Can accreting isolated neutron stars be detected?

Anton Biryukov; Marina Afonina; Sergei Popov

arxiv: 2512.10666 · v2 · pith:VQTN2MSRnew · submitted 2025-12-11 · 🌌 astro-ph.HE

Can accreting isolated neutron stars be detected?

Marina Afonina , Anton Biryukov , Sergei Popov This is my paper

Pith reviewed 2026-05-21 17:45 UTC · model grok-4.3

classification 🌌 astro-ph.HE

keywords isolated neutron starsaccretionpropeller stagepopulation synthesiseROSITAinterstellar mediumGaia binaries

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The pith

Isolated neutron stars may accrete from the interstellar medium in numbers detectable by eROSITA if the propeller stage lasts long enough.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper runs population synthesis for isolated neutron stars across the Milky Way's lifetime with updated interstellar medium and magneto-rotational models. It identifies the spin-down rate during the propeller stage as the main uncertainty that controls whether these stars can transition to accreting gas from space. When that stage is sufficiently long and accretion efficiency is high, the model predicts up to a few thousand such sources in eROSITA observations. Different choices for the spin-down rate or lower efficiency can cut the number sharply. The authors note that Gaia data on neutron stars in wide low-mass binaries could help pin down the uncertain parameters.

Core claim

With more detailed models of the interstellar medium and the magneto-rotational evolution of neutron stars, the spin-down rate at the propeller stage remains the dominant uncertain factor. If the propeller stage duration allows neutron stars to begin accreting matter from the interstellar medium and if the efficiency of accretion is high, then the number of accreting isolated neutron stars in eROSITA data can reach a few thousand. Uncertainties in spin-down at the propeller stage and in the accretion process can drastically decrease this number.

What carries the argument

The spin-down rate at the propeller stage, treated as the primary free parameter that sets whether a neutron star can begin accreting from the interstellar medium.

If this is right

Under favorable propeller-stage conditions and high accretion efficiency, thousands of accreting isolated neutron stars become visible to eROSITA.
Variations in the propeller spin-down rate or accretion efficiency can reduce the predicted number to much smaller values.
Gaia observations of neutron stars in wide low-mass binaries offer a route to constrain the propeller-stage spin-down and accretion efficiency.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

A confirmed large population would imply that isolated neutron stars can overcome magnetic barriers to accrete at observable rates.
Non-detection would tighten limits on how quickly neutron stars spin down while in the propeller regime.
The same modeling choices could affect predictions for the contribution of isolated accretors to the galactic X-ray background.

Load-bearing premise

The spin-down rate during the propeller stage controls whether accretion from the interstellar medium can start.

What would settle it

A count of isolated accreting neutron star candidates in eROSITA X-ray data that is either near a few thousand or far below that range would test whether the propeller-stage spin-down rate permits significant accretion.

Figures

Figures reproduced from arXiv: 2512.10666 by Anton Biryukov, Marina Afonina, Sergei Popov.

**Figure 1.** Figure 1: Evolutionary stages – ejector (E), propeller (P), accretor (A), and georotator (G) – and transition conditions between them. For direct transitions (E-P, [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗

**Figure 2.** Figure 2: Duration of the propeller stage over the characteristic velocity [PITH_FULL_IMAGE:figures/full_fig_p009_2.png] view at source ↗

**Figure 3.** Figure 3: Corner diagram illustrating the fraction of time an NS with specific initial values of magnetic field, kick velocity, and spin period spends at the accretor [PITH_FULL_IMAGE:figures/full_fig_p011_3.png] view at source ↗

**Figure 4.** Figure 4: Extended corner diagram of the accretion time [PITH_FULL_IMAGE:figures/full_fig_p012_4.png] view at source ↗

**Figure 5.** Figure 5: The number of the accreting neutron stars [PITH_FULL_IMAGE:figures/full_fig_p014_5.png] view at source ↗

read the original abstract

We perform population synthesis modeling of isolated neutron stars in the Milky Way over its lifetime. Compared with previous studies, we use more detailed models of the interstellar medium and the magneto-rotational evolution of neutron stars. We demonstrate that presently, the spin-down rate at the propeller stage is the main uncertain factor that influences the number of accreting isolated neutron stars. If the propeller stage duration allows neutron stars to begin accreting matter from the interstellar medium and if the efficiency of accretion is high, then the number of accreting isolated neutron stars in eROSITA data can reach ~a few thousand. Still, uncertainties in spin-down at the propeller stage and in the accretion process can drastically decrease this number. We suggest that future observations of neutron stars in wide low-mass binaries recently discovered by Gaia can clarify these issues.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Desk Editor's Note private letter to a colleague

This updates population synthesis for accreting isolated NS with better ISM and magneto-rotational details, but the eROSITA count still swings from near zero to a few thousand based on an uncalibrated propeller-stage spin-down rate.

read the letter

Hi colleague, the main thing to know is that this paper runs an updated population synthesis for isolated neutron stars that could accrete from the interstellar medium. They bring in more detailed ISM structure and magneto-rotational evolution than some prior studies, and they show that if the propeller stage lasts long enough for accretion to start and if efficiency is high, eROSITA could see up to a few thousand such objects. Change the assumptions and the number drops sharply. They flag the propeller-stage spin-down rate as the dominant uncertainty right away and tie the outcomes to explicit choices on duration and efficiency. The suggestion to use Gaia wide low-mass binaries for future calibration is a sensible pointer. What they do well is keep the modeling transparent and build directly on earlier work without overclaiming. The soft spot is that the propeller spin-down rate is not derived from the detailed evolution equations used elsewhere in the model and is not constrained by new data or first-principles calculation here. It functions as a tunable input, so the headline range is essentially set by how that parameter is chosen rather than emerging as a robust output. The abstract also skips quantitative sensitivity tests and direct comparison to existing observational upper limits. This paper is for specialists in high-energy astrophysics who track neutron-star demographics or plan eROSITA and similar surveys. A reader in that niche can use the updated modeling to map parameter space, but the dependence on one free parameter limits how much weight the numbers carry. The work shows clear thinking and honest engagement with the literature. I would bring it to a reading group if the group has people working on compact-object populations. It deserves peer review because the updates are real, the limitations are stated plainly, and referees could help tighten the uncertainty or suggest better constraints.

Referee Report

2 major / 2 minor

Summary. The manuscript performs population synthesis modeling of isolated neutron stars in the Milky Way over its lifetime, incorporating more detailed models of the interstellar medium and magneto-rotational evolution than previous studies. It identifies the spin-down rate at the propeller stage as the dominant uncertainty controlling whether neutron stars reach the accretor regime from the interstellar medium. Conditional on sufficient propeller-stage duration and high accretion efficiency, the model predicts that the number of accreting isolated neutron stars detectable in eROSITA data could reach a few thousand, though uncertainties in spin-down and accretion can reduce this number substantially. The authors propose that observations of neutron stars in wide low-mass binaries recently discovered by Gaia could help constrain these parameters.

Significance. If the central assumptions hold, this work usefully updates estimates for the population of accreting isolated neutron stars by emphasizing detailed physical modeling and explicitly flagging the propeller-stage spin-down rate as the key uncertainty. The suggestion to use Gaia-discovered binaries for future calibration is a constructive step toward falsifiable constraints. The detailed ISM and magneto-rotational evolution components represent a strength over prior population synthesis efforts. However, the headline prediction remains conditional on tunable inputs, which limits its immediate significance for observational planning until quantitative bounds are provided.

major comments (2)

[Abstract and § on propeller stage] Abstract and propeller-stage modeling: The central claim that the number of accreting isolated neutron stars can reach ~a few thousand is load-bearing on the propeller stage allowing accretion to begin, yet the spin-down rate during this stage is explicitly identified as the main uncertain factor and dominant free parameter without derivation from the magneto-rotational evolution equations or new observational calibration within the model; this reduces the predicted count to a tunable input whose range can drive the result from near zero to thousands.
[Results and discussion sections] Results and discussion: No quantitative sensitivity analysis, error bars, or direct comparison to existing observational upper limits on isolated accreting neutron stars is presented for the eROSITA prediction, which is required to assess whether the 'few thousand' figure is robust given the acknowledged uncertainties in spin-down rate and accretion efficiency.

minor comments (2)

[Abstract] The abstract would benefit from briefly stating the specific improvements in the ISM and magneto-rotational models relative to prior work to clarify the advances.
[Methods] Notation for accretion efficiency and propeller-stage parameters could be defined more explicitly when first introduced to aid readability for readers outside the immediate subfield.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive and detailed comments. We address each major point below, clarifying our approach to uncertainties and outlining planned revisions to strengthen the presentation of results.

read point-by-point responses

Referee: [Abstract and § on propeller stage] Abstract and propeller-stage modeling: The central claim that the number of accreting isolated neutron stars can reach ~a few thousand is load-bearing on the propeller stage allowing accretion to begin, yet the spin-down rate during this stage is explicitly identified as the main uncertain factor and dominant free parameter without derivation from the magneto-rotational evolution equations or new observational calibration within the model; this reduces the predicted count to a tunable input whose range can drive the result from near zero to thousands.

Authors: We agree that the propeller-stage spin-down rate is the dominant uncertainty controlling the transition to accretion, as stated explicitly in the abstract and discussion. Our magneto-rotational evolution modeling covers the earlier evolutionary phases in detail, but the propeller regime involves complex interactions (e.g., magnetic field coupling and angular momentum transfer) that are not fully captured by standard equations and lack direct observational constraints for isolated neutron stars. We therefore treat the spin-down rate as a parameter to map the plausible range of outcomes rather than a derived quantity. The 'few thousand' figure is presented as an upper value under favorable assumptions, with the text already noting that uncertainties can reduce the number substantially. In revision we will add a short paragraph referencing the theoretical limitations on deriving this rate and emphasize that the parameter values are chosen to bracket optimistic versus pessimistic scenarios, not tuned to observations. revision: partial
Referee: [Results and discussion sections] Results and discussion: No quantitative sensitivity analysis, error bars, or direct comparison to existing observational upper limits on isolated accreting neutron stars is presented for the eROSITA prediction, which is required to assess whether the 'few thousand' figure is robust given the acknowledged uncertainties in spin-down rate and accretion efficiency.

Authors: We acknowledge that a more explicit quantitative sensitivity analysis would improve the manuscript. Although the current text discusses the qualitative impact of varying the propeller spin-down rate and accretion efficiency, we will add a new figure and accompanying text in the results section showing the predicted number of detectable sources as a function of these parameters over their plausible ranges. This will include illustrative bounds rather than formal statistical error bars, given the systematic nature of the uncertainties. We will also expand the discussion to compare our predictions against existing upper limits from ROSAT and other X-ray surveys, noting consistency with non-detections when parameters are not at the most optimistic end. revision: yes

Circularity Check

0 steps flagged

No significant circularity; central estimate is conditional on explicitly uncertain parameters without reduction to inputs by construction.

full rationale

The paper conducts population synthesis of isolated neutron stars using detailed ISM and magneto-rotational evolution models. The headline number (~a few thousand in eROSITA) is presented strictly as conditional on propeller-stage duration allowing accretion and high accretion efficiency. The spin-down rate at the propeller stage is identified as the dominant uncertain factor whose plausible values can drive the count from near zero to thousands, but this parameter is not fitted to the target observable and then renamed as a prediction, nor is any central result shown to equal its inputs via the paper's own equations. No self-citation chains, uniqueness theorems, or ansatzes are invoked in a load-bearing manner for the main claim. The derivation remains a self-contained modeling exercise that explores parameter space against external benchmarks such as eROSITA sensitivity, qualifying for a low circularity score.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The central prediction rests on standard assumptions about neutron-star birth rates, magnetic-field decay, and interstellar-medium structure, plus two key modeling choices whose values are not fixed by the paper itself.

free parameters (2)

propeller-stage spin-down rate
Identified in the abstract as the main uncertain factor that controls whether accretion begins; its value is not derived from first principles within the model.
accretion efficiency
Treated as a variable that can be high or low; the headline number of a few thousand requires the high-efficiency case.

axioms (2)

domain assumption Standard Milky Way star-formation history and initial neutron-star mass and velocity distributions
Invoked implicitly as the basis for the population synthesis over the Galaxy lifetime.
domain assumption Magneto-rotational evolution follows previously published prescriptions with updated parameters
The abstract states use of more detailed models but does not re-derive the underlying spin-down equations.

pith-pipeline@v0.9.0 · 5666 in / 1560 out tokens · 43381 ms · 2026-05-21T17:45:43.696258+00:00 · methodology

discussion (0)

Lean theorems connected to this paper

Citations machine-checked in the Pith Canon. Every link opens the source theorem in the public Lean library.

IndisputableMonolith/Cost/FunctionalEquation.lean washburn_uniqueness_aczel unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

We demonstrate that presently, the spin-down rate at the propeller stage is the main uncertain factor that influences the number of accreting isolated neutron stars... uncertainties in spin-down at the propeller stage and in the accretion process can drastically decrease this number.
IndisputableMonolith/Foundation/DimensionForcing.lean reality_from_one_distinction unclear

?

unclear
Relation between the paper passage and the cited Recognition theorem.

We consider several evolutionary models: the models of the spin-down at the propeller stage and magnetic field behavior... two models of the ISM

What do these tags mean?

matches: The paper's claim is directly supported by a theorem in the formal canon.
supports: The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
extends: The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
uses: The paper appears to rely on the theorem as machinery.
contradicts: The paper's claim conflicts with a theorem or certificate in the canon.
unclear: Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.

Reference graph

Works this paper leans on

7 extracted references · 7 canonical work pages · 5 internal anchors

[1]

Davies, R.E., Pringle, J.E., 1981

doi:10.1093/mnras/186.4.779. Davies, R.E., Pringle, J.E., 1981. Spindown of neutron stars in close binary systems - II. MNRAS 196, 209–224. doi:10.1093/mnras/196.2.209. De, K., Daly, F.A., Soria, R., 2024. Infrared spectroscopy of SWIFT J0850.8-4219: identification of the second red su- pergiant X-ray binary in the Milky Way. MNRAS 528, L38– L44. doi:10.1...

work page doi:10.1093/mnras/186.4.779 1981
[2]

The Vertical Structure of Warm Ionised Gas in the Milky Way

The Vertical Structure of Warm Ionised Gas in the Milky Way. PASA 25, 184–200. doi:10.1071/AS08004, arXiv:0808.2550. Gourgouliatos, K.N., Cumming, A., 2014a. Hall At- tractor in Axially Symmetric Magnetic Fields in Neu- tron Star Crusts. Phys. Rev. Lett. 112, 171101. doi:10.1103/PhysRevLett.112.171101,arXiv:1311.7345. Gourgouliatos, K.N., Cumming, A., 201...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1071/as08004 2024
[3]

Modified pulsar current analysis: probing magnetic field evolution

doi:10.1093/mnras/stu1496,arXiv:1407.6269. Igoshev, A.P., Popov, S.B., 2015. Magnetic field decay in nor- mal radio pulsars. Astronomische Nachrichten 336, 831. doi:10.1002/asna.201512232,arXiv:1507.07962. Igoshev, A.P., Popov, S.B., Hollerbach, R., 2021b. Evolu- tion of Neutron Star Magnetic Fields. Universe 7, 351. doi:10.3390/universe7090351,arXiv:2109...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1093/mnras/stu1496 2015
[4]

Astronomy Reports 65, 615–630

Observability of Single Neutron Stars at SRG/eROSITA. Astronomy Reports 65, 615–630. doi:10.1134/S1063772921080060. Khoperskov, S., Gerhard, O., 2022. Chemo-kinematics of the Milky Way spiral arms and bar resonances: Con- nection to ridges and moving groups in the solar vicin- ity. A&A 663, A38. doi:10.1051/0004-6361/202141836, arXiv:2111.15211. Lam, C.Y ...

work page doi:10.1134/s1063772921080060 2022
[5]

N., Hobbs G

The Australia Telescope National Facility Pulsar Catalogue. AJ 129, 1993–2006. doi:10.1086/428488, arXiv:astro-ph/0412641. Manning, R.A., Jeffries, R.D., Willmore, A.P., 1996. Are there any isolated old neutron stars in the ROSAT Wide Field Camera survey? MNRAS 278, 577–585. doi:10.1093/mnras/278.2.577. Marasco, A., Fraternali, F., 2011. Modelling the H i...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1086/428488 1993
[6]

Bondi Accretion and the Problem of the Missing Isolated Neutron Stars

Bondi Accretion and the Problem of the Missing Iso- lated Neutron Stars. ApJ 594, 936–942. doi:10.1086/377091, arXiv:astro-ph/0305421. Pfahl, E., Rappaport, S., Podsiadlowski, P., Spruit, H., 2002. A New Class of High-Mass X-Ray Binaries: Implications for Core Collapse and Neutron Star Recoil. ApJ 574, 364–376. doi:10.1086/340794,arXiv:astro-ph/0109521. P...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1086/377091 2002
[7]

MHD Simulations of Bondi Accretion to a Star in the "Propeller" Regime

doi:10.1086/373990,arXiv:astro-ph/0209548. Ronchi, M., Rea, N., Graber, V ., Hurley-Walker, N., 2022. Long-period Pulsars as Possible Outcomes of Supernova Fallback Accretion. ApJ 934, 184. doi:10.3847/1538- 4357/ac7cec,arXiv:2201.11704. Rozwadowska, K., Vissani, F., Cappellaro, E., 2021. On the rate of core collapse supernovae in the milky way. New A 83,...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1086/373990 2022

[1] [1]

Davies, R.E., Pringle, J.E., 1981

doi:10.1093/mnras/186.4.779. Davies, R.E., Pringle, J.E., 1981. Spindown of neutron stars in close binary systems - II. MNRAS 196, 209–224. doi:10.1093/mnras/196.2.209. De, K., Daly, F.A., Soria, R., 2024. Infrared spectroscopy of SWIFT J0850.8-4219: identification of the second red su- pergiant X-ray binary in the Milky Way. MNRAS 528, L38– L44. doi:10.1...

work page doi:10.1093/mnras/186.4.779 1981

[2] [2]

The Vertical Structure of Warm Ionised Gas in the Milky Way

The Vertical Structure of Warm Ionised Gas in the Milky Way. PASA 25, 184–200. doi:10.1071/AS08004, arXiv:0808.2550. Gourgouliatos, K.N., Cumming, A., 2014a. Hall At- tractor in Axially Symmetric Magnetic Fields in Neu- tron Star Crusts. Phys. Rev. Lett. 112, 171101. doi:10.1103/PhysRevLett.112.171101,arXiv:1311.7345. Gourgouliatos, K.N., Cumming, A., 201...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1071/as08004 2024

[3] [3]

Modified pulsar current analysis: probing magnetic field evolution

doi:10.1093/mnras/stu1496,arXiv:1407.6269. Igoshev, A.P., Popov, S.B., 2015. Magnetic field decay in nor- mal radio pulsars. Astronomische Nachrichten 336, 831. doi:10.1002/asna.201512232,arXiv:1507.07962. Igoshev, A.P., Popov, S.B., Hollerbach, R., 2021b. Evolu- tion of Neutron Star Magnetic Fields. Universe 7, 351. doi:10.3390/universe7090351,arXiv:2109...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1093/mnras/stu1496 2015

[4] [4]

Astronomy Reports 65, 615–630

Observability of Single Neutron Stars at SRG/eROSITA. Astronomy Reports 65, 615–630. doi:10.1134/S1063772921080060. Khoperskov, S., Gerhard, O., 2022. Chemo-kinematics of the Milky Way spiral arms and bar resonances: Con- nection to ridges and moving groups in the solar vicin- ity. A&A 663, A38. doi:10.1051/0004-6361/202141836, arXiv:2111.15211. Lam, C.Y ...

work page doi:10.1134/s1063772921080060 2022

[5] [5]

N., Hobbs G

The Australia Telescope National Facility Pulsar Catalogue. AJ 129, 1993–2006. doi:10.1086/428488, arXiv:astro-ph/0412641. Manning, R.A., Jeffries, R.D., Willmore, A.P., 1996. Are there any isolated old neutron stars in the ROSAT Wide Field Camera survey? MNRAS 278, 577–585. doi:10.1093/mnras/278.2.577. Marasco, A., Fraternali, F., 2011. Modelling the H i...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1086/428488 1993

[6] [6]

Bondi Accretion and the Problem of the Missing Isolated Neutron Stars

Bondi Accretion and the Problem of the Missing Iso- lated Neutron Stars. ApJ 594, 936–942. doi:10.1086/377091, arXiv:astro-ph/0305421. Pfahl, E., Rappaport, S., Podsiadlowski, P., Spruit, H., 2002. A New Class of High-Mass X-Ray Binaries: Implications for Core Collapse and Neutron Star Recoil. ApJ 574, 364–376. doi:10.1086/340794,arXiv:astro-ph/0109521. P...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1086/377091 2002

[7] [7]

MHD Simulations of Bondi Accretion to a Star in the "Propeller" Regime

doi:10.1086/373990,arXiv:astro-ph/0209548. Ronchi, M., Rea, N., Graber, V ., Hurley-Walker, N., 2022. Long-period Pulsars as Possible Outcomes of Supernova Fallback Accretion. ApJ 934, 184. doi:10.3847/1538- 4357/ac7cec,arXiv:2201.11704. Rozwadowska, K., Vissani, F., Cappellaro, E., 2021. On the rate of core collapse supernovae in the milky way. New A 83,...

work page internal anchor Pith review Pith/arXiv arXiv doi:10.1086/373990 2022