Abstract
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 8 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 3 of them. There are 4 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 low-mass X-ray binary candidates, but it is also possible that these 2 sources are CVs. One of the sources is a known high-mass X-ray binary (HMXB), and NuSTAR J105008–5958.8 is a new HMXB candidate that 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 (log N–log S) 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.
Highlights
The Nuclear Spectroscopic Telescope Array (NuSTAR) serendipitous source survey is a systematic analysis of all NuSTAR observations excluding the core Galactic Survey programs, the Galactic center and the Norma regions (Hong et al 2016; Fornasini et al 2017), and excluding the dedicated Extragalactic Survey programs: COSMOS, ECDFS, EGS, GOODS-N, and UDS (e.g., Civano et al 2015; Mullaney et al 2015)
The source types that INTEGRAL and Swift/BAT have detected in the largest numbers (Bird et al 2016; Voss & Ajello 2010; Krivonos et al 2012) are Active Galactic Nuclei (AGN), Low-Mass X-ray Binaries (LMXBs), High-Mass Xray Binaries (HMXBs), and Cataclysmic Variables (CVs)
CVs, and LMXBs For the 8 stars, the classifications either come from the SIMBAD database or the optical spectra we show in this work
Summary
The NuSTAR serendipitous source survey is a systematic analysis of all NuSTAR observations excluding the core Galactic Survey programs, the Galactic center and the Norma regions (Hong et al 2016; Fornasini et al 2017), and excluding the dedicated Extragalactic Survey programs: COSMOS, ECDFS, EGS, GOODS-N, and UDS (e.g., Civano et al 2015; Mullaney et al 2015). The source types that INTEGRAL and Swift/BAT have detected in the largest numbers (Bird et al 2016; Voss & Ajello 2010; Krivonos et al 2012) are Active Galactic Nuclei (AGN), Low-Mass X-ray Binaries (LMXBs), High-Mass Xray Binaries (HMXBs), and Cataclysmic Variables (CVs).
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