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 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 ± 0.03 M _⊙ 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 H _α 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 ^60 Fe 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.

Keywords

• Cataclysmic variable stars
• Neutron stars