X-ray binaries are the brightest X-ray sources in the sky. Binary systems are systems where two stars orbit around their common centre of mass. In X-ray binaries, a typically normal star (companion) is rotating around a collapsed star (a neutron star or a black hole). If the stars are close enough, matter falls from the companion onto the compact object in form a of a disk (or wind for high-mass companion stars). The innermost regions of this accretion disk are so hot that it reaches temperatures in excess of a million degrees and emit in X-rays.
One of the principal motivations for studying X-ray binaries is the unique window that accretion onto neutron stars and black holes provides on the physics of strong gravity and dense matter. The radiation from the plasma orbiting neutron stars and black holes carries the signature of these compact objects and the effects on the surrounding space-time. Astronomical techniques allow us to study these signatures and test to the extreme theories that cannot be tested in laboratories on earth, such as ultra dense matter equations of state or Einstein's theory of general relativity.
Close to the accreting compact object, an important fraction of the in-falling matter is ejected in form of collimated, relativistic outflows or jets. Jets are possibly the most spectacular and powerful consequences of accretion onto compact objects; they have been observed in white dwarfs; neutron stars; and black holes of all mass scales, from stellar-mass in X-ray binaries to super-massive black hole in Active Galactic Nuclei; and are thought to be at the origin of the most energetic phenomena in the Universe, the gamma-ray bursts.
Relativistic jets in these systems extract a large, possibly dominant, fraction of the total available accretion energy. They are formed very close to the compact object, deep in the strong-gravity field regions. Studies of plasma dynamics and radiative processes in relativistic jet, and especially the comparison among the different classes of compact objects, give us a unique opportunity to investigate the effects of the depth of the gravitational potential well in the strong-gravity regime; the effects of the stellar surface and of the presence of an event horizon (the existence of which is one of the most important quests of high-energy astrophysics); the effects of the stellar magnetic field and the shock acceleration mechanisms on leptons and nuclei.
The HEAUB is actively involved in addressing the most fundamental quests in relativistic outflows and accretion physics. Our efforts include coordinated multi-wavelength observations with radio/IR/optical/X-ray/gamma-ray observatories, with high spatial, spectral and temporal resolution, and a contemporaneous development of detailed theoretical models.