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We have searched for optical identifications for 79 Chandra X-ray sourcesthat lie within the half-mass radius of the nearby, core-collapsed globularcluster NGC 6397, using deep Hubble Space Telescope Advanced Camera for SurveysWide Field Channel imaging in H-alpha, R, and B. Photometry of these imagesallows us to classify candidate counterparts based on color-magnitude diagramlocation. In addition to recovering nine previously detected cataclysmicvariables (CVs), we have identified six additional faint CV candidates, a totalof 42 active binaries (ABs), two millisecond pulsars (MSPs), one candidateactive galactic nucleus, and one candidate interacting galaxy pair. Of the 79sources, 69 have a plausible optical counterpart. The 15 likely and possibleCVs in NGC 6397 mostly fall into two groups: a brighter group of six for whichthe optical emission is dominated by contributions from the secondary andaccretion disk, and a fainter group of seven for which the white dwarfdominates the optical emission. There are two possible transitional objectsthat lie between these groups. The faintest CVs likely lie near the minimum ofthe CV period distribution, where an accumulation is expected. The spatialdistribution of the brighter CVs is much more centrally concentrated than thoseof the fainter CVs and the active binaries. This may represent the result of anevolutionary process in which CVs are produced by dynamical interactions, suchas exchange reactions, near the cluster center and are scattered to largerorbital radii, over their lifetimes, as they age and become fainter.

astrophysical journal/the astrophysical journal

Identification of Faint Chandra X-ray Sources in the Core-Collapsed  Globular Cluster NGC 6397: Evidence for a Bimodal Cataclysmic Variable  Population