Accretion onto the white dwarf may eventually
lead to a dwarf nova outburst. Identifications of globular cluster
CVs have been made through such outbursts in the cores of
M5 [103], 47 Tuc [118], and NGC
6624 [148
]. With the exception
of V101 in M5 [103], original
searches for dwarf novae performed with ground based telescopes
proved unsuccessful. This is primarily due to the fact that
crowding obscured potential dwarf novae up to several core radii
outside the center of the cluster [145, 147]. Since binaries
tend to settle into the core, it is not surprising that none were
found significantly outside of the core. Subsequent searches using
the improved resolution of the Hubble Space Telescope eventually
revealed a few dwarf novae close to the cores of selected globular
clusters [144, 146, 148].
A more productive approach has been to look for
direct evidence of the accretion around the white dwarf. This can
be in the form of excess UV emission and strong emission [44
, 63, 93
] from the accretion
disk. This technique has resulted in the discovery of candidate CVs
in 47 Tuc [44, 93], M92 [47], NGC
6397 [26
, 41
, 156], and NGC
6712 [45]. The
accretion disk can also be discerned by very soft X-ray emissions.
These low luminosity X-ray binaries are characterized by a
luminosity
, which distinguishes them
from the low-mass X-ray binaries with
.
Initial explanations of these objects focused on accreting white
dwarfs [10], and a
significant fraction of them are probably CVs. However, some of the
more energetic sources may be LMXBs in quiescence.
Early searches performed with ROSAT data (which
had a detection limit of ) revealed
roughly 30 sources in 19 globular clusters [87
]. A more recent
census of the ROSAT low luminosity X-ray sources, published by
Verbunt [159], lists 26 such
sources that are probably related to globular clusters. Recent
observations with the improved angular resolution of Chandra have
begun to uncover numerous low luminosity X-ray candidates for
CVs [64
, 65
, 73
, 79
]. An attempt to
identify an IR counterpart to the LMXB X-ray burster in Liller 1
with Chandra also resulted in the serendipitous discovery of 3 low
luminosity X-ray sources [79], which may be
quiescent LMXBs. Another probable quiescent LMXB has been observed
in NGC 5139 with Chandra [140].
Observations of NGC 6652 have also discovered 3 new low luminosity
X-ray sources, two of which may be CVs [73
]. The most
comprehensive survey of X-ray sources in a globular cluster is that
of Grindlay et al. [64
], which presents
results of high resolution Chandra images of 47 Tuc. In
addition to numerous other X-ray sources, they report 13 candidate
CVs.
The state of the field at this time is one of rapid change as Chandra results come in and optical counterparts are found for the new X-ray sources. A living catalog of CVs has been created by Downes et al. [39] and may be the best source for confirmed CVs in globular clusters.