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3.1 Cataclysmic variables

Cataclysmic variables (CVs) are white dwarfs accreting matter from a companion. They have been detected in globular clusters through identification of the white dwarf itself or through evidence of the accretion process. White dwarfs managed to avoid detection until recent observations with the Hubble Space Telescope revealed photometric sequences in several globular clusters [2726Jump To The Next Citation Point119135137136156Jump To The Next Citation Point]. Spectral identification of white dwarfs in globular clusters has begun both from the ground with the VLT [110] and in space with the Hubble Space Telescope [26Jump To The Next Citation Point41Jump To The Next Citation Point156Jump To The Next Citation Point]. With spectral identification, it will be possible to identify those white dwarfs in hard binaries through Doppler shifts in the Hb line. This approach has promise for detecting a large number of the expected double white dwarf binaries in globular clusters.

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 [103Jump To The Next Citation Point], 47 Tuc [118], and NGC 6624 [148Jump To The Next Citation Point]. 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 [145147]. 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 [144146148].

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 Ha emission [44Jump To The Next Citation Point6393Jump To The Next Citation Point] from the accretion disk. This technique has resulted in the discovery of candidate CVs in 47 Tuc [4493], M92 [47], NGC 6397 [26Jump To The Next Citation Point41Jump To The Next Citation Point156], 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 LX < 1034.5 erg/s, which distinguishes them from the low-mass X-ray binaries with LX > 1036 erg/s. 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 1031 erg/s) revealed roughly 30 sources in 19 globular clusters [87Jump To The Next Citation Point]. 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 [64Jump To The Next Citation Point65Jump To The Next Citation Point73Jump To The Next Citation Point79Jump To The Next Citation Point]. 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 [73Jump To The Next Citation Point]. The most comprehensive survey of X-ray sources in a globular cluster is that of Grindlay et al. [64Jump To The Next Citation Point], 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.

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