A Complete Census of Hα Emitters in NGC 6397

We used a data set of archival Hubble Space Telescope images obtained through the F555W, F814W, and F656N filters to perform a complete search for objects showing $H\alpha$ emission in the globular cluster NGC 6397. As a photometric diagnostic, we used the ${(V-H\alpha )}_0$ color excess in the ${(V-H\alpha )}_0$–${(V-I)}_0$ color–color diagram. In the analyzed field of view, we identified 53 $H\alpha$ emitters. In particular, we confirmed the optical counterpart to 20 X-ray sources (7 cataclysmic variables, 2 millisecond pulsars and 11 active binaries) and identified 33 previously unknown sources, thus significantly enlarging the population of known active binaries in this cluster. We report the main characteristics for each class of objects. Photometric estimates of the equivalent width of the $H\alpha$ emission line were derived from the ${(V-H\alpha )}_0$-excess and, for the first time, compared to the spectroscopic measurements obtained from the analysis of MUSE spectra. The very good agreement between the spectroscopic and photometric measures fully confirmed the reliability of the proposed approach to measure the $H\alpha$ emission. The search demonstrated the efficiency of this novel approach to pinpoint and measure $H\alpha$-emitters, thus offering a powerful tool to conduct a complete census of objects with formations and evolutions that can be strongly affected by dynamical interactions in star clusters.