The Faint, the Poor, and the Steady: studies of low-luminosity, metal-poor, and non-pulsating populations of high-mass X-ray binaries

High-mass X-ray binaries (HMXBs) consist of a black hole or neutron star accreting material from a high-mass stellar companion. Although these systems are very rare, with only about 100 having been discovered in the Milky Way Galaxy, they provide crucial insights into the evolution of high-mass stars and may have played an important role in the early Universe, heating the gas in the intergalactic medium and facilitating its reionization by the ultraviolet light produced by the first stars and galaxies. The advent of gravitational wave astronomy further motivates a more thorough understanding of HMXB populations, since HMXBs are the likely progenitors of many of the double compact binaries whose mergers will be detected by gravitational wave observatories. This dissertation presents three studies of HMXB populations, addressing open questions about the faint end of the HMXB luminosity function, the metallicity dependence of HMXB evolution, and the nature of compact objects in non-pulsating HMXBs.In order to identify low-luminosity HMXBs and study their properties and Galactic number density, we surveyed a square-degree region in the direction of the Norma spiral arm with the Chandra and NuSTAR X-ray telescopes. We discovered three low-luminosity HMXB candidates, all of which have main-sequence Be/B-type counterparts and X-ray luminosities equal to 0.1-1 solar luminosities. The Chandra and NuSTAR surveys of the Norma region also provided the opportunity to study other low-luminosity X-ray populations in the Galaxy. We found that the majority of sources detected at energies above 2 keV are cataclysmic variables (CVs), which likely dominate the hard X-ray component of the Galactic Ridge X-ray Emission. The CV candidates in the Norma region have plasma temperatures of kT = 10-20 keV, whereas the CVs found in similar surveys of the Galactic Center region have temperatures of kT = 20-50 keV. The lower temperatures of Norma CVs may result from a significant number of them being nonmagnetic CVs, polars, or symbiotic binaries whereas the Galactic Center CVs are likely dominated by intermediate polars. The second part of this dissertation discusses the X-ray emission of star-forming galaxies at redshifts between z=1.4 and z=2.6. Simulated models of HMXB populations predict that luminous HMXBs should be more numerous in low-metallicity environments. Studies of nearby galaxies have found an excess of luminous HMXBs in very metal-poor blue compact dwarf galaxies, and it has been suggested that the observed increase of the X-ray luminosity per star formation rate (SFR) of star-forming galaxies at higher redshifts is a result of the metallicity dependence of HMXBs. In order to test this hypothesis, we divided a sample of high-redshift galaxies from the MOSDEF survey into different metallicity bins, and stacked the X-ray data from deep Chandra extragalactic surveys to measure the average X-ray luminosity of the galaxies in each bin. Our preliminary results confirm the increase of the X-ray luminosity per SFR with redshift but do not find a significant correlation between the X-ray luminosity per SFR and the metallicity of galaxies. The third part of this dissertation investigates the nature of the compact object in a non-pulsating HMXB. X-ray pulsations provide strong evidence that an HMXB hosts a neutron star (NS), but the absence of pulsations does not rule out the possibility that an HMXB hosts a NS. Using XMM-Newton and NuSTAR observations of IGR J18214-1318, we study the timing and spectral properties of this supergiant HMXB in detail. Our analysis rules out the presence of pulsations with periods shorter than an hour and detects an exponential cutoff to the power-law spectrum of the source with e-folding energy lower than 25 keV. This low cutoff energy indicates that the compact object in this HMXB is most likely a NS. This study exemplifies the powerful diagnostics provided by the combination of XMM-Newton and NuSTAR observations for the identification of compact objects in HMXBs