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3 Observations

Observations of relativistic binaries in globular clusters are hampered by the fact that most of the binaries have segregated to the cores of the clusters where crowding is a problem. Furthermore, most of these systems are only visible during mass transfer or accretion stages of the evolution. One tracer of the processes that may lead to relativistic binaries is the population of “blue stragglers.” These stars appear on the main sequence to the left of the turn-off in the CMD of a globular cluster (see Figure  3View Image). They are hot and massive enough that they should have already evolved off the main sequence. Blue stragglers are thought to arise from stellar coalescences either through the gradual coalescence of the components of binaries in the cluster or through direct collisions. These objects are some of the most visible and populous evidence of dynamical interactions in globular clusters that lead to the formation of ultracompact binary systems. (See Hut [82Jump To The Next Citation Point] and Bailyn [11Jump To The Next Citation Point] for good reviews of the implications of blue stragglers on the dynamics of globular clusters.)

White dwarfs are generally too cool to have their binary properties readily measured. For example, the binary properties of double degenerate cataclysmic variables of the AM CVn type are determined from observing the hot spot where the accreting matter from the donor dwarf collides with the accretion disk around the accretor (see Warner [162] and references therein). With an expected M ~ 10 V, these objects would be virtually invisible at globular cluster distances. Searches for cataclysmic variables generally focus on low-luminosity X-ray sources [87Jump To The Next Citation Point64Jump To The Next Citation Point159Jump To The Next Citation Point] and on ultraviolet-excess stars [6293Jump To The Next Citation Point103Jump 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 [26Jump To The Next Citation Point156Jump To The Next Citation Point] have been explained as He white dwarfs in binaries containing dark CO white dwarfs [41Jump To The Next Citation Point65Jump To The Next Citation Point].

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” [108Jump To The Next Citation Point], which assumes a constant acceleration of the pulsar during the observation period, more short orbital period binary pulsars are being discovered [20Jump To The Next Citation Point2129Jump To The Next Citation Point30Jump To The Next Citation Point48Jump To The Next Citation Point51Jump To The Next Citation Point130Jump To The Next Citation Point]. For a good review and description of this technique, see Lorimer [98Jump To The Next Citation Point]. The progenitors of the ultracompact millisecond pulsars are thought to pass through a low-mass X-ray binary (LMXB) phase [37Jump To The Next Citation Point64Jump To The Next Citation Point131Jump To The Next Citation Point134Jump To The Next Citation Point]. These systems are very bright and all of them in the globular cluster system are known. There may, however, be some additional LMXBs that are currently quiescent [64Jump To The Next Citation Point]. In addition, a very recent observation has shown that the LMXB in M15 is, in fact, two bright sources [164].

With the exception of M15 [5366], no black holes have been identified in globular clusters. Theoretical predictions of black holes in globular clusters indicate that there may be intermediate-mass black holes (M ~ 103Mo .) in as many as ~ 20 globular clusters [109Jump To The Next Citation Point], or that stellar mass binary black holes may be generated and subsequently ejected from most globular clusters [127Jump 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 [169Jump To The Next Citation Point].

Recent observations and catalogs of known binaries are presented in the following sections.

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

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