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The globular cluster population of white dwarfs can be used to determine the ages of globular
clusters [162], 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
Centauri has revealed over
2000 white dwarfs [163
], while Hansen et al. [95
] 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 [125
, 90
, 230
] and on ultraviolet-excess stars [88, 134
, 152
], but these systems are usually a white
dwarf accreting from a low mass star. The class of “non-flickerers” which have been detected
recently [37
, 224
] have been explained as He white dwarfs in binaries containing dark CO white
dwarfs [58
, 91
, 94].
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 [26, 27, 40
, 43, 67, 73
, 192
]. For a good review
and description of this technique, see Lorimer [143
]. The progenitors of the ultracompact millisecond
pulsars are thought to pass through a LMXB phase [50
, 90
, 195
, 198
]. 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 [90
, 231
].
Although there are many theoretical predictions of the existence of black holes in globular clusters (see,
e.g., [159, 187
, 158
, 44]), there are very few observational hints of them. Measurements of the kinematics of
the cores of M15 [76, 92] 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 [148, 173]. 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 [36
]. 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
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.
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