1 Introduction

Relativistic binaries containing white dwarfs (WDs), neutron stars (NSs), and black holes (BHs) in compact orbits are over-represented in globular clusters compared with their population in the galactic field. Observations of this population reveal a host of exotic objects such as ultracompact cataclysmic variables, non-flickering X-ray and UV sources, low-mass X-ray binaries (LMXBs), and millisecond pulsars. These objects and their dark counterparts in the population of relativistic binaries are also likely to be observable sources of gravitational radiation for low-frequency gravitational wave detectors such as the planned space-borne interferometer LISA. In the field, a relativistic binary is a product of the interplay between stellar evolution and the gravitational interaction of a tight binary. In globular clusters, the population of tight binaries is also a product of the dynamical evolution of an $N$ -body gravitational system. Thus, relativistic binaries result from a combination of several of the more interesting processes in astrophysics. In keeping with the focus of this review article on relativistic binaries in globular clusters, we shall only touch on the aspects of globular clusters, observations, binary evolution, and $N$ -body dynamics as they relate to populations of this specific class of binaries in globular clusters.

We begin by looking at the physical structure and general history of the galactic globular cluster system that leads to the concentration of evolved stars, stellar remnants, and binary systems in the cores of the clusters. Current observations of globular clusters that have revealed numerous populations of relativistic binaries and their tracers are also presented. We also look at the prospects for future observations in this rapidly changing area. Many of these relativistic binaries are the product of stellar evolution in compact binaries. We will look at how mass transfer from one star in the presence of a nearby companion can dramatically alter the evolution of both stars in the process of binary evolution. The enhanced production of relativistic binaries in globular clusters results from dynamical processes that drive binaries toward tighter orbits and that preferentially exchange more massive and degenerate objects into binary systems. Numerical simulations of globular cluster evolution, which can be used to predict the rate at which relativistic binaries are formed, are discussed. These models are compared with the observable members of the population of relativistic binaries. Finally, we conclude with a brief discussion of the prospects for observing these systems in gravitational radiation.

Readers interested in further studies of the structure and evolution of globular clusters are invited to look at Binney and Tremaine [24 ], Spitzer [219 ], and Volumes I and II of Padmanabhan’s Theoretical Astrophysics [169 , 170 ] for a good introduction to the physical processes involved. Review articles of Meylan and Heggie [156 ] and Meylan [155 ] also provide a comprehensive look at the internal dynamics of globular clusters. Although our focus is solely on the Galactic globular cluster system, the physics of globular cluster systems associated with other galaxies is well covered in the review article by Harris [96 ] as well as his lecture notes from the Saas-Fee course on star clusters [29 ]. Carney has a thorough introduction to evolution of stars in globular clusters [30 ]. An observational perspective on the role of binaries in globular clusters is presented in an excellent review by Bailyn [11 ], while a good introduction to the details of observing binary systems in general can be found in An Introduction to Close Binary Stars [107 ]. Although slightly out of date, the review of binaries in globular clusters by Hut et al. [118 ] is an excellent introduction to the interaction between globular cluster dynamics and binary evolution, as is a short article on globular cluster binaries by McMillan, Pryor, and Phinney [154 ]. Rappaport et al. [196 ] and Rasio et al. [197 ] have written reviews of numerical simulations of binary populations in globular clusters. An excellent introduction to the astrophysics and numerical techniques relevant to globular cluster dynamics can be found in the book by Heggie and Hut [100 ].