Binary stellar systems are critical in revealing the nature of compact objects. Black holes especially, reveal themselves only when they are member of an X-ray binary (XRB). XRBs are mass-transferring binary systems composed of a compact object accretor and a donor star. They emit radiation in X- rays, when mass is transferred from the donor star onto the accreting compact object. In the last 15 years, space-based X-ray observatories (e.g. Chandra, NUSTAR, XMM-Newton) have revolutionized our understanding of XRB formation and evolution by allowing us to study XRBs in extragalactic environments. Extragalactic XRBs probe the compact object populations and accretion processes within parent stellar populations that can differ considerably from those represented in the Milky Way (e.g., starbursts and massive ellipticals). For example, due to the relatively young stellar ages and primordial metallicities in the early Universe (z > 3), it is predicted that XRBs were more luminous than today and played a significant role in the heating of the intergalactic medium and perhaps even contributed to a smoother reionization of the Universe.

Observational studies of the statistical properties of XRB population in the local Universe show that their total energy output is dominated by the relatively few, most luminous XRBs, also known as ultraluminous X-ray sources (ULXs). ULXs have been suggested to not only play a crucial role in shaping the thermal evolution of the early universe, but also regulating, through their radiative and mechanical energy output, star-formation in dwarf galaxies and massive stellar clusters. Nevertheless, binary population synthesis modeling has not advanced significantly in the past decade. Current simulation tools incorporate old analytical fits to single-star models , leading to ad-hoc assumptions and hampering physical, self-consistent treatment of binary interaction, especially in the extreme regime of ULXs. As part of the proposed collaborative project, we intent to bring together an international team of world experts in (i) binary evolution and population synthesis studies, (ii) accretion physics, (iii) extragalactic observations of XRB populations, (iv) observations of stellar populations, and (v) X-ray spectral modeling of XRBs.

The proposing team combines all the necessary expertise in order to address the following key points: (1) Compile a comprehensive sample of ULXs in the local Universe for which we will derive the properties (star-formation history and metallicity) of host galaxy’s parent stellar population. (2) Develop population models where the crucial ULX phase will be model through detailed simulations, avoiding the simplifications of traditional modeling tools, and constrain them to the local observed ULX sample. (3) Construct a library of physically self-consistent X-ray spectral models that will be incorporated in the population models in order to accurately predict the spectral energy distributions of ULX populations. (4) Use these synthetic models to estimate the properties of ULXs in the early Universe, constrain the models to current deep X-ray surveys, and make predictions about what future X-ray missions, like Athena, will observe. (5) Reexamine the role of ULXs as a feedback mechanism that can regulate both the thermal evolution of the early Universe and star-formation in the local Universe.