Galactic Sources Detected in the NuSTAR Serendipitous Survey

The Nuclear Spectroscopic Telescope Array (NuSTAR) provides an improvement in sensitivity at energies above 10 keV by two orders of magnitude over non-focusing satellites, making it possible to probe deeper into the Galaxy and Universe. Lansbury and collaborators recently completed a catalog of 497 sources serendipitously detected in the 3-24 keV band using 13 square degrees of NuSTAR coverage. Many of these NuSTAR "serendips" have counterparts at soft X-ray and other wavelengths, and about half of them have been classified, primarily via ground-based optical spectroscopy. While Active Galactic Nuclei (AGN) are, by far, the largest group within the classified sources, Galactic sources have also been identified based on optical spectra showing emission or absorption lines at zero redshift, previous classifications, or other observed features. We have carried out an optical and X-ray study of 16 Galactic serendips that include X-ray binaries, Cataclysmic Variables, and active stars. We focus, in particular, on constraints on the population of High-Mass X-ray Binaries (HMXBs) as their overall numbers and fraction that include black holes vs. neutron stars is relevant to predictions for the types of compact object mergers that we expect to see with gravitational wave detectors. Also, X-rays from HMXBs may be important for heating the early Universe. In addition to the HMXBs, we will report on results of observations of other serendips, including a relatively bright and variable source with unusual properties that may be an ultracompact X-ray binary. Finally, we discuss on-going work to classify more of the serendips in the Galactic plane.