X-ray Luminosity–Age Relation in Stars

• X-ray luminosity/age relationship is a correlation where stellar magnetic activity and rotation drive the X-ray output decay over time.
• Empirical studies using eROSITA, Chandra, and XMM-Newton data reveal a power-law decay with significant scatter influenced by mass, metallicity, and binary evolution.
• Precise age-dating of stars requires multi-epoch observations to account for intrinsic variability and systematic uncertainties in X-ray measurements.

The relationship between X-ray luminosity ($L_X$) and stellar age is a fundamental diagnostic of magnetic activity evolution in stars, populations, and galaxies. The X-ray/age relation encapsulates the effects of stellar rotation, magnetic dynamo efficiency, binary evolution, and high-energy feedback processes, and serves as a clock for age-dating individual stars, stellar populations, and environments hosting exoplanets. Empirical calibration of this relation has advanced with precise ages from asteroseismology and cluster membership, as well as large X-ray surveys (eROSITA, Chandra, XMM-Newton), revealing both broad correlations and substantial astrophysical scatter.

1. Physical Foundations and Formalism

Stellar X-ray emission originates primarily from magnetically heated coronae in single stars and accretion onto compact objects (primarily in binaries) in populations. For single field stars and cluster members, magnetic activity is regulated by rotation and interior structure. Major formalisms include:

• Surface-area normalized relation: log $L_{X,n} = m\,\log \tau + b$ with $L_{X,n} \equiv L_X/(R_*/R_\odot)^2$, age $\tau$ in Gyr, intercept $b$, and slope $m$ ($\equiv \beta$).
• Fractional luminosity: $R_X = L_X / L_{\rm bol}$, with $R_{X,\rm sat}$ indicating a saturated "plateau" (typically $10^{-3.1}$ to $10^{-4.3}$ across spectral types for young rapid rotators (Jackson et al., 2011)).
• Population scaling: $L_X / M_*$ for integrated binaries and field populations, linked via $L_X(t) = L_0 (t/t_0)^{-\beta}$ or broken power-law luminosity functions (XLFs).

In X-ray binary populations, two principal regimes are recognized:

• High-mass X-ray binaries (HMXBs) dominate at $t \lesssim 100$ Myr, tracking recent star formation.
• Low-mass X-ray binaries (LMXBs) emerge at $t \gtrsim 1$ Gyr, with normalization and slope steepening as donor-mass declines (Lehmer et al., 25 Oct 2024, Lehmer et al., 2017, Lehmer et al., 2014).

2. Empirical Results for Single and Main-Sequence Stars

Age and Mass Dependence

Aldarondo Quiñones et al. (Quiñones et al., 10 Nov 2025) provide updated eROSITA-based calibrations for main-sequence F–M stars older than 1 Gyr:

• Best-fit slope: $\beta = -1.37 \pm 0.47$, intercept $b=40.74 \pm 4.22$ (surface-area normalized).
• Intrinsic scatter: $\sigma_j = 0.48 \pm 0.09$ dex (factor 2–3 in $L_X$).
• With mass term: log $L_{X,n} = \beta \log \tau + b + c(M_* - 1 M_⊙)$, $c = -1.06 \pm 0.48$, and $\beta$ steepens to $-2.11 \pm 0.54$.

A prior paper by Booth et al. (Booth et al., 2017) found a steeper decay in old stars ($\beta = -2.80 \pm 0.72$), but eROSITA data indicate that accounting for variability ("jitter") the decay slope matches that of younger stars ($\beta \sim -1.1$ to $-1.4$, Jackson et al. 2012 (Jackson et al., 2011)).

Astrophysical implications:

• X-ray luminosity decays with age, but intrinsic scatter and variability dominate, limiting age precision to ≳30–50%.
• Shallow decay plus scatter implies $L_X$ should not be used in isolation for field star ages beyond 1 Gyr (Quiñones et al., 10 Nov 2025).

Activity "Plateau" and Dynamo Evolution

Young ($<$1 Gyr) and pre-main-sequence (PMS) stars show a saturated X-ray regime with $L_X$ and $L_X/L_{\rm bol}$ nearly constant:

• Saturation values range from $\log R_{X,\rm sat} \approx -3.14$ (late K) to $-4.28$ (early F) (Jackson et al., 2011).
• Saturation timescale $\tau_{\rm sat} \sim 50$–200 Myr, not monotonic with spectral type.
• After saturation, fractional luminosity decays as $R_X(t) = R_{X,\rm sat} (t/τ_{\rm sat})^{-\alpha}$, $\alpha = 1.22 \pm 0.10$.

For PMS stars, X-ray output is constant for $t \lesssim 5$ Myr; decay ($\beta$) steepens with mass and radiative-core development (Getman et al., 2022):

• $\beta \sim 0.5$ for 0.75–1 M$_\odot$; $\sim$1.8 for 1–3.5 M$_\odot$; up to $\sim$3–4 for 3.5–7 M$_\odot$.

3. Population and Binary Scaling Relations

Galaxy and Cluster Populations

Star-forming galaxies and open clusters reveal a pronounced decline in integrated X-ray output per unit mass with stellar population age, traced both by individual source XLFs and by aggregate $L_X/M_*$:

• Empirical frameworks (e.g., (Lehmer et al., 25 Oct 2024)): XLF normalization per mass drops by 2–3 dex from 10 Myr to 10 Gyr; metallicity modulates this decline (slower at low $Z$).
• Quantitatively, (Gilbertson et al., 2021) find $\log (L_X/M_*)$ drops from $\sim$32.4 (young; $<$10 Myr) to $\sim$29 for $>$3 Gyr—a $\sim10^3$ decline.
• M51 spatially-resolved analysis (Lehmer et al., 2017): $L_X/M_*$ declines by $\sim$3 dex over 10 Myr–10 Gyr, and the bright-end XLF slope steepens from $\sim$1.4 (HMXB-like) to 3 (LMXB-dominated).

LMXBs in Early-Type Galaxies

Zhang et al. (Zhang et al., 2012) systematically show $\sim$50% more LMXBs per unit mass in older ($>$6 Gyr) compared to younger galaxies, with a two-parameter scaling: $$f_{\rm XLF}(t, S_N) = (0.044 \pm 0.008)t + (0.049 \pm 0.012)S_N + (0.385 \pm 0.047)$$ where $t$ is age (Gyr) and $S_N$ is globular cluster frequency. The cumulative field data (Lehmer et al., 2014) reveal field LMXB $L_X/L_K$ declines as $\sim t^{-0.9 \pm 0.4}$; excesses in young galaxies are factors $2$–$3$ above old ones, in agreement with population synthesis models (Fragos et al.).

4. Spectral Type, Mass, and Metallicity Dependencies

• Spectral type: Faster $L_X$ decay for higher-mass stars, longer "saturation" for late-Ms (e.g. $L_X$ drops with $\beta \sim 4$ post 2.1 Gyr for M2.5–6.5 vs. $\sim2$ for early Ms (Engle, 2023)).
• Metallicity: Lower $Z$ enhances normalization and delays the decline in $L_X/M_*$ due to reduced stellar wind mass-loss and higher donor mass in binaries (Lehmer et al., 25 Oct 2024, Gilbertson et al., 2021).
• Mass: Weak $3\sigma$ evidence for steeper $L_X$ decay in high-mass stars ($c \sim -1.1$ in Aldarondo Quiñones et al. (Quiñones et al., 10 Nov 2025)).

5. Astrophysical Variability and Systematic Uncertainties

Intrinsic variability ("jitter") in $L_X$ (magnetic cycles, rotational modulation, flares) introduces an irreducible scatter (0.3–0.5 dex) above formal errors (Quiñones et al., 10 Nov 2025):

• This scatter exceeds the age-driven trend in old stars, limiting age inference for individuals to $\gtrsim30$–$50\%$ uncertainty.
• Systematics: Instrument cross-calibration; age estimation errors (asteroseismology, WD companions); unresolved binaries and contaminating flux.
• For population studies: completeness corrections, cosmic variance, and globular cluster LMXB contributions affect normalization.

6. Applications, Limitations, and Future Prospects

• X-ray luminosity is a robust ensemble age diagnostic, notably for population studies (e.g., field galaxies, clusters), but is too variable for precise single-star ages at $>$1 Gyr except when multi-epoch data are available (Quiñones et al., 10 Nov 2025).
• Calibration of exoplanet evaporation histories must include the early saturated phase, which delivers the bulk (≥75%) of irradiation-driven mass loss within the first Gyr (Jackson et al., 2011).
• Multi-dimensional frameworks that incorporate metallicity, mass, and SFH (e.g. (Lehmer et al., 25 Oct 2024)) are now available and required for accurate modeling.
• Larger time-resolved X-ray samples, especially multi-epoch monitoring, are needed to deconvolve variability from intrinsic age trends and tighten precision.
• Combination with complementary indicators (rotation, chromospheric lines, asteroseismology) yields the strongest constraints for stellar dating and environmental effects.

7. Summary Table: Key Empirical Age–$L_X$ Relations

Regime	Best-fit Relation	Slope/Exponent (β)	Typical Scatter (dex)
PMS plateau (0–5 Myr)	$\log L_X\simeq30.3$ (0.75–1 M$_\odot$)	~0	$<$0.3
Early MS (6–625 Myr, G/K/M)	$L_X \propto t^{b}$	G: –0.61; K: –0.82; M: –0.4	0.2–0.4
Field stars ($>$1 Gyr, F–M)	$\log L_{X,n} = \beta\log\tau + b$	$\beta = -1.37 \pm 0.47$	$0.48\pm0.09$
LMXBs in field galaxies	$L_X/L_K \propto t^{-0.9\pm0.4}$	–0.9	$\sim$0.2–0.3
Integrated galaxy $L_X/M_*$	Decline by $\sim$3 dex, 10 Myr–10 Gyr

The X-ray luminosity/age relationship is thus characterized by a plateau at early times, a subsequent power-law decay with an exponent near –1 for field stars and steeper in select regimes, and substantial astrophysical jitter. Population effects, metallicity, mass, and intrinsic variability all modulate this decay and must be accounted for in precise applications. The relation remains a core input for stellar astrophysics, galactic evolution modeling, and exoplanet habitability studies.