A candidate to the long sought optical counterpart to the Rapid Burster in the bulge fossil fragment Liller 1
Cristina Pallanca, Francesco R. Ferraro, Barbara Lanzoni, Mario Cadelano, Craig O. Heinke, Maureen van den Berg, Jeroen Homan, Chiara Crociati, Sebastien Guillot
Published: 2025/9/12
Abstract
We report on 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 the presence of a star with a position possibly compatible with the X-ray and radio band coordinates of the Rapid Burster, and showing significant optical variability. According to its location in the color-magnitude diagram, the candidate companion appears to belong to the young (~ 1-2 Gyr old) super-solar metallicity ([M/H]= +0.3) sub-population 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, also highlighting the need for further coordinated observations to solidly confirm their association and better clarify their physical properties.