What is the Nature of High Energy X-ray Sources in our Galaxy?

The Nuclear Spectroscopic Telescope Array (NuSTAR) is the first focusing telescope in the hard X-ray band, covering 3-79 keV. Since its launch in 2012, there has been an ongoing effort to study sources serendipitously detected in the NuSTAR observations, that is, detected sources that are not the target of the observation. These serendipitous sources were presented in a published catalog following 40 months of observations (Lansbury et al. 2017), and 822 new sources were proposed for the updated 80-month NuSTAR Source catalog (NSS80, Klindt et al. submitted). This project is an investigation of the 58 sources listed as having potential (or in some cases definite) Galactic optical counterparts in the NSS80. Nine of these sources were classified with their optical counterparts through the SIMBAD Database: three standard stars, a T Tauri star, a rotating variable, a pulsating variable, a long period variable candidate, a cataclysmic variable binary, and a classical nova system. Gaia counterparts and distances from Bailer-Jones et al. (2021) were used to calculate source luminosities. Two sources were also previously classified (Tomsick et al. 2017): an M-type star and a black hole low mass X-ray binary candidate. We compared the 3-8 keV soft X-ray flux (data provided by the NSS80, specifically from telescopes XMM-Newton, Chandra, and Swift) against the 3-24 keV NuSTAR flux in order to identify particularly bright or potentially variable sources for further investigation. One spectra was produced for NuSTAR J183052+0925.3, the second brightest Galactic source in the catalog. Our analysis suggests that this source is an accreting white dwarf candidate, rather than an X-ray bright star. Overall, we are able to classify 11 of the 58 Galactic sources from the NSS80.