Erupting Cataclysmic Variable Stars in the Nearest Globular Cluster, NGC 6397: Intermediate Polars?

Abstract

NGC 6397 is the closest globular cluster and hence the ideal place to search for faint stellar populations such as cataclysmic variables (CVs). Hubble Space Telescope (HST) and Chandra observers have identified nine certain and likely CVs in this nearby cluster, including several magnetic CV candidates. We have combined our recent UV imagery with archival HST images of NGC 6397 to search for new CV candidates and especially to look for dwarf nova–like eruptive events. We find remarkable and somewhat unexpected dwarf nova–like eruptions of the two well-known cataclysmic systems CV2 and CV3. These two objects have been claimed to be magnetic CVs, as indicated by their helium emission-line spectra. Magnetic fields in CVs are usually expected to prevent the disk instability that leads to dwarf nova eruptions. In fact, most field magnetic CVs are observed to not undergo eruptions. Our observations of the dwarf nova eruptions of CV2 and CV3 can be reconciled with these objects' He II emission lines if both objects are infrequently erupting intermediate polars, similar to EX Hya. If this is the case for most globular cluster CVs, then we can reconcile the many X-ray and UV-bright CV candidates seen by Chandra and HST with the very small numbers of erupting dwarf novae observed in cluster cores.