Introduction

Since their discovery in 1962 (Giacconi et al. 1962), accreting compact objects in the Galaxy have offered unique insights into the astrophysics of the end stages of stellar evolution and the physics of matter at extreme physical conditions. During the first three decades of exploration, new phenomena were discovered and understood, such as the periodic pulsations in the X-ray lightcurve of spinning neutron stars (Giacconi et al. 1971) and the thermonuclear flashes on neutron-star surfaces that are detected as powerful X-ray bursts (see, e.g., Grindlay et al. 1976; Chapter 3). Moreover, the masses of the compact objects were measured in a number of systems, providing the strongest evidence for the existence of black holes in the Universe (McClintock & Remillard 1986; Chapter 4).

During the past ten years, the launch of X-ray telescopes with unprecedented capabilities, such as RXTE, BeppoSAX, the Chandra X-ray Observatory, and XMM-Newton opened new windows onto the properties of accreting compact objects. Examples include the rapid variability phenomena that occur at the dynamical timescales just outside the neutron-star surfaces and the black-hole horizons (van der Klis et al. 1996; Strohmayer et al. 1996; Chapters 2 and 4) as well as atomic lines that have been red- and blue-shifted by general relativistic effects in the vicinities of compact objects (Cottam et al. 2001; Miller et al. 2002b).