IY Lyr: A Thick-Disk first-overtone RR Lyrae Star with a Possible Neutron Star Companion

Abstract: IY Lyr, historically misclassified as an eclipsing binary, is now established as a first-overtone RR Lyrae star (RRc star). Using multi-band photometry (ASAS-SN, ZTF, TESS, and our BVRI data), LAMOST spectroscopy, and Gaia astrometry, we investigate its pulsation, binarity, and Galactic population. From O-C analysis, we detect a long-term period decrease and a light-travel time effect with an orbital period of 3.94 years, eccentricity of 0.46, and a mass function of 0.65 M${\odot}$. The companion is independently confirmed by radial velocity residuals and Gaia proper motions. Combined constraints yield an orbital inclination of 94.8$^{\circ}$ and a companion mass of 1.37 M${\odot}$. Chemical abundances ([Fe/H] $\simeq$ -1.0, [$α$/Fe] $\simeq$ +0.27, Xiang et al. 2019) and dynamics ($L_{\rm z}$ $\simeq$ 1250 kpc km s$^{-1}$, $Z_{\rm max}$ $\simeq$ 1.31 kpc) identify IY Lyr as an old, high-$α$, thick-disk star. The companion mass lies at the peak of the neutron star mass distribution, and the system's age excludes a main-sequence star; we conclude the companion is most likely a typical neutron star, although a massive white dwarf near the Chandrasekhar limit cannot be ruled out. IY Lyr is among the few RRc binaries with a compact companion verified by multiple methods, and it has important implications for thick-disk binary evolution and neutron star formation.