An X-Ray-dim "Isolated'' Neutron Star in a Binary?

Abstract

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 modelling of the multiband light curve, one obtains a mass function of $\simeq 0.131{\mathrm{M}}_{\odot }$, an inclination of $45.20^{\circ }{}_{-0.20^{\circ }}^{+0.13^{\circ }}$, and a mass ratio of $0.631_{-0.003}^{+0.014}$, which demonstrate the binary nature of the dark companion with mass of $0.98\pm 0.03{\mathrm{M}}_{\odot }$ and a main-sequence K9-M0 star of $0.62\pm 0.01{\mathrm{M}}_{\odot }$. LAMOST optical spectra at a range of orbital phases reveal extra-peaked Halpha 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, a kinematic analysis of the system's orbit in the galaxy may suggest its supernova event is associated with the radionuclide ${}^{60}\mathrm{Fe}$ signal observed from the 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.

17 pages, 9 figures; Accepted for publication in ApJL

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