2017MNRAS.469..267D

We present an observational far-ultraviolet (FUV) and near-ultraviolet (NUV) study of the core region of the globular cluster (GC) NGC 6397. The observations were obtained with the Space Telescope Imaging Spectrograph (STIS, FUV) and the Wide Field Camera 3 (NUV) on board the Hubble Space Telescope. Here, we focus on the UV-bright stellar populations such as blue stragglers (BSs), white dwarfs (WDs) and cataclysmic variables (CVs). We present the first FUV - NUV colour-magnitude diagram (CMD) for this cluster. To support our classification of the stellar populations, we compare our FUV - NUV CMD with optical data from the ACS Survey of Galactic Globular Clusters. The FUV - NUV CMD indicates 16 sources located in the WD area, and 10 BSs within the 25 x 25 arcsec² of the STIS FUV data. 18 Chandra X-ray sources are located within the FUV field of view. 13 of those have an NUV counterpart, of which 9 sources also have an FUV counterpart. Out of those, five sources are previously suggested CVs, and indeed, all five are located in the WD/CV region in our FUV - NUV CMD. Another CV has only an FUV but no NUV counterpart. We also detect an NUV (but no FUV) counterpart to the millisecond pulsar (MSP) located in the core of this cluster. The NUV light curves of the CVs and MSP show flickering behaviour typical of CVs. We found that the BSs and CVs are the most centrally concentrated populations. This might be an effect of mass segregation or it might indicate the preferred birth place of BSs and CVs via dynamical interactions in the dense core region of GCs. Horizontal branch stars are the least centrally concentrated population and absent in the innermost area of the core.