X-RayBinaries and Black Hole Candidates: A Review of Optical Properties

Abstract

This chapter summarizes the optical properties of X-Ray Binaries, with special emphasis on the class of Low Mass X-ray Binaries and Soft X-Ray Transients. The latter provide the most compelling evidence for the existence of black holes in the Universe, with nine well-established dynamical studies. We review the techniques employed to extract the component masses and discuss the importance of systematic effects. Despite the growing number of black hole cases, the uncertainties involved are still too large to draw statistical conclusions on the mass distribution of collapsed objects. We also present new observational techniques which may help to improve the mass determinations and set constraints on the theory of Supernovae and black hole formation. 6.1 X-ray Binaries Our Galaxy is populated with about 10 3 powerful X-ray sources with typical lu-minosities L X 10 36 − 10 39 erg s −1. This energy output is most easily explained invoking a model of mass-exchanging binaries with a compact star (i.e. neutron star or black hole) [164],[152]. This scenario is solidly supported by many empirical results obtained since the dawn of X-ray astronomy. Before highlighting the most significant contributions let me summarize the basis of the canonical model. 6.1.1 Introduction X-ray binaries are close binaries consisting of a compact object and an optical (companion) star. The optical star fills its Roche lobe 1 and transfers gas onto the compact object. Material shares the angular momentum of the companion star and this needs to be reduced for accretion to take place. This is done by the action of viscous processes which take place around the compact object, in the so-called accretion disc. The net effect of the accretion disc is both the removal of angular momentum and the extraction of gravitational potential energy (ΔE acc = GM m R with m the accreted mass, M and R the mass and radius of the accretting object) from the accreted gas which is the ultimate source of X-rays. Matter being accreted at a rate ˙ m will power an accretion luminosity L acc = GM ˙ m R (6.1) 1 Roche lobes are the binary equipotential surfaces intersecting in a single point, the so-called inner Lagrangian Point L1.