An X-Ray-dim “Isolated” Neutron Star in a Binary?

We report the discovery of a dark companion to 2MASS J15274848+3536572 with an orbital period of 6.14 hr. Combining the radial velocity from LAMOST observations and modeling of the multiband light curve, one obtains a mass function of ≃0.131 M⊙, an inclination of $45\buildrel{\circ}\over{.} {20}_{-0\buildrel{\circ}\over{.} \,20}^{+0\buildrel{\circ}\over{.} \,13}$, and a mass ratio of ${0.631}_{-0.003}^{+0.014}$, which demonstrate the binary nature of the dark companion with a mass of 0.98 ± 0.03M⊙ and a main-sequence K9-M0 star of 0.62 ± 0.01 M⊙. LAMOST optical spectra at a range of orbital phases reveal extra-peaked Hα emission that suggests the presence of an accretion disk. The dark companion does not seem to be a white dwarf because of the lack of any observed dwarf nova outbursts in the long-term data archive, although a magnetic white dwarf cannot be excluded. Alternatively, we propose a scenario wherein the dark companion is a neutron star, but we have not detected radio pulsations or a single pulse from the system with the FAST (Five-hundred-meter Aperture Spherical radio Telescope), which hints at a radio-quiet compact object. If the dark companion is identified as a neutron star, it will be the nearest (∼118 pc) and lightest neutron star. Furthermore, kinematic analysis of the system's orbit in the galaxy may suggest its supernova event is associated with the radionuclide 60Fe signal observed from deep-sea crusts. This radio-quiet and X-ray-dim nearby neutron star may resemble an XDINS (X-ray-dim isolated neutron star) but in a binary.