Investigating X-ray Binaries in the Low-metallicty Galaxy NGC 4631

Environmental conditions during the Epoch of Reionization (EoR) favor the formation of HMXBs due to lower metallicity and high star formation rates (SFR). These effects require a reevaluation of the HMXB luminosity function for these unique conditions. However, analyzing HMXBs from the early Universe is problematic due to extreme red-shifting restricting access to only the harder X-ray bands. A clear understanding of the spectral shape of HMXBs is necessary to reconstruct the lower energy range for these red-shifted sources. Through the study of local low-metallicity high-SFR analog galaxies, we can better understand how HMXB properties and energy distributions change for conditions like those during the EoR. NGC 4631 is one such analog galaxy, with a metallicity of Z/Z_Solar ∼ 0.2, and features high SFR (~3 solar masses per year). NGC 4631 is also fairly luminous, causing it to deviate from the mass-metallicity relation. These conditions make it an excellent analog galaxy. By combining temporally overlapping XMM-Newton (30 ks, 0.3 - 7.0 keV) and NuSTAR (280 ks, 3.0 - 25.0 keV) observations of NGC 4631 we can study HMXB spectra over a wide X-ray energy range to better calibrate the HMXB luminosity function, as well as study the properties and types of three potential HMXBs detected in NGC 4631.