High-Mass X-ray Binaries in the Milky Way: A closer look with INTEGRAL

TL;DR

This paper reviews the advancements in understanding high-mass X-ray binaries in the Milky Way facilitated by INTEGRAL, highlighting discoveries of obscured systems, transient behaviors, and spectral features, and emphasizing the importance of multi-observatory data and future modeling efforts.

Contribution

It provides a comprehensive overview of INTEGRAL's role in discovering and characterizing diverse high-mass X-ray binary systems in the Milky Way, including obscured, eccentric, and transient types.

Findings

Increased known super-giant systems by nearly threefold.

Identification of heavily obscured and fast transient systems.

Detection of cyclotron resonant scattering features in X-ray pulsars.

Abstract

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 broad- ened significantly our understanding in particular for the super-giant systems in the Milky Way, which 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 in three categories: classical/obscured, eccentric and fast transient. The classical systems feature low eccentricity and variability factor of about 1000, 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 pro- gressed 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 ...