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 SZ~Lyncis, identified through a combination of asteroseismology, spectroscopy, pulsation timing, and astrometry. The visible $\delta$ 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. SZ~Lyn has the potential to be the first $\delta$ Scuti binary with a NS candidate identified through asteroseismology and dynamics, demonstrating the potential of this approach to uncover non-accreting compact objects.