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 , these objects would be
virtually invisible at globular cluster distances. Searches for
cataclysmic variables generally focus on low-luminosity X-ray
sources [87
, 64
, 159
] and on
ultraviolet-excess stars [62, 93
, 103
], but these systems
are usually a white dwarf accreting from a low mass star. The class
of “non-flickerers” which have been detected recently [26
, 156
] have been explained
as He white dwarfs in binaries containing dark CO white
dwarfs [41
, 65
].
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” [108], which assumes a
constant acceleration of the pulsar during the observation period,
more short orbital period binary pulsars are being
discovered [20
, 21, 29
, 30
, 48
, 51
, 130
]. For a good review
and description of this technique, see Lorimer [98
]. The progenitors of
the ultracompact millisecond pulsars are thought to pass through a
low-mass X-ray binary (LMXB) phase [37
, 64
, 131
, 134
]. 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 [64
]. 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 [53, 66], 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 () in as
many as
globular clusters [109
], or that stellar
mass binary black holes may be generated and subsequently ejected
from most globular clusters [127
]. 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 [169
].
Recent observations and catalogs of known binaries are presented in the following sections.