0.2–2.0 keV band, consistent with the faint flux state of the eROSITA data. The optical spectrum reveals a galaxy-like s...

    work page 2021

  2. [806]

    A.1 with additional ZTF r-band data in the third panel

    The data is similar to the data of Fig. A.1 with additional ZTF r-band data in the third panel. faint flux state. The optical ATLAS data in Fig. A.6 seems to follow the pattern established by the X-ray data, especially be- tween eRASS1 and eRASS2 and during the NICER monitoring program of early 2023. eRASSt J1522-3722 (or 2MASS J15221541-3722488) is class...

    work page 2023

  3. [1988]

    where the soft X-ray emission seen by SRG/eROSITA is likely produced by the repeated accretion of the debris material onto the SMBH after the disruption event. There is a 4-year window between the tentative start of the TDE and the lack of detection on the Swift-XRT monitoring ob- servations where the source might have been detected in X-rays. This window...

    work page 2020

  4. [2159]

    A.2 with additional r-band ZTF optical data

    The data and markers are similar to those in Fig. A.2 with additional r-band ZTF optical data. eRASSt J0458-2159 (ESO 552-39; z = 0.040) is a Seyfert 1 Galaxy monitored by NICER. The source was ob- Article number, page 25 of 28 A&A proofs: manuscript no. aa54749-25 served with the ROSAT all-sky survey, ROSAT-PSPC, and ROSAT-HRI pointed observations with f...

    work page 2014

  5. [2252]

    A.1 with additional ZTF r-band data in the third panel

    The data is similar to the data of Fig. A.1 with additional ZTF r-band data in the third panel. The X-ray panel also displays NICER data with orange markers. Appendix A.14: eRASSt J0600-2939 eRASSt J0600-2939 is a galaxy at z = 0.104 with an optical spectrum that shows weak Hβ+[O iii] emission lines. Only the broad Hα line was used to estimate a single-ep...

    work page 2014

  6. [2607]

    Additional NICER observations (orange markers) were ob- tained to monitor the X-ray evolution of the source

    Only Swift-XRT (purple markers in top panel) and Swift- UVOT (bottom panel) observations in different bands were pre- viously available for this object, covering almost 10 years of data. Additional NICER observations (orange markers) were ob- tained to monitor the X-ray evolution of the source. 4500 5000 5500 6000 6500 7000 7500 8000 Wavelength [Å] 0.2 0....

    work page 2016

  7. [2939]

    The data is similar to the data of Fig. A.1. source was detected in X-ray by the ROSAT all-sky survey with a flux of∼ 4.5× 10−13 erg s−1 cm−2 on the 0.2–2.0 keV band. 57000 58000 59000 60000 61000 MJD Obs dates [years] 10 13 10 12 Flux0.2 2.3 keV eROSITA Swift-XRT 2014 2016 2018 2020 2022 2024 2026 Obs dates [years] 12.0 12.5 13.0Magnitude W1 W2 16.50 16....

    work page 2014

  8. [3023]

    The data is similar to the data of Fig. A.1. The X-ray panel also displays NICER data shown with orange markers. Appendix A.5: eRASSt J1130-0806 eRASSt J1130-0806 (MCG-01-29-027: z = 0.037) shows a galaxy-like spectrum with faint emission lines and the ab- sence of broad Balmer lines. The optical counterpart of the SRG/eROSITA coordinates corresponds to a...

    work page 2014

Show all 13 references
  1. [3125]

    Data and markers similar as in Fig. A.1. Appendix A.12: eRASSt J1003-2607 eRASSt J1003-2607 (RX J1003.2-2607) was classified as AGN by Kahabka et al. (2000) while studying the gas content of the dwarf galaxy NGC 3109 using ROSAT PSPC observations of background X-ray sources. N...

    2000

  2. [3313]

    The data is similar to the data of Fig. A.1. Appendix A.16: eRASSt J0614-3835 eRASSt J0614-3835 shows a Seyfert II optical spectrum hosted by a galaxy at z = 0.054. The source was detected in X-ray by the ROSAT all-sky survey with a flux of∼ 4×10−13 erg s−1 cm−2 on the 0.2–2.0...

    2014

  3. [3722]

    The data is similar to the data of Fig. A.4. Appendix A.7: eRASSt J1141+0635 eRASSt J1141 +0635 (or 2MASS J11415445 +0635096; z = 0.101) is one of the most monitored sources in our sample. Ini- tially, the X-ray data visually suggested a periodic behavior with a tentative peri...

    2021

  4. [3835]

    The data is similar to the data of Fig. A.1. Article number, page 28 of 28

  5. [4333]

    Data similar than in Fig. A.1. eRASSt J2227-4333 (6dFGS gJ222755.8-433339) is a galaxy at redshift z = 0.198 with a strong [O iii]λ5007Å emis- sion line but without very prominent broad Balmer lines, indi- cating a type II AGN. The optical and IR light curves of the source (Fi...

    2014

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