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arxiv: 2606.12732 · v1 · pith:W26VUCM6new · submitted 2026-06-10 · 🌌 astro-ph.SR · astro-ph.HE

X-ray Emission and Stellar Ages of Sun-Like Stars

Breanna A. Binder , Edward W. Schwieterman , Alison Farrish , Sarah Peacock , Margaret C. Turnbull , Stephen R. Kane , Katherine Garcia-Sage This is my paper

Pith reviewed 2026-06-27 07:59 UTC · model grok-4.3

classification 🌌 astro-ph.SR astro-ph.HE
keywords X-ray emissionstellar agesFGK starscoronal temperatureage-activity relationmain-sequence starsXMM-NewtonChandra
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The pith

X-ray observations of 85 sun-like stars show three plasma components in quiescent spectra and confirm soft-band activity decay as t to the -1.5 with more scatter in harder bands beyond 4 Gyr.

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

The paper examines XMM-Newton and Chandra data for 85 nearby main-sequence FGK stars with ages from 0.2 to 12 Gyr. It finds that quiescent X-ray spectra are typically modeled with three thermal plasma components at roughly 0.1, 0.4, and 0.8 keV. Relations are derived linking the emission measure-weighted coronal temperature to X-ray luminosity and surface flux. Inferred soft-band luminosities follow the standard age decay, while harder-band luminosities display greater scatter for stars older than 4 Gyr. Several stars appear as outliers, which the paper links to possible age estimate issues, inclination, or companions.

Core claim

Quiescent spectra are typically described by three characteristic plasma components (kT≈0.1, 0.4, 0.8 keV); relations are derived between emission measure-weighted coronal temperature and both L_X and F_X, and inferred ROSAT-band L_X broadly follows the canonical t^{-1.5} decay while the harder band exhibits increased scatter at >4 Gyr.

What carries the argument

Three-component thermal plasma spectral model for quiescent emission, with emission measure-weighted coronal temperature serving as the link to luminosity, flux, and age relations.

If this is right

  • Temperature-informed count-rate conversions become available for faint sources.
  • Bandpass conversions between ROSAT and XMM-Newton depend on the coronal temperature.
  • Several stars show excess activity that may indicate age errors, inclination effects, or unresolved companions.
  • Some outlier stars are potential direct imaging targets whose activity could influence planetary atmospheric evolution.

Where Pith is reading between the lines

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

  • Better age determination techniques could reduce observed scatter in the harder X-ray band for older stars.
  • The temperature relations may improve X-ray property estimates for stars lacking detailed spectra.
  • Outlier identification highlights the need to account for multiplicity and viewing angle when using activity as an age proxy.

Load-bearing premise

The age estimates for the 85 stars are accurate and unbiased enough to support the reported age-activity relations and outlier identifications.

What would settle it

Independent high-precision age measurements for the outlier stars that place them at ages consistent with their observed activity levels would undermine the attribution of excess activity to age errors.

Figures

Figures reproduced from arXiv: 2606.12732 by Alison Farrish, Breanna A. Binder, Edward W. Schwieterman, Katherine Garcia-Sage, Margaret C. Turnbull, Sarah Peacock, Stephen R. Kane.

Figure 1
Figure 1. Figure 1: Comparison of stellar ages derived from different methods for stars with more than one age estimate. Composite ages (τcomp) are taken from Mamajek & Hillenbrand (2008), SPOCS stars with stellar parameters (including ages) derived from YREC are taken from Takeda et al. (2007, τYREC), Ca II H+K age estimates are taken from Lorenzo-Oliveira et al. (2016, 2018), and age estimates from Hα line fluxes are from S… view at source ↗
Figure 3
Figure 3. Figure 3: Distribution of ages for stars in our sample. The F star distribution is shown in cyan, the G star distribution is in yellow, and the K star distribution is in red-orange. constrain Z. For some spectra, allowing Z to vary produced poorly constrained or unphysical values, while the best-fit temperatures and normalizations remained insensitive to the assumed abundance. We therefore assumed Z = 1 as the de￾fa… view at source ↗
Figure 2
Figure 2. Figure 2: Our adopted age compared to the those derived from the four methods described in the text. The red line shows a one-to-one correspondence [PITH_FULL_IMAGE:figures/full_fig_p007_2.png] view at source ↗
Figure 4
Figure 4. Figure 4: Distribution of best-fit APEC temperature com￾ponents from the quiescent spectral fits from [PITH_FULL_IMAGE:figures/full_fig_p009_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: The flux fraction originating from the coolest (≤2.7 MK) thermal plasma component (top) and the hottest (≥7.0 MK) thermal plasma component (bottom) as a func￾tion of X-ray surface flux. Stars are color coded as in Fig￾ure 3. the leverage of the hottest component relative to a linear￾temperature weighted mean (Johnstone & G¨udel 2015; John￾stone et al. 2021a) [PITH_FULL_IMAGE:figures/full_fig_p009_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: Left: The distribution of emission measure-weighted coronal temperatures TEM (in MK) for all stars in our sample. Middle: The relationship between TEM and surface 0.3-10 keV flux FX. Right: The relationship between TEM and 0.3-10 keV LX of the star. The black dashed lines in the middle and right panels show the best fit relationships given by Equation 3 (middle panel) and Equation 4 (right panel), discusse… view at source ↗
Figure 7
Figure 7. Figure 7: The 0.1-2.4 keV flux makes up >90% of the full X-ray flux for thermal plasmas below ∼1 keV in tempera￾ture. The 0.3-10 keV band, meanwhile, captures ∼80% of the total flux for plasmas above ∼0.25 keV, and drops pre￾cipitously (to ∼15%) at cooler temperatures. Only above plasma temperatures of ∼1.5 keV does the 0.3-10 keV flux exceed the 0.1-2.4 keV flux. As a result, the 0.1-2.4 keV to 0.3-10 keV flux rati… view at source ↗
Figure 8
Figure 8. Figure 8: The evolution of LX (top panels), FX (middle panels), and FX/v sin i with age (bottom panels). The dashed black line shows the measured LX-age relationship from G¨udel (2004, see Equation 1). Left panels are constructed using the observed 0.3-10 keV XMM-Newton energy band. Right panels show the inferred 0.1-2.4 keV ROSAT energy band (using the black curve from the bottom panel of [PITH_FULL_IMAGE:figures/… view at source ↗
Figure 9
Figure 9. Figure 9: Left: photon arrival time cumulative distribution function of HD 192020 (XMM-Newton observation ID 0721570101, black) compared to a constant count rate (red dashed line). The A 2 statistic and critical values are shown in the upper-left corner. Right: the light-curve data (black circles) compared to a constant median count rate (red). The reduced χ 2 r is shown in the upper-left corner. Gaps in the data at… view at source ↗
read the original abstract

We present an analysis of XMM-Newton and Chandra observations of 85 nearby main-sequence FGK stars with age estimates ranging from 0.2-12 Gyr. We measure quiescent 0.3-10 keV luminosities, variability metrics, and multi-temperature thermal plasma spectral parameters. Quiescent spectra are typically described by three characteristic plasma components ($kT\approx0.1$, 0.4, 0.8 keV); the fraction of flux from $T\ge7$ MK rises with X-ray surface flux, reaching $\sim$50% for $F_X\gtrsim10^6$ erg cm$^{-2}$ s$^{-1}$. We derive relations between emission measure-weighted coronal temperature and both $L_X$ and $F_X$, enabling temperature-informed count-rate conversions for faint sources. We quantify how bandpass conversions (ROSAT 0.1-2.4 keV vs. XMM-Newton 0.3-10 keV) depend on temperature, and show that inferred ROSAT-band $L_X$ broadly follows the canonical $t^{-1.5}$ decay, while the harder band exhibits increased scatter at $>$4 Gyr. Several stars show excess activity suggestive of age errors, inclination effects, or unresolved companions. Some of these "outlier" stars are potential direct imaging targets for the Habitable Worlds Observatory, and detailed characterization of these stars is needed to inform their likely influence on the atmospheric evolution of orbiting planets.

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.

Referee Report

2 major / 2 minor

Summary. The paper analyzes XMM-Newton and Chandra observations of 85 nearby main-sequence FGK stars (ages 0.2-12 Gyr) to measure quiescent 0.3-10 keV luminosities, variability, and multi-temperature plasma spectral parameters. It reports that quiescent spectra are typically fit by three plasma components (kT≈0.1, 0.4, 0.8 keV), derives relations between emission-measure-weighted coronal temperature and both L_X and F_X, quantifies bandpass conversion factors, and finds that inferred ROSAT-band L_X broadly follows the canonical t^{-1.5} decay while the harder band shows increased scatter beyond 4 Gyr. Several outliers with excess activity are attributed to possible age errors, inclination effects, or companions.

Significance. If the supplied age estimates prove reliable and independent of the X-ray quantities, the work supplies updated empirical relations for coronal temperature and activity decay that are directly useful for interpreting faint sources and for modeling exoplanet atmospheric evolution around solar analogs. The three-component spectral characterization and temperature-dependent conversion factors are practical contributions to the field.

major comments (2)
  1. [Sample selection and age estimates] The headline result that ROSAT-band L_X follows the t^{-1.5} decay (and the identification of outliers) rests entirely on plotting the measured luminosities against the supplied ages for the 85 stars. The manuscript gives no information on the provenance of these ages (isochrone fitting, gyrochronology, literature compilation, etc.), their typical uncertainties, or any test for correlation between age indicators and the X-ray activity metrics; the abstract itself invokes age errors to explain outliers without quantification.
  2. [Observations and data analysis] The data-reduction pipeline, background-subtraction choices, definition of quiescent states, and variability metrics are not described in sufficient detail to assess whether post-hoc sample cuts or analysis decisions affect the reported relations or the scatter at >4 Gyr.
minor comments (2)
  1. [Spectral fitting] Clarify the exact energy bands used for the reported L_X and F_X values and how the emission-measure-weighted temperature is computed from the three-component fits.
  2. [Results] The statement that the fraction of flux from T≥7 MK reaches ~50% for F_X ≳10^6 erg cm^{-2} s^{-1} should be accompanied by the number of stars in that flux bin and the associated uncertainty.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the constructive comments, which highlight areas where additional detail will improve the manuscript's clarity and utility. We address each major comment below and commit to revisions that incorporate the requested information.

read point-by-point responses

  1. Referee: [Sample selection and age estimates] The headline result that ROSAT-band L_X follows the t^{-1.5} decay (and the identification of outliers) rests entirely on plotting the measured luminosities against the supplied ages for the 85 stars. The manuscript gives no information on the provenance of these ages (isochrone fitting, gyrochronology, literature compilation, etc.), their typical uncertainties, or any test for correlation between age indicators and the X-ray activity metrics; the abstract itself invokes age errors to explain outliers without quantification.

    Authors: We agree that provenance, uncertainties, and correlation tests for the age estimates are essential for interpreting the t^{-1.5} relation and outliers. The revised manuscript will add a dedicated subsection in the sample description that specifies the sources and methods for the 85 age estimates (primarily compiled literature values based on isochrones and gyrochronology), reports typical uncertainties, and includes explicit checks for any correlation between age indicators and X-ray metrics. We will also quantify the possible contribution of age errors to the outliers. revision: yes

  2. Referee: [Observations and data analysis] The data-reduction pipeline, background-subtraction choices, definition of quiescent states, and variability metrics are not described in sufficient detail to assess whether post-hoc sample cuts or analysis decisions affect the reported relations or the scatter at >4 Gyr.

    Authors: We concur that insufficient methodological detail limits assessment of potential biases. The revised manuscript will substantially expand the 'Observations and Data Analysis' section to provide a complete description of the XMM-Newton and Chandra data-reduction pipeline, background-subtraction procedures, criteria used to define quiescent intervals, and the computation of all variability metrics. Any analysis choices that could influence the reported relations or scatter will be explicitly noted. revision: yes

Circularity Check

0 steps flagged

No significant circularity; observational analysis with external age inputs

full rationale

The paper measures quiescent X-ray luminosities, variability, and multi-temperature plasma parameters directly from XMM-Newton and Chandra observations of 85 FGK stars. It derives empirical relations between emission measure-weighted temperature and L_X/F_X from these measurements, quantifies bandpass conversions, and observes that inferred ROSAT-band L_X broadly follows the referenced canonical t^{-1.5} decay when plotted against the supplied external age estimates (0.2-12 Gyr). No step reduces the reported decay trend, temperature relations, or outlier identifications back to quantities defined by the same fit or self-citation; ages are treated as independent inputs rather than derived quantities, and the canonical decay is invoked only for comparison. This matches the default expectation of a non-circular observational study.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The analysis rests on standard X-ray astronomy assumptions about thermal plasma emission and instrument response; no new entities are postulated. The three-temperature components and age estimates function as fitted or external inputs.

free parameters (2)
  • three characteristic plasma temperatures (0.1, 0.4, 0.8 keV)
    Described as typical values used to fit quiescent spectra; these are chosen to describe the data rather than derived from first principles.
  • age estimates for the 85 stars
    External inputs spanning 0.2-12 Gyr used to bin and interpret the activity trends.
axioms (2)
  • domain assumption X-ray emission from main-sequence FGK stars can be adequately modeled as multi-temperature thermal plasma in collisional ionization equilibrium
    Invoked when fitting the 0.3-10 keV spectra with three components.
  • domain assumption The provided stellar age estimates are reliable enough to trace evolutionary trends
    Required to interpret the t^{-1.5} decay and identify outliers.

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discussion (0)

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