# Galactic Sources Detected in the NuSTAR Serendipitous Survey

## TL;DR

This study analyzes 16 Galactic sources detected by NuSTAR, identifying various types including stars, CVs, and HMXBs, and discusses their implications for understanding the population and distribution of high-mass X-ray binaries in the Galaxy.

## Contribution

The paper provides the first detailed optical and X-ray characterization of a subset of NuSTAR Galactic sources, revealing potential new HMXB candidates and insights into their population density.

## Key findings

- Identification of 8 stars, some with binary companions.
- Discovery of 4 CV or CV candidates, including a strong candidate.
- Detection of 2 possible LMXBs and 2 HMXB candidates, with implications for Galactic HMXB population.

## Abstract

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 deg2 of NuSTAR coverage. Here, we report on an optical and X-ray study of 16 Galactic sources in the catalog. We identify eight of them as stars (but some or all could have binary companions), and use information from Gaia to report distances and X-ray luminosities for three of them. There are four CVs or CV candidates, and we argue that NuSTAR J233426-2343.9 is a relatively strong CV candidate based partly on an X-ray spectrum from XMM-Newton. NuSTAR J092418-3142.2, which is the brightest serendipitous source in the Lansbury catalog, and NuSTAR J073959-3147.8 are LMXB candidates, but it is also possible that these two sources are CVs. One of the sources is a known HMXB, and NuSTAR J105008-5958.8 is a new HMXB candidate, which has strong Balmer emission lines in its optical spectrum and a hard X-ray spectrum. We discuss the implications of finding these HMXBs for the surface density (logN-logS) and luminosity function of Galactic HMXBs. We conclude that, with the large fraction of unclassified sources in the Galactic plane detected by NuSTAR in the 8-24 keV band, there could be a significant population of low luminosity HMXBs.