High-mass X-ray binaries in the Milky Way

High-mass X-ray binaries are fundamental in the study of stellar evolution, nucleosynthesis, structure and evolution of galaxies and accretion processes. Hard X-rays observations by INTEGRAL and Swift have broadened significantly our understanding in particular for the super-giant systems in the Milky Way, whose number has increased by almost a factor of three. INTEGRAL played a crucial role in the discovery, study and understanding of heavily obscured systems and of fast X-ray transients. Most super-giant systems can now be classified into three categories: classical/obscured, eccentric and fast transient. The classical systems feature low eccentricity and variability factor of , mostly driven by hydrodynamic phenomena occurring on scales larger than the accretion radius. Among them, systems with short orbital periods and close to Roche-Lobe overflow or with slow winds appear highly obscured. In eccentric systems, the variability amplitude can reach even higher factors because of the contrast of the wind density along the orbit. Four super-giant systems, featuring fast outbursts, very short orbital periods and anomalously low accretion rates, are not yet understood. Simulations of the accretion processes on relatively large scales have progressed and reproduce parts of the observations. The combined effects of wind clumps, magnetic fields, neutron star rotation and eccentricity ought to be included in future modelling work. Observations with INTEGRAL in combination with other observatories were also important for detecting cyclotron resonant scattering features in spectra of X-ray pulsars, probing their variations and the geometry of the accretion column and emission regions. Finally, the unique characteristics of INTEGRAL and its long life time played a fundamental role for building a complete catalogue of HXMBs, to study the different populations of these systems in our Galaxy and to constrain some of the time scales and processes driving their birth and evolution.