Search for Radio Pulsations from Neutron Star Candidates in Detached Binaries

TL;DR

This study conducted sensitive radio observations of 31 neutron star candidates in detached binaries, finding no pulsations, which suggests they may be radio-quiet or not beaming towards Earth, complicating their confirmation.

Contribution

First targeted radio survey of optically identified neutron star candidates in detached binaries, setting stringent flux limits and discussing implications for their nature.

Findings

No radio pulsations detected from any candidate

Flux density upper limits are below typical Galactic pulsars

Non-detections imply possible radio-quietness or unfavorable beaming geometry

Abstract

Recent optical astrometric and spectroscopic surveys have identified numerous neutron star (NS) candidates in non-accreting detached binary systems, but their compact-object nature remains unconfirmed. In this work, we present targeted radio observations of 31 such candidates using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the Robert C. Byrd Green Bank Telescope, and the Shanghai TianMa Radio Telescope. Over a total of 46.65 hours of observing time, we detected neither periodic nor single-pulse radio emissions. These nondetections place stringent upper limits on the flux densities of any potential radio signals, reaching ~4 $\mu$Jy for periodic emission and ~10 mJy for single pulses with FAST. Since our observations are highly sensitive and the flux density upper limits are well below the median fluxes of known Galactic pulsars, this suggests that geometric beaming is the most likely explanation for the non-detections if these objects are indeed pulsars. Alternatively, the NSs may be sufficiently old ($\gtrsim$ 10 Gyr) and have become intrinsically radio-quiet. In this case, our findings highlight the inherent difficulty of confirming NSs in such old detached binary systems through radio pulsation searches.