Black Holes and Neutron Stars in Nearby Galaxies: Insights from NuSTAR

Abstract

Abstract Nearby galaxy surveys have long classified X-ray binaries (XRBs) by the mass category of their donor stars (high-mass and low-mass). The NuSTAR observatory, which provides imaging data at E > 10 keV, has enabled the classification of extragalactic XRBs by their compact object type: neutron star (NS) or black hole (BH). We analyzed NuSTAR/Chandra/XMM-Newton observations from a NuSTAR-selected sample of 12 galaxies within 5 Mpc having stellar masses (M ⋆) 107–11 M ⊙ and star formation rates (SFRs) ≈ 0.01–15 M ⊙ yr−1. We detected 128 NuSTAR sources to a sensitivity of ≈1038 erg s−1. Using NuSTAR color-intensity and color–color diagrams we classified 43 of these sources as candidate NSs and 47 as candidate BHs. We further subdivide BHs by accretion states (soft, intermediate, and hard) and NSs by weak (Z/Atoll) and strong (accreting pulsar) magnetic field. Using eight normal (Milky Way-type) galaxies in the sample, we confirm the relation between the SFR and galaxy X-ray point source luminosity in the 4–25 and 12–25 keV energy bands. We also constrained galaxy X-ray point source luminosity using the relation L X = α M ⋆ + βSFR, finding agreement with previous work. The X-ray luminosity function (XLF) of all sources in the 4–25 and 12–25 keV energy bands matches the α = 1.6 slope for high-mass XRBs. We find that NS XLFs suggest a decline beginning at the Eddington limit for a 1.4 M ⊙ NS, whereas the BH fraction shows an approximate monotonic increase in the 4–25 and 12–25 keV energy bands. We calculate the overall ratio of BH to NS to be ≈1 for 4–25 keV and ≈2 for 12–25 keV.