Unveiling the nature of donor stars of ULXs in NGC 1559

A. Akyuz; S. Allak

REVIEW 3 major objections 4 minor 56 references

Unveiling the nature of donor stars of ULXs in NGC 1559

T0 review · 3 major / 4 minor · reviewed 2026-08-10 · deepseek-v4-flash

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Pith's one-line read The paper identifies donor stars of four ULXs in NGC 1559 and shows their optical light sources differ.

desk verdict Solid, careful counterpart identifications for four ULXs in NGC 1559, but the X-24 period rejection rests on a thin extinction treatment. read the letter →

arxiv 2501.09974 v1 pith:FQ4C6NCI submitted 2025-01-17 astro-ph.HE

S. Allak , A. Akyuz This is my paper · ORCID

classification astro-ph.HE

keywords ultraluminousX-raysourcesdonorstarsNGC1559binariesHubbleSpaceTelescopeJamesWebbcolor-magnitudediagramsspectralenergydistributions

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 asks which stars feed the eight ultraluminous X-ray sources (ULXs) in the nearby galaxy NGC 1559, using archival Hubble and James Webb imaging. For four of the eight it finds unique optical counterparts, and for two of those the counterparts are also detected in the infrared. From spectral energy distributions and color-magnitude diagrams, the authors conclude that the optical emission of X-14 and X-24 comes from their donor stars, with masses near 18 and 12 solar masses and ages near 7 and 12 million years, while the optical light of X-1 and X-18 is dominated by their accretion disks. They further argue that the 7500-second periodicity previously reported for X-24 is too short to be the orbital period of a high-mass X-ray binary, so it is likely a quasi-periodic oscillation or spin signal. If correct, the paper turns four anonymous ULX counterparts into characterized stellar systems.

What carries the argument

The analysis rests on four tools. First, astrometric alignment of Chandra X-ray positions with GAIA and HST star catalogs, using three reference sources, yields a 0.38 arcsecond error circle that defines which optical and infrared point sources count as counterparts. Second, photometry from HST/WFC3 and JWST/NIRCam images, including long-baseline HST F160W monitoring, provides light curves and spectral energy distributions. Third, SEDs are fitted with power-law and blackbody models to distinguish donor-star emission from disk or jet emission. Fourth, color-magnitude diagrams with PARSEC isochrones at the Tully-Fisher distance modulus of 30.5 magnitudes pin down donor ages and masses. Finally, the Eggleton Roche-lobe formula converts the donor mass and the proposed 7500 s period into a geometry that cannot accommodate Roche-lobe overflow.

What would settle it

A high-resolution spectrum of the X-24 counterpart that shows it to be a background star, without the radial-velocity signature of a binary, would falsify the donor identification. Equally, detecting X-ray or optical timing that confirms the 7500 s signal as the binary orbital period would overthrow the central interpretation, since a 12 solar-mass supergiant cannot fit in that orbit. The cleanest test is measuring the counterpart's radial velocity over several nights: a true 7500 s orbit produces a large semi-amplitude, while a quasi-periodic oscillation or jet signature would not.

Watch

Extended reading notes

Core claim

The central claim is that the four ULXs in NGC 1559 with unique optical counterparts have different emission origins. For X-14 and X-24 the optical light is constant over years and the objects sit on stellar isochrones in color-magnitude diagrams, placing them as a roughly 18 solar-mass, 7 million-year donor and a 12 solar-mass, 12 million-year supergiant donor respectively; the X-24 optical SED is a 7000 K blackbody. For X-1 and X-18 the optical SEDs are power laws and the sources are faint, indicating that the accretion disk or jet dominates the optical emission. The paper also rejects the previously proposed 7500 s orbital period for X-24: with a 12 solar-mass supergiant donor at that period the Roche-lobe radius is far too small for mass transfer, so the X-ray modulation is more plausibly a quasi-periodic oscillation, spin period, or random variability. The authors therefore classify X-14 and X-24 as high-mass X-ray binaries and leave the compact-object nature of X-1 open between a stellar-mass black hole and a neutron star.

Load-bearing premise

The identification of donor stars assumes that the single optical and infrared sources found inside the 0.38 arcsecond error circles are the actual companions, not unrelated stars that happen to lie close to the X-ray positions; the estimated false-positive rate is about 5.7 percent for optical and 4.2 percent for infrared counterparts.

Editorial extensions

If this is right

X-14 and X-24 are established as high-mass X-ray binaries with donor masses and ages, letting future work target them for direct spectral classification.
The 7500 s X-ray modulation of X-24 should be searched in other bands and in later epochs; if it is a quasi-periodic oscillation or spin, it becomes a probe of the accretion flow rather than of the binary orbit.
The X-1 and X-18 counterparts being disk-dominated explains their faintness and power-law SEDs; deeper ultraviolet or infrared coverage could still expose their unseen donors.
The demonstration that JWST can separate ULX counterparts in a 12.6 megaparsec star-forming galaxy extends the method to other ULX populations in similarly crowded fields.
Multi-epoch infrared variability, as seen in X-24, is a potential marker of jets or circumbinary dust around ULXs even when the optical donor looks perfectly quiet.

Reading between the lines

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

If the 7500 s signal of X-24 is a spin or quasi-periodic oscillation, X-24 may belong to the growing class of ULX pulsars; a direct search for coherent pulsations in the existing Chandra data would test this.
With a false-positive rate near 5.7 percent for optical counterparts and four candidates, the chance of at least one spurious association is roughly one in five; verifying each candidate with timing, color, or spectral evidence is a natural next step.
The same constant-optical-plus-variable-infrared signature found in X-24 could be used as a selection criterion to find jet-dominated ULX donors in other JWST-observed galaxies.
A direct spectral type for the X-14 and X-24 donors, for example an O or B supergiant versus a cooler supergiant, would sharpen the mass and age estimates, since color-magnitude placement alone carries systematic isochrone and extinction uncertainties.

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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 paper delivers honest, limited-scope progress: four new optical counterparts for ULXs in NGC 1559, two with matching IR counterparts, and a plausible split between donor-dominated and disk-dominated systems. The astrometry is careful, with a Gaia-based correction and a false-positive check that puts the chance-match rate around 5% for optical and 4% for NIR. That is the right way to handle the crowding problem, and the X-1 X-ray analysis, while secondary, is competent. The X-24 reinterpretation of the 7500 s period as non-orbital is interesting and physically motivated.

The soft spots are real but not fatal. The largest is the extinction treatment in the CMD/SED work. The authors correct with a fixed AV=0.04 and no uncertainty, while also reporting that X-24 sits in a dense dusty NIRCam region, coincides with an H II region, and shows an F814W excess. The observed colors are far redder than the 12 Msun/12 Myr isochrone they adopt. A modest reddening, say E(B-V)=0.3, shifts the CMD position enough that the derived donor mass and age change, and the 7500 s period rejection depends on that mass. So the paper's most striking claim is the one most exposed to a single unconstrained parameter. The X-14 classification is also a bit thin: the SED could not be fitted with physically meaningful parameters, yet it is classified as donor-dominated based on constancy and CMD position. There are no quoted uncertainties on the CMD ages or masses, which would help the reader judge how firm the classifications are.

The chance-superposition worry is only minor given the false-positive rates they quote. I'd send this to a serious referee. The counterpart identifications and the X-24 period challenge deserve to be on the record, but the referee should push for a proper reddening treatment and error propagation before the donor masses and the period rejection are taken as established.

Referee Report

3 major / 4 minor

Summary. This manuscript reports a multi-wavelength study of eight ULXs in the spiral galaxy NGC 1559, combining HST/WFC3 optical photometry, JWST/NIRCam infrared imaging, and archival Chandra and Swift/XRT X-ray data. Using GAIA-based astrometric corrections, the authors identify unique optical counterparts for X-1, X-14, X-18, and X-24, and NIR counterparts for X-14 and X-24. They construct optical SEDs (finding power-law shapes for X-1 and X-18, a 7000 K blackbody for X-24, and no acceptable fit for X-14), build CMDs to derive donor masses and ages for X-14 and X-24, and analyze the long-term X-ray variability and time-averaged spectrum of X-1. The central claims are that the optical emission from X-14 and X-24 originates from their donor stars, while for X-1 and X-18 it is dominated by the accretion disk, and that the 7500 s periodicity of X-24 reported by Ma et al. (2023) is not an orbital period because the donor is a massive supergiant in an HMXB. The paper also reports order-of-magnitude X-ray variability in X-1 and a tentative 130.5 d period.

Significance. If the conclusions hold, the paper provides a valuable set of donor-star characterizations for ULXs in an external galaxy, exploiting JWST's spatial resolution to recover NIR counterparts that are often blended in ground-based data. The explicit astrometric calibration using GAIA reference sources, the false-positive rate calculation for counterpart identification, the multi-epoch variability monitoring, and the detailed X-ray timing/spectral analysis of X-1 are concrete strengths that increase confidence in the observational results. The interpretation that the 7500 s period of X-24 is not an orbital period is an interesting and testable claim that would, if correct, have implications for the nature of that source and for ULX donor population studies. The paper is within the scope of the journal and addresses a topic of active interest.

major comments (3)

[Section 4.4, Fig. 7] The CMD analysis for X-24 uses a fixed extinction of A_V = 0.04 mag with no quoted uncertainty and no justification, despite the same section describing the source as embedded in a dense, dusty NIRCam region, coincident with an H II region in F657N, and exhibiting an F814W excess attributed to gas/dust or a circumbinary disk. The observed colors (F438W-F555W = 0.63, F555W-F814W = 0.92) are far redder than the unreddened 12 Msun/12 Myr isochrone point. A modest additional reddening, e.g., E(B-V) = 0.3 (A_V ~ 0.9), shifts the dereddened CMD position by a significant fraction of the isochrone spacing, changing the inferred donor mass and age. Since the rejection of the 7500 s orbital period in this section depends directly on the donor being a ~12 Msun supergiant, the extinction treatment is load-bearing. The authors should derive A_V from independent evidence (e.g., the surrounding stellar population or the Balmer decrement) or explicitly propagate a realistic range of A_V through the CMD analysis and demonstrate that the mass/age conclusion, and hence the orbital-period argument, remains robust.
[Section 4.2 and Section 3.4] For X-14, the paper states that a physically meaningful SED model could not be fitted, yet X-14 is one of the two sources for which the central claim asserts donor-dominated optical emission. The classification for X-14 therefore rests entirely on the observed constancy in optical and NIR bands and on the CMD location. This is weaker evidence than an SED shape that is inconsistent with a disk-dominated spectrum. In addition, the CMD for X-14 uses the same fixed A_V = 0.04, so the reported age (7 Myr) and mass (18 Msun) carry the same extinction uncertainty as X-24. The paper should either obtain a usable SED fit (e.g., with a reddened stellar atmosphere model) or explicitly phrase the X-14 conclusion as preliminary and dependent on the assumed zero/negligible extinction.
[Section 4.4, Roche-lobe argument] The quantitative argument against the 7500 s orbital period is presented in a way that appears logically inverted. With a donor mass of 12 Msun and a 7500 s period, Kepler's third law gives an orbital separation of roughly 2 solar radii (for a total mass near 15 Msun), and the Eggleton formula yields a Roche lobe radius of only about 1 solar radius for a compact-object mass of 3 Msun. A supergiant donor with a radius of tens of solar radii would enormously overflow such a lobe; the correct statement is that the Roche lobe is too small to contain the donor, not that 'the separation remains too small for the donor to fill its Roche lobe.' The argument as written does not support the intended conclusion, and the numerical check should be redone and stated clearly.

minor comments (4)

[Section 3.2] The MIRI pixel scale is quoted as '0.111 pixels/arcsec' in the text; the unit should be arcsec/pixel (0.111 arcsec/pixel would be the standard expression).
[Table 4 and Section 4.1] The unabsorbed X-ray luminosity quoted in the text of Section 4.1 (7.87e39 erg/s) does not match either of the values in Table 4 (8.87 and 7.72, for power-law and diskbb, respectively); this inconsistency should be corrected.
[Fig. 6 caption] The caption states 'SEDs of four counterparts' but only three SEDs are displayed (X-1, X-18, X-24), because no acceptable fit was found for X-14. The caption should be amended to avoid confusion.
[Section 3.4 and Fig. 7] The CMDs are presented for the donor candidates X-14 and X-24, but the surrounding stellar population is not shown or used to independently constrain the reddening or the age. Showing the field-star distribution in the CMD panels would help the reader assess the significance of the isochrone placement relative to the assumed extinction.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the donor/disk classifications and the X-24 period argument follow from external SED and CMD fits, not from quantities defined by the claimed result.

full rationale

This is an observational characterization paper, not a derivation in which a claimed output is built into a defining input. The central classifications (donor-dominated for X-14 and X-24; disk-dominated for X-1 and X-18) are based on measured photometry, SED shapes fitted with power-law or blackbody models, variability behavior, and CMD placement against external PARSEC isochrones. The X-24 donor mass of 12 Msun and age of 12 Myr are inferred from the observed CMD position and adopted isochrones; the subsequent argument against the 7500 s orbital period uses that inferred mass together with the Eggleton Roche-lobe formula, so the period rejection is an application of the inferred donor properties rather than a circular restatement of them. Self-citations to Allak (2022, 2023, 2024) are methodological: they are used for the astrometric error radius, photometry reduction, and false-positive-rate estimation, and those steps are supported in-paper by GAIA reference sources and computed FPR values, so they are not load-bearing in the sense of importing an unverified uniqueness claim. The fixed AV=0.04 extinction correction flagged in the skeptical reading is a possible robustness limitation, but it is not circular: the extinction value is an independent assumption, and no equation in the paper defines the donor mass or the period conclusion in terms of that extinction. No fitted parameter is renamed as a prediction, and no known result is repackaged under new coordinates. The derivation chain is therefore self-contained against external benchmarks, and any concerns about extinction, crowding, or counterpart uniqueness belong to correctness risk rather than circularity.

Assumptions & free parameters 5 free parameters · 6 assumptions · 0 invented entities

The central claims rest on the adopted distance and extinction, solar-metallicity isochrones, and a simple SED model dictionary. These are standard tools in this field but are not independently verified for NGC 1559. No new physical entities are introduced.

free parameters (5)

Extinction correction A_V = 0.04 mag
Applied to the PARSEC isochrones in the CMDs (Figure 7). The value is asserted without derivation and shifts donor age and mass estimates if it is wrong.
X-24 optical SED blackbody temperature = 7000 K
Fitted to HST optical photometry and used to argue that the optical emission is donor-dominated.
X-24 NIR SED blackbody temperature = 300 K
Fitted to JWST F277W, F300M, and F335M photometry and used to argue that the NIR emission is gas and dust, not the donor star.
X-1 optical SED power-law index = -0.66 +/- 0.11
Fitted to the HST optical SED and used to argue that the accretion disk dominates the optical emission.
X-18 optical SED power-law index = -2.1 +/- 0.13
Fitted to the HST optical SED and used to argue that the accretion disk dominates the optical emission.

assumptions (6)

domain assumption All eight targets are ULXs with L_X above 10^39 erg/s as defined by Ma et al. (2023)
Section 2.1 adopts the Ma et al. source list and luminosity criterion. If some of these sources are background AGN or transient contaminants, the donor associations would fail.
domain assumption Adopted distance of 12.6 Mpc (Tully-Fisher) is correct, despite the NED range of 9 to 23 Mpc
Section 2.1 and Section 3.4 use 12.6 Mpc for luminosities, absolute magnitudes, and the distance modulus of 30.5 used in the CMDs. A different distance shifts donor masses, ages, and the HMXB classification.
domain assumption PARSEC solar-metallicity isochrones (Z = 0.02) are appropriate for the donor candidates
Section 3.4 uses these isochrones to derive ages and masses from the CMDs. Different metallicities or age spreads would change the estimates.
domain assumption Blackbody and power-law SED models can separate donor-star emission from accretion-disk emission
Section 3.4 fits a blackbody for X-24 and power laws for X-1 and X-18. The interpretation assumes these are the relevant emission laws and ignores alternatives such as irradiated disk models.
domain assumption The astrometric error radius of 0.38 arcsec at 90 percent confidence, taken from Allak (2022), applies to this dataset
Section 3.2 derives this radius using only one matched Chandra source plus GAIA references. If the error radius is underestimated, the unique counterparts may include chance alignments.
standard math The Eggleton (1983) Roche-lobe formula and the CMD-derived donor mass can rule out the 7500 s orbital period
Section 4.4 uses the Eggleton equation to argue that a 12 solar mass donor cannot fit in a 7500 s orbit. This is standard physics, but it depends on the CMD-derived donor mass being correct.

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Cite this review

Pith. "Pith review of Unveiling the nature of donor stars of ULXs in NGC 1559." pith.science (2026). https://pith.science/paper/FQ4C6NCI

@misc{pith2026250109974,
  author       = {Pith},
  title        = {Pith review of: Unveiling the nature of donor stars of ULXs in NGC 1559},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/FQ4C6NCI}},
  note         = {Machine review of arXiv:2501.09974}
}

read the original abstract

X-ray data provide insights into accretion processes and the compact objects of ultraluminous X-ray sources (ULXs), while optical and infrared (IR) observations help identify the donor star and surrounding environment. Together, these approaches shed light on the origins of the high X-ray luminosities observed in ULXs. This study examines the optical and infrared properties of eight ULXs in NGC 1559 using archival data from the Hubble Space Telescope (HST) and James Webb Space Telescope (JWST). To constrain the nature of the donor stars of the ULXs, photometric results were obtained from the temporal, spectral energy distributions (SEDs), and color-magnitude diagrams (CMDs). Furthermore, the long-term and spectral characteristics of only a ULX X-1 were investigated. ULX counterparts were determined from astrometric calculations. The long-term light curves and SEDs were constructed to interpret the origin of the optical and IR emissions. The masses and ages of donor star candidates were determined using CMDs. To constrain the mechanism of X-ray emission, the time-averaged spectrum and long-term light curve of the X-1 were obtained. Unique optical and IR counterparts for ULXs X-14 and X-24 were determined, while only optical counterparts were detected for X-1 and X-18. Our findings indicate that the optical emission originates from the donor stars of X-14 and X-24, whereas for X-1 and X-18, it is dominated by the accretion disk. In addition, the X-1 exhibits long-term X-ray variability spanning orders of magnitude.

Figures

Figures reproduced from arXiv: 2501.09974 by the authors.

**Figure 1.** A false-color Chandra image of the galaxy NGC 1559. The energy ranges used for the color image are highlighted. The image has been smoothed using a 5 arcsecond Gaussian, and the ULXs are indicated with white circles. HST NGC 1559 JWST (1" = 61 pc) 0.5 arcmin N E X5 X24 X17 X18 X14 X6 X3 X1 [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗

**Figure 2.** The RGB (red:green:blue) images of NGC 1559 from HST (left) and JWST (right). The filters used for the HST RGB image are F814W, F555W, and F438W, and for JWST, they are F355M, F300W, and F275W, respectively. The ULXs are marked with red circles on both images. Note that in the JWST image, the X-1 source is outside the area. 2.2. X-ray and multi-wavelength observations of NGC 1559 NGC 1559 was observed by Swift/XRT a… view at source ↗

**Figure 3.** [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗

Figures from the paper (9 more)

**Figure 4.** Figure 4: The HST F555W image of corrected X-ray positions (solid red circles) and also Chandra source detection error ellipses (dashed blue ellipses) of the ULXs. Since X-1 is not observed in HST F555W filter, its position is shown on the F438W image. The numbers shown for the …

**Figure 5.** Figure 5: 3.4. SEDs and CMDs SEDs of the optical counterparts were constructed to obtain the spectral characteristics of the counterparts using flux derived from values given in [PITH_FULL_IMAGE:figures/full_fig_p005_5.png]

**Figure 5.** Figure 5: Long-term light curves for X-14 (above) and X-24 (below) using 13 observations taken with the HST F160W IR images. The dashed red lines represent the average magnitudes derived from these observations. variability, following the procedures outlined in Allak (2022), a t…

**Figure 7.** Figure 7: HST/WFC3 color-magnitude diagrams for counterparts of X-14 (filled star) and X-24 (filled triangle). The isochrones were corrected for an extinction of AV = 0.04 mag. 0.5’ N E X-6 X-3 X-17 X-5 X-24 X-18 X-14 X-1 [PITH_FULL_IMAGE:figures/full_fig_p007_7.png]

**Figure 8.** Figure 8: The stack Swift/XRT image of the galaxy NGC 1559. The image is smoothed with a 2 arcsec Gaussian. The ULXs are indicated with red circles on the image. 4. Results and discussion Following astrometric corrections, ULX counterparts of the eight previously identified ULXs…

**Figure 9.** Figure 9: Panel (a): Long-term Swift/XRT light curve of the ULX X-1. The epoch-1 (panel b), epoch-2, and epoch-3 (panel c) observations are indicated by red, blue, and black-filled circles, respectively. Panel (d): Lomb-Scargle periodograms of the X-1. The periodic signal that p…

**Figure 11.** Figure 11: Upper two panels show X-ray soft (0.3-1 keV) and hard (1- 4 keV) count rates vs time diagrams, and the lower panel presents the hardness-intensity diagram of the ULX X-1. The optical SED of X-24 is modeled as a blackbody with a temperature of 7000 K, suggesting that t…

**Figure 12.** Figure 12: JWST/NIRCam image of the region around ULX X-24, in the broad-band filters F335M (red), F300M (green), and F277W (blue). The position of X-4 is also shown on the HST UVIS F657N image in the upper right corner. White and green bars indicate the position of the donor st…

**Figure 13.** Figure 13: The NIR SED of the donor star candidate for X-24. The dashed red line indicates a blackbody temperature of 300 K. The errors, taken as a systematic 0.05 mag, match the symbol size. the surroundings, which may obscure the optical emission and result in a lower observed…

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