Potential discovery of the long-sought optical counterpart to the Rapid Burster in the bulge fossil fragment Liller 1

We report the possible identification of the optical counterpart of the Rapid Burster MXB 1730-335 in the stellar system Liller 1. The identification was performed by taking advantage of a set of images acquired with the Hubble Space Telescope/Advanced Camera for Surveys in the optical band, and with the Gemini South Telescope in the near-infrared. The analysis of these images revealed a star with a position that might be compatible with the X-ray and radio-band coordinates of the Rapid Burster, and it varies significantly in the optical. According to its location in the color-magnitude diagram, the candidate companion appears to belong to the young (∼ 1−2 Gyr old) supersolar metallicity ([M/H]=+0.3) subpopulation recently discovered in Liller 1. We discuss the main characteristics of the candidate counterpart and the Rapid Burster binary system as derived from the available data. We also highlight the need for further coordinated observations to solidly confirm their association and better clarify their physical properties.