1 Introduction

For the purposes of this review, relativistic binaries are systems containing two stellar mass degenerate or collapsed objects that are in close orbits. In the Galactic field, where these systems evolve in relative isolation, their final properties are set solely by their initial conditions and are the result of mass transfer, common envelope evolution, or other interactions that may interrupt the course of single star evolution due to the presence of a nearby neighbor. When considered as a fraction of the total stellar mass, the number of relativistic binaries in Galactic globular clusters is overrepresented compared to the Galactic field. Thus, the dynamical interactions found in the environment of dense stellar clusters provide additional channels for the formation of these systems. Relativistic binaries reveal themselves observationally as UV or X-ray sources and are potential sources of gravitational radiation.

This review will concentrate on the Galactic globular cluster system for the bulk of the text. We shall touch on extra-galactic globular cluster systems briefly in the sections on observations (Section 3) and gravitational radiation prospects (Section 6).

We begin in Section 2 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 these clusters. Current observations of globular clusters that have revealed numerous populations of relativistic binaries and their tracers are presented in Section 3. We also consider the prospects for future observations in this rapidly changing area. Many relativistic binaries are the product of stellar evolution in close binaries. In Section 4, we will look at how mass transfer between one star and a nearby companion can dramatically alter the evolution of both stars. The enhanced production of relativistic binaries in globular clusters results from dynamical processes that drive binaries toward tighter orbits and that preferentially exchange massive, 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 in Section 5. These models can be compared with the observable members of the population of relativistic binaries in order to try and constrain the entire population. Finally, we conclude with a brief discussion of the prospects for observing these systems in gravitational radiation in Section 6.

Readers interested in further studies of the structure and evolution of globular clusters are invited to look at Binney and Tremaine [57], Spitzer [443], and Volumes I and II of Padmanabhan’s Theoretical Astrophysics [362, 363] for a good introduction to the physical processes involved. Review articles of Meylan and Heggie [326] and Meylan [325] also provide a comprehensive look at the internal dynamics of globular clusters. Although our focus is mainly 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 [192] as well as his lecture notes from the Saas-Fee course on star clusters [65]. Carney has a thorough introduction to evolution of stars in globular clusters [66]. An observational perspective on the role of binaries in globular clusters is presented in an excellent review by Bailyn [25], while a good introduction to the details of observing binary systems in general can be found in An Introduction to Close Binary Stars [215]. Although slightly out of date, the review of binaries in globular clusters by Hut et al. [241] 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 [323]. Rappaport et al. [402] and Rasio et al. [404] 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 [199]. Finally, a shorter and more observationally focused review of compact objects in globular clusters can be found in Maccarone and Knigge [306].