A dynamically discovered and characterized non-accreting neutron star -- M dwarf binary candidate

TL;DR

This paper reports the discovery of a non-accreting neutron star candidate in a binary system with an M dwarf, identified through optical spectroscopy and photometry, with follow-up observations supporting its neutron star nature.

Contribution

The study demonstrates a new method to identify non-accreting neutron stars in binaries using optical surveys combined with follow-up spectroscopy and photometry.

Findings

Companion mass estimated at ~1.24 solar masses.

No radio, X-ray, or gamma-ray emission detected.

Supports the neutron star hypothesis over white dwarf.

Abstract

Optical time-domain surveys can unveil and characterize exciting but less-explored non-accreting and/or non-beaming neutron stars (NS) in binaries. Here we report the discovery of such a NS candidate using the LAMOST spectroscopic survey. The candidate, designated LAMOST J112306.9+400736 (hereafter J1123), is in a single-lined spectroscopic binary containing an optically visible M star. The star's large radial velocity variation and ellipsoidal variations indicate a relatively massive unseen companion. Utilizing follow-up spectroscopy from the Palomar 200-inch telescope and high-precision photometry from TESS, we measure a companion mass of $1.24_{-0.03}^{+0.03}~M_{\odot}$. Main-sequence stars with this mass are ruled out, leaving a NS or a massive white dwarf (WD). Although a massive WD cannot be ruled out, the lack of UV excess radiation from the companion supports the NS hypothesis. Deep radio observations with FAST yielded no detections of either pulsed or persistent emission. J1123 is not detected in numerous X-ray and gamma-ray surveys. These non-detections suggest that the NS candidate is not presently accreting and pulsing. Our work exemplifies the capability of discovering compact objects in non-accreting close binaries by synergizing the optical time-domain spectroscopy and high-cadence photometry.