Classifying High Energy X-Ray Sources near the Galactic Plane

Since 2012, the Nuclear Spectroscopic Telescope Array (NuSTAR) has identified many hard X-Ray sources at energies above 10 keV that were previously undetected, thanks to the high sensitivity of the instrument. The NuSTAR Serendipitous Survey has catalogued about 1000 sources that were serendipitously detected by NuSTAR while it was carrying out observations of other targets. The majority of these are likely to be supermassive black holes in distant galaxies, but the nature of the sources close to the Milky Way’s Galactic Plane are less clear. Some of these sources may be associated with stellar remnants in our Galaxy, such as cataclysmic variables or X-ray binaries, while others may be extragalactic AGN. The identification of new Galactic hard X-ray sources can lead to the discovery of new, relatively rare sources and provide insights on stellar evolution. Determining the most likely classification of NuSTAR sources near the Galactic Plane often requires soft X-ray observations with better angular resolution to localize the sources with sufficient precision to identify unique optical/infrared counterparts and to perform broadband X-ray spectral analysis. We have been working on classifying twelve sources from the NuSTAR Serendipitous Survey located within twenty degrees of the Galactic Plane for which soft X-ray observations were obtained through the Chandra Cool Targets program. We have identified the most likely soft X-ray counterparts of these NuSTAR sources and searched optical/infrared catalogs for possible matches. We have also analyzed their broadband X-ray spectra and studied their X-ray variability when sufficient data were available. We present the results of these analyses and the most likely classifications of the twelve NuSTAR sources.