Weighing Hidden Companions of Compact Object Candidates via Rotational Broadening

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.

Figures (27)

• Figure 1: Orbital period vs visible star's radius for compact object candidates with $f(M_{1}) > 0.05~M_\odot$. The radii are obtained from Gaia DR3. The dashed line corresponds to $V_{\mathrm{rot}} = 60~\mathrm{km\,s^{-1}}$. The sources above the line are shown as blue points for $\mathrm{SNR} < 30$, red points for $\mathrm{SNR} > 30$, and a red triangle for double-lined spectroscopic binaries. Three identified candidates, J2354 2023SCPMA..6629512Z, J1729 2022ApJ...936...33Z, and J0419 2022ApJ...933..193Z, are highlighted with star markers.
• Figure 2: Spectral fits for J0341 in our sample. The observed LAMOST MRS spectra (blue) are shown with best-fit model spectra (orange) over the wavelength range $4950$–$6530~\text{\AA}$.
• Figure 3: SED fitting result for J0341. Top panel: Colored circles with error bars indicate observed photometric measurements, while the gray curve represents the best-fit model spectrum derived from the SED fitting. Filter labels are placed adjacent to their corresponding data points. Bottom panel: The residuals of the fit, normalized by the photometric uncertainties.
• Figure 4: Posterior distributions of stellar parameters for target J0341 from SED fitting using AstroARIADNE.
• Figure 5: Example of spectral disentangling for the target J0341. Top panel: the original observed spectrum (blue), the reconstructed spectrum from the disentangling procedure (red), and the individual spectra of the primary (green) and secondary (orange) components. Secondary spectrum rescaled by a factor of 2. Stellar template (black dashed line) corresponds to 1.39 $M_\odot$ ($T_{\rm eff}$ = 7017 K, $\log g$ = 4.0, $v \sin i = 80~\mathrm{km\,s^{-1}}$). Bottom panel: the residuals between the reconstructed and observed spectra, showing that the disentangling reproduces the observed spectrum within the noise level.