Weighing Hidden Companions of Compact Object Candidates via Rotational Broadening

TL;DR

This paper demonstrates that measuring rotational broadening ($v \, \sin i$) in spectra can effectively estimate the masses of unseen companions in binary systems, aiding the identification of compact objects like neutron stars or white dwarfs.

Contribution

It introduces a systematic method using $v \sin i$ measurements combined with stellar radii to determine companion masses in compact object candidates.

Findings

Five sources have mass ratios > 2/3 with no spectral signatures of companions.

Two targets host companions with masses near the Chandrasekhar limit.

The method can identify potential Type Ia supernova progenitors.

Abstract

The determination of unseen companion masses ($M_1$) is essential for identifying compact objects in binary systems, yet obtaining reliable orbital inclinations remains one of the most difficult challenges. In this study, we focus on ten targets selected from a sample of 89 compact object candidates characterized by large mass functions, rapid rotation, and high-quality Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) spectra. We measure their projected rotational velocities ($v \sin i$) from the LAMOST medium-resolution spectra and, combined with stellar radii, derive orbital inclinations and the corresponding companion masses. Our results show that five sources exhibit mass ratios $M_1 / M_2 > 2/3$, with no detectable spectral signatures of the unseen companions, providing strong evidence for their compact nature. Two particularly notable cases, J0341 and J0359, host companions with inferred masses of $1.39^{+0.09}_{-0.10}$ $M_\odot$ and $1.34^{+0.08}_{-0.09}$ $M_\odot$, respectively. These masses suggest that the invisible objects are either neutron stars or massive white dwarfs with masses close to the Chandrasekhar limit. If they are white dwarfs, these two targets are highly likely to be Type Ia supernova progenitors. This study highlights the potential of $v \sin i$ measurements as a systematic approach to unveiling compact objects in binaries.