We propose a method to search for stellar-mass black hole (BH) candidates with giant companions from spectroscopic observations. Based on the stellar spectra of LAMOST Data Release 6, we obtain a sample of seven giants in binaries with large radial velocity variation ∆V R > 80 km s −1 . With the effective temperature, surface gravity, and metallicity provided by LAMOST, and the parallax given by Gaia, we can estimate the mass and radius of the giant, and therefore evaluate the possible mass of the optically invisible star in the binary. We show that the sources in our sample are potential BH candidates, and are worthy of dynamical measurement by further spectroscopic observations. Our method may be particularly valid for the selection of BH candidates in binaries with unknown orbital periods.

Most dynamically confirmed stellar-mass black holes and the candidates were originally selected from X-ray outbursts. In the present work, we search for black hole candidates in the LAMOST survey by using the spectra along with photometry from the ASAS-SN survey, 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 9 binaries, where each source contains a giant star with large radial velocity variation (∆V R 70 km s −1 ) and periods known from light curves. We focus on the 9 sources with long periods (T ph > 5 days) and evaluate the mass M 2 of the optically invisible companion. Since the observed ∆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 ∆V R from LAMOST and T ph from ASAS-SN, particularly by the ongoing LAMOST Medium Resolution Survey.

We present a novel, iterative method using an empirical Bayesian approach for modeling the limb-darkened WASP-121b transit from the TESS light curve. Our method is motivated by the need to improve R p /R * estimates for exoplanet atmosphere modeling and is particularly effective with the limb-darkening (LD) quadratic law requiring no prior central value from stellar atmospheric models. With the nonlinear LD law, the method has all the advantages of not needing atmospheric models but does not converge. The iterative method gives a different R p /R * for WASP-121b at a significance level of 1σ when compared with existing noniterative methods. To assess the origins and implications of this difference, we generate and analyze light curves with known values of the LD coefficients (LDCs). We find that noniterative modeling with LDC priors from stellar atmospheric models results in an inconsistent R p /R * at a 1.5σ level when the known LDC values are the same as those previously found when modeling real data by the iterative method. In contrast, the LDC values from the iterative modeling yield the correct value of R p /R * to within 0.25σ. For more general cases with different known inputs, Monte Carlo simulations show that the iterative method obtains unbiased LDCs and correct R p /R * to within a significance level of 0.3σ. Biased LDC priors can cause biased LDC posteriors and lead to bias in the R p /R * of up to 0.82%, 2.5σ for the quadratic law and 0.32%, 1.0σ for the nonlinear law. Our improvement in R p /R * estimation is important when analyzing exoplanet atmospheres.

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Searching for compact objects (black holes, neutron stars, or white dwarfs) in the Milky Way is essential for understanding the stellar evolution history, the physics of compact objects, and the structure of our Galaxy. Compact objects in binaries with a luminous stellar companion are perfect targets for optical observations. Candidate compact objects can be achieved by monitoring the radial velocities of the companion star. However, most of the spectroscopic telescopes usually obtain stellar spectra at a relatively low efficiency, which makes a sky survey for millions of stars practically impossible. The efficiency of a large-scale spectroscopic survey, the Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST), presents a specific opportunity to search for compact object candidates, i.e., simply from the spectroscopic observations. Late-type K/M stars are the most abundant populations in our Galaxy. Owing to the relatively large Keplerian velocities in the close binaries with a K/M-dwarf companion, a hidden compact object could be discovered and followed-up more easily. In this study, compact object candidates with K/M-dwarf companions are investigated with the LAMOST low-resolution stellar spectra. Based on the LAMOST Data Release 5, we obtained a sample of 56 binaries, each containing a K/M-dwarf with a large radial velocity variation ∆V R > 150 km s −1 . Complemented with the photometric information from the Transiting Exoplanet Survey Satellite, we derived a sample of 35 compact object candidates, among which, the orbital periods of 16 sources were revealed by the light curves. Considering two sources as examples, we confirmed that a compact object existed in the two systems by fitting the radial velocity curve. This study demonstrates the principle and the power of searching for compact objects through LAMOST.