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

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 ⋆ ) 10 7−11 M ⊙ and star formation rates (SFR) ≈ 0.01 − 15 M ⊙ yr −1 . We detect 128 NuSTAR sources to a sensitivity of ≈ 10 38 erg s −1 . Using NuSTAR color-intensity and color-color diagrams we classify 43 of these sources as candidate NS and 47 as candidate BH. We further subdivide BH by accretion states (soft, intermediate, and hard) and NS by weak (Z/Atoll) and strong (accreting pulsar) magnetic field. Using 8 normal (Milky Way-type) galaxies in the sample, we confirm the relation between SFR and galaxy X-ray point source luminosity in the 4 − 25 and 12 − 25 keV energy bands. We also constrain galaxy X-ray point source luminosity using the relation L X = αM ⋆ + βSFR, finding agreement with previous work. The XLF of all sources in the 4 − 25 and 12 − 25 keV energy bands matches with 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.

Neutron Stars and Black Holes in the Small Magellanic Cloud: The SMC NuSTAR Legacy Survey

“…The compmag model is used by Ballhausen et al (2017) to fit the 4-25 keV NuS-TAR spectrum of a low luminosity pulsar observed with NuSTAR, A 0535+26. The model is cited as a more Vulic et al (2018). We note the small error bars on the sources in our sample due to the large number of source counts.…”

“…In crowded regions, emission from point sources can be contaminated by the PSF wings of other nearby sources. To account for this, we fit point source count rates and hardness ratios using simultaneous PSF fitting for an input source catalog, using the method presented in Wik et al (2014) and following the methodology outlined in Vulic et al (2018). The steps of this PSF fitting analysis are described below.…”

“…With the launch of the Nuclear Spectroscopic Telescope Array (NuSTAR) in 2012 (Harrison et al 2013), we are now able to use the 4-25 keV energy range to study extragalactic populations (e.g., Wik et al 2014;Yukita et al 2016;Vulic et al 2018). An entire population of XRBs can be separated into groups according to compact-object type using NuSTAR because of spectral differences in the hard band (E < 10 keV).…”

Identifying new X-ray binary candidates in M31 using random forest classification

“…If time-resolved data are not available, then classification is typically done using a combination of X-ray brightness and X-ray colours or hardness ratio. X-ray colour-colour diagrams or colourcolour-intensity diagrams in the 0.5 − 10 keV bands show that certain types of objects tend to cluster together based on, for example, compact object type and pulsating versus non-pulsating neutron stars (Prestwich et al 2003;Vrtilek & Boroson 2013); as discussed above, adding hard X-ray data can help separate some X-ray source types, for exam-ple black hole versus neutron star binaries (Vulic et al 2018). Sensitivity limits mean that only the brightest sources are detected in hard X-rays, so the utility of hard X-ray data for classifying large samples of extragalactic X-ray sources is limited.…”

“…XRBs detected in nearby galaxies are generally well-described by an absorbed power-law with spectral index Γ ≈ 1.7 (e.g., Colbert et al 2004), although this depends on numerous physical properties that affect their observed spectral states (for a detailed review, see Done et al 2007). XRBs in different spectral states have been successfully identified in nearby galaxies with the inclusion of NuSTAR data; the additional spectral curvature in the hard energy band at 10 < E < 25 keV can serve to identify XRB types and differentiate them from AGN (Wik et al 2014;Yukita et al 2016;Vulic et al 2018;Lazzarini et al 2018). This type of classification is beyond the scope of this paper and is only feasible in the nearest galaxies because of the lower angular resolution of hard X-ray observations.…”