X-ray Transients and Transits Hidden in the Chandra Source Catalog

The Chandra Source Catalog version 2.0 (CSC 2.0) provides spectroscopic, photometric, and variability information for hundreds of thousands of X-ray sources across the sky, including numerous serendipitously observed sources. A fraction of these sources show anomalous variability patterns in their short-term light curves, including fast transient and transiting phenomena, that can be associated with diverse physical phenomena, such as micro-lensing events, stellar flares, exoplanet transits, and explosive events of unknown nature. In this project, we mine CSC 2.0 data in order to identify compelling transient-like and transit-like X-ray light curves, and characterize the most promising sources. We use manual thresholding in the CSC 2.0 properties and a machine-learning anomaly detection algorithm to find these previously unidentified transient and transiting sources. We compile a list of potential candidates and examine some of the most scientifically compelling sources based on their anomalous variability and spectral properties. We construct their X-ray light curves with the Chandra Interactive Analysis of Observations (CIAO) software and further characterize some of the most promising sources by fitting their X-ray spectra with appropriate physical models in order to investigate the potential source of X-ray emission. Among the sources we find, one is likely an X-ray binary in the hard, quiescent state whose stellar companion appears to be eclipsing the X-ray emission from the compact source. This X-ray eclipsing behavior has not been previously reported for this source, but optical eclipses of the binary have been reported. We also report a second source that is likely to be a binary system composed of a dim White Dwarf being accreted onto an even more compact object. We provide the method and the catalog of candidates to the community as a baseline for investigating unidentified transient and transiting X-ray sources.