Searching for Black Hole Candidates by LAMOST and ASAS-SN

Most dynamically confirmed stellar-mass black holes (BHs) and their candidates were originally selected from X-ray outbursts. In the present work, we search for BH candidates in the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey using the spectra along with photometry from the All Sky Automated Survey for SuperNovae (ASAS-SN), where the orbital period of the binary may be revealed by the periodic light curve, such as the ellipsoidal modulation type. Our sample consists of nine binaries, where each source contains a giant star with large radial velocity variation (Delta V-R greater than or similar to 70 km s(-1)) and periods known from light curves. We focus on the nine sources with long periods (T-ph > 5 days) and evaluate the mass M-2 of the optically invisible companion. Since the observed Delta V-R from only a few repeating spectroscopic observations is a lower limit of the real amplitude, the real mass M-2 can be significantly higher than the current evaluation. It is likely an efficient method to place constraints on M-2 by combining Delta V-R from LAMOST and Tph from ASAS-SN, particularly by the ongoing LAMOST Medium Resolution Survey.