Searching for Compact Object Candidates from LAMOST Time-Domain Survey of Four K2 Plates

The time-domain (TD) surveys of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) yield high-cadence radial velocities, paving a new avenue to study binary systems including compact objects. In this work, we explore LAMOST TD spectroscopic data of four K2 plates and present a sample of six single-lined spectroscopic binaries that may contain compact objects. We conduct analyses using phase-resolved radial velocity measurements of the visible star, to characterize each source and to infer the properties of invisible companion. By fitting the radial velocity curves for the six targets, we obtain accurate orbital periods, ranging from $\sim$ (0.6-6) days, and radial velocity semi-amplitudes, ranging from $\sim$ (50-130) km s$^{-1}$. We calculate the mass function of the unseen companions to be between 0.08 and 0.17 $M_{\odot}$. Based on the mass function and the estimated stellar parameters of the visible star, we determine the minimum mass of the hidden star. Three targets, J034813, J063350, and J064850, show ellipsoidal variability in the light curves from K2, ZTF, and TESS surveys. Therefore, we can put constraints on the mass of the invisible star using the ellipsoidal variability. We identify no X-ray counterparts for these targets except for J085120, of which the X-ray emission can be ascribed to stellar activity. We note that the nature of these six candidates is worth further characterization utilizing multi-wavelength follow-up observations.