X-ray binaries are excellent laboratories to study the physics of relativistic stars. On the one hand, they provide robust evidence for the existence of black holes (BHs), the pinnacle of extreme gravity. On the other, they allow investigations on neutron stars (NS) the densest form of matter known. In addition, accretion on matter (provided by the companion) onto the compact star triggers some of the most energetic phenomena observed in the Universe, including X-ray outbursts and the ejection of matter and particles at relativistic speeds.Despite the increased knowledge gathered over the last five decades our understanding on the formation and evolution of X-ray binaries is strongly limited by scarce numbers. In this PNAYA project we have assembled a strong group of internationally renowned scientists that seeks to make a large impact in the field by discovering hidden populations of X-ray binaries with accreting NSs and BHs.

Our first goal will be to unveil the missing population of dormant BH Xray binaries in the Milky Way. The current census of 19 BHs (dynamically confirmed) is only the tip of the iceberg of a vast population of hibernating BHs whose properties are argely unexplored. By exploiting new techniques (founded on a fundamental FWHM_alpha-K2 correlation discovered by our group) we expect to double the current sample and lay the grounds for more efficient methods of discovering BHs. These include extending the correlation to outburst epochs (when the accretion disc is ~6 mag brighter), to NIR wavelengths (appropriate for highly extinguished BH candidates in the Galactic Plane) and the use of narrow-band photometry (ideal for blind searches). The expanded census will inform us on the size of the population and the distribution of scale-heights and orbital periods. These are key observables to constrain BH formation through supernova explosions and X-ray binary evolution. In addition, the new census will provide a unique laboratory of Galactic BH properties at solar metallicity, a unique reference to understand the population of extragalactic BHs discovered by recent LIGO-VIRGO observations.

Our second goal will be to hunt for the tantalising, but scarce, population of ultra-compact X-ray binaries (UCXBs). This Xray binary population, comprised by systems with orbital periods shorter than 1 hour, is of particular interest, as they harbour degenerate donor starsthat feed the black-hole/neutrón star with hydrogen poor material. Thus, they are unique laboratories to study accretion processes in hydrogen deficient environments as well as some of the fundamental stages of binary evolution. In addition, since they are composed by two ultra-dense objects in a tight binary orbit, UCXBs play a crucial role in the new era of gravitational wave astronomy, as they are primary targets for gravitational waves studies at low frequencies by the forthcoming LISA mission. THE-HUNT project aims at significantly growing the known population of UCXBs by using novel observational techniques, as well as performing a global characterization of their general properties.