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3 Observations

Observational evidence for relativistic binaries in globular clusters has undergone an explosion in recent years, thanks to concentrated pulsar searches, improved X-ray source positions from Chandra, and optical follow-ups with HST and ground-based telescopes. There are challenges to detecting most binaries since they have generally segregated to the cores of the clusters where crowding can be a problem. Nonetheless, numerous observations of both binaries and their tracer populations have been made in several globular clusters.
View Image

Figure 5: CMD of M3 from the Hubble Space Telescope WFPC2. Note the stars above and to the left of the turn-off. These are the blue stragglers. Figure taken from Zhao and Bailyn [242].
One tracer population of the dynamical processes that may lead to the formation of relativistic binaries is the population of blue stragglers. These are stars that appear on the main sequence above and to the left of the turn-off in the CMD of a globular cluster (see Figure 5View Image). These stars are hot and massive enough that they should have already evolved off the main sequence. Consequently, these objects are thought to arise from stellar coalescences either through the gradual merger of the components of binaries or through direct collisions [64Jump To The Next Citation Point178]. Blue stragglers are some of the most visible and populous evidence of the dynamical interactions that can also give rise to relativistic binaries. For a good description of the use of far-ultraviolet surveys in detecting these objects, see Knigge [133]. For somewhat older but still valuable reviews on the implications of blue stragglers on the dynamics of globular clusters, see Hut [117Jump To The Next Citation Point] and Bailyn [11Jump To The Next Citation Point].

The globular cluster population of white dwarfs can be used to determine the ages of globular clusters [162Jump To The Next Citation Point], and so they have been the focus of targeted searches despite the fact that they are arguably the faintest objects in a globular cluster. These searches have yielded large numbers of globular cluster white dwarfs. For example, a recent search of w Centauri has revealed over 2000 white dwarfs [163Jump To The Next Citation Point], while Hansen et al. [95Jump To The Next Citation Point] have detected 222 white dwarfs in M4. In general, however, these searches uncover single white dwarfs. Optical detection of white dwarfs in binary systems tends to rely on properties of the accretion process related to the binary type. Therefore, searches for cataclysmic variables generally focus on low-luminosity X-ray sources [125Jump To The Next Citation Point90Jump To The Next Citation Point230Jump To The Next Citation Point] and on ultraviolet-excess stars [88134Jump To The Next Citation Point152Jump To The Next Citation Point], but these systems are usually a white dwarf accreting from a low mass star. The class of “non-flickerers” which have been detected recently [37Jump To The Next Citation Point224Jump To The Next Citation Point] have been explained as He white dwarfs in binaries containing dark CO white dwarfs [58Jump To The Next Citation Point91Jump To The Next Citation Point94].

Pulsars, although easily seen in radio, are difficult to detect when they occur in hard binaries, due to the Doppler shift of the pulse intervals. Thanks to an improved technique known as an “acceleration search” [157], which assumes a constant acceleration of the pulsar during the observation period, more short orbital period binary pulsars are being discovered [26Jump To The Next Citation Point2740Jump To The Next Citation Point436773Jump To The Next Citation Point192Jump To The Next Citation Point]. For a good review and description of this technique, see Lorimer [143Jump To The Next Citation Point]. The progenitors of the ultracompact millisecond pulsars are thought to pass through a LMXB phase [50Jump To The Next Citation Point90Jump To The Next Citation Point195Jump To The Next Citation Point198Jump To The Next Citation Point]. These systems are very bright and all of them in the globular cluster system are known. There are, however, several additional LMXBs that are currently quiescent [90Jump To The Next Citation Point231Jump To The Next Citation Point].

Although there are many theoretical predictions of the existence of black holes in globular clusters (see, e.g., [159Jump To The Next Citation Point187Jump To The Next Citation Point158Jump To The Next Citation Point44]), there are very few observational hints of them. Measurements of the kinematics of the cores of M15 [7692] and NGC 6752 [54] provide some suggestions of a large, compact mass. However, these observations can also be explained without requiring an intermediate mass black hole [148173]. The unusual millisecond pulsar in the outskirts of NGC 6752 has also been argued to be the result of a dynamical interaction with a possible binary intermediate mass black hole in the core [36Jump To The Next Citation Point]. If the velocity dispersion in globular clusters follows the same correlation to black hole mass as in galactic bulges, then there may be black holes with masses in the range 1 - 103Mo . in many globular clusters [243]. Stellar mass black hole binaries may also be visible as low luminosity X-ray sources, but if they are formed in exchange interactions, they will have very low duty cycles and hence are unlikely to be seen [128].

Recent observations and catalogs of known binaries are presented in the following Sections 3.1, 3.2, 3.3, and 3.4.


 3.1 Cataclysmic variables
 3.2 Low-mass X-ray binaries
 3.3 Millisecond pulsars
 3.4 Black holes

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