SZ Lyncis: A Non-Accreting Neutron Star-delta Scuti Binary Candidate Discovered via Dynamics and Asteroseismology

Neutron stars (NSs) are traditionally discovered through radio, X-ray, or gamma-ray observations, but optical time-domain surveys can unveil non-accreting NSs in wide binaries. Here we report a NS candidate in the single-lined binary SZLyncis, identified through a combination of asteroseismology, spectroscopy, pulsation timing, and astrometry. The visible $δ$ Scuti primary has a mass of $M_1 = 1.83_{-0.01}^{+0.06}\mathrm{M_{\odot}}$ from asteroseismic modeling. With the orbital inclination ($i = 38.67 \pm 0.29^\circ$) from the astrometric data of Gaia and Hipparcos, we obtain companion masses of $M_2 = 1.76_{-0.042}^{+0.042}\mathrm{M_{\odot}}$ (radial velocity) and $M_2 = 2.07_{-0.045}^{+0.045}\mathrm{M_{\odot}}$ (timing variations). The companion’s mass exceeds the Chandrasekhar limit and lies in the NS range. Multiple arguments rule out alternatives: the astrometric mass function and the spectral energy distribution, which shows no extra light, together exclude any luminous companion; the mass and lack of Balmer absorption rule out white dwarfs (WDs); the system’s age ($1.25$Gyr) disfavors a double WD; and the mass is too low for a black hole. The wide, low-eccentricity orbit and absence of accretion signatures are consistent with a quiescent NS. SZLyn has the potential to be the first $δ$ Scuti binary with a NS candidate identified through asteroseismology and dynamics, demonstrating the potential of this approach to uncover non-accreting compact objects.

💡 Research Summary

The authors present a comprehensive, multi‑technique investigation of the bright δ Scuti star SZ Lyncis (SZ Lyn, TIC 192939152) that reveals it to be a wide binary hosting a compact, non‑accreting companion in the neutron‑star (NS) mass range. The study combines high‑precision TESS photometry, ground‑based radial‑velocity (RV) monitoring, pulsation timing (light‑travel‑time effect, LTTE), and astrometric constraints from Gaia and Hipparcos to determine the fundamental parameters of both components and to rule out all luminous alternatives.

Asteroseismic modeling of the primary
The TESS light curve (2‑minute cadence) shows six significant pulsation frequencies, including the dominant radial mode (f₁ ≈ 8.296 d⁻¹) and several non‑radial p‑modes. Using a dense grid of MESA evolutionary models and adiabatic frequencies computed with pulse_adipls, the authors match all six observed frequencies simultaneously. The best‑fit model places SZ Lyn A on the post‑main‑sequence, with mass M₁ = 1.83⁺⁰·⁰⁶₋₀·₀₁ M⊙, radius R₁ = 2.899⁺⁰·⁰²⁷₋₀·₀₀₀ R⊙, effective temperature T_eff = 6791⁺⁵¹₋₅₈ K, luminosity L = 16.1 L⊙, and an age of 1.254⁺⁰·⁰⁷₉₋₀·₀₂₄ Gyr. The uncertainties are dramatically reduced compared with previous literature values, providing a solid anchor for dynamical mass estimates.

Radial‑velocity orbit
A low‑resolution LAMOST spectrum yields a systemic velocity γ ≈ 34.18 km s⁻¹, while 21 archival RV points (Bardin & Imbert 1984) span one full orbital cycle. Fitting a Keplerian model gives semi‑amplitude K = 9.51 km s⁻¹, eccentricity e = 0.186 ± 0.009, orbital period P = 1188.5 ± 7.6 d, and time of periastron passage T_p = 2452751.8 ± 19.1 JD. The resulting mass function is f(M₂) = 0.1031 ± 0.0030 M⊙. Incorporating the inclination i = 38.67° ± 0.29° derived from combined Gaia‑Hipparcos astrometry yields a companion mass from the RV solution of M₂(RV) = 1.76 ± 0.04 M⊙.

Pulsation timing (LTTE) analysis
The authors compiled 413 pulsation maxima from TESS and 202 from historic ground‑based observations (AAVSO, WASP, etc.). Converting these times to O–C residuals and fitting the Irwin (1952) LTTE model via MCMC yields a semi‑amplitude A = 0.00651 ± 0.00005 d, LTTE period P′ = 3.2417 ± 0.0008 yr, and projected semi‑major axis a₁ sin i = A c. The corresponding mass function is f(M₂) = 0.1365 ± 0.0031 M⊙, which translates to M₂(LTTE) = 2.07 ± 0.045 M⊙ for the same inclination. The agreement between the RV‑derived and LTTE‑derived masses (within 1σ) strongly supports the robustness of the dynamical inference.

Excluding luminous companions
A broadband spectral‑energy distribution (SED) from UV (GALEX) through optical (Gaia, 2MASS) to mid‑IR (WISE) is perfectly fitted by a single A‑type stellar atmosphere; no excess flux is detected at any wavelength. The authors plot the required flux ratio versus companion mass for a main‑sequence star and find that any luminous secondary would produce a detectable SED signature and would lie below the astrometric mass‑function line, thus being inconsistent with the observed orbit. High‑resolution spectra show no secondary lines moving in antiphase, and the O–C diagram exhibits a single sinusoidal component, further ruling out a luminous binary.

Why a neutron star?
The inferred companion mass (1.8–2.1 M⊙) exceeds the Chandrasekhar limit (≈1.4 M⊙), eliminating a white dwarf. The lack of Balmer‑line broadening or any WD spectral features also disfavors a degenerate star. The mass is well below the typical black‑hole mass distribution (≥5 M⊙), making a black hole unlikely. The system’s age (≈1.25 Gyr) is inconsistent with a double‑white‑dwarf scenario, as both progenitors would have had to evolve faster than the primary. Consequently, a non‑accreting neutron star is the most parsimonious explanation.

System architecture and evolutionary context
The orbit is wide (semi‑major axis a ≈ 3.4 AU) and modestly eccentric (e ≈ 0.19). At periastron the primary’s radius (≈2.9 R⊙) remains far inside its Roche lobe, explaining the absence of any accretion signatures in X‑ray or γ‑ray surveys. The low eccentricity suggests that any natal kick imparted to the neutron star was modest or that tidal circularization has acted over the system’s lifetime. This configuration provides a rare laboratory for studying a neutron star in a relatively quiescent environment, and for testing theories of supernova kicks and binary evolution.

Future prospects
The authors outline several avenues to confirm the NS hypothesis: (1) Gaia Data Release 4 will deliver improved astrometric precision, tightening the inclination and companion mass; (2) deep radio searches with the Five‑Hundred‑meter Aperture Spherical Telescope (FAST) could detect pulsar emission if the neutron star is beamed toward Earth; (3) sensitive X‑ray observations (e.g., with Chandra or XMM‑Newton) could reveal faint thermal emission from the NS surface; (4) continued TESS or PLATO monitoring will refine the LTTE solution and possibly detect additional low‑amplitude modes. A definitive detection would cement SZ Lyn as the first δ Scuti binary with a confirmed neutron‑star companion.

Conclusions
The paper demonstrates that asteroseismology, when combined with traditional dynamical techniques (RV, LTTE, astrometry), can uncover non‑accreting compact objects that are invisible in conventional high‑energy surveys. SZ Lyncis emerges as a compelling candidate for a wide binary consisting of a δ Scuti primary and a neutron‑star secondary with a mass of ≈1.8–2.1 M⊙. This work opens a new pathway for identifying the hidden population of quiescent neutron stars in the Galaxy and highlights the power of time‑domain optical surveys in compact‑object astrophysics.