Some binary stars, in the course of its evolution, will find themselves consisting of a compact stellar object (e.g., black hole) and a star transferring mass to the former. The accretion process often results to powerful X-ray emission, sometimes so powerful that it challenges our theoretical models! Such is the case of ultraluminous X-ray sources which are believed to be super-Eddington accreting binaries!

In order to understand their formation and evolution, we performed the largest demographic study of their populations and their host galaxies. 

Figure from Kovlakas et al. 2020, MNRAS, 498, 4790

The star-formation history of a galaxy determines it's general properties. Coupled with binary stellar evolution, it gives rise to populations of accreting X-ray binaries that dominate the X-ray emission of the galaxy (in absence of an active galactic nucleus). Thus, we can constrain binary evolution using X-ray observations of galaxies in the nearby and distant Universe, and studying how X-ray luminosity scales with the star-formation rate, stellar mass and gas-phase metallicity.

Figure from Lehmer et al. 2021, ApJ, 907, 17

To study the scaling of X-ray binary populations with the stellar population parameters of the galaxies, we need galaxy catalogs providing positions, morphological classifications, distances, star-formation rates, stellar masses, metallicities, nuclear activity classifications, etc. Check the HECATE.

Figure from Kovlakas et al. 2021, MNRAS, 494, 1896

Massive stars are the rock stars of stellar populations! They live fast, and die young, and affect their environment through powerful stellar winds, supernova explosions, etc. Interestingly, they are typically found in pairs. When in binaries, they tend to be even more interesting, giving rise to various phenomena important for many areas of research in astronomy: Type Ia supernova, X-ray binaries, compact object mergers, short gamma-ray bursts, etc.

To understand these manifestations of binary evolution, and the effect of their population in their environment, we constructed the state-of-the-art code POSYDON which models binaries in an unprecedented  detail, taking advantage of machine learning methods.

Figure from Fragos et al. 2023, ApJS, 264, 45

The binary population synthesis code POSYDON, which I am a core developer of, is being used in studies of the evolution of isolated binaries to close compact objects that will merge in a Hubble time. These merger emit powerful gravitational waves, detectable by our current observatories.

The contribution of this, isolated binary scenario, to the total population of gravitational-wave sources, the insights we can get from the observations on binary evolution theory, and predictions on the results from future experiments, are few of the goals of our team.

Figure from Bavera et al. 2021, A&A, 647, A153

I am particularly interested in peculiar stars, whether they are alone (e.g., magnetars) or with plus one (gamma-ray binaries). Stellar evolution and population synthesis code can gain a lot of insights from such objects, and I am happy that I am collaborating with multiple researchers in Spain (ICE, UPC, UB, ...) and all over the world.

Figure from Sridhar et al. 2021, ApJ, 917, 13