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

¹ Dipartimento di Fisica e Astronomia “Augusto Righi”, Alma Mater Studiorum Università di Bologna, via Piero Gobetti 93/2, 40129 Bologna, Italy
² INAF-Osservatorio di Astrofisica e Scienze dello Spazio di Bologna, Via Piero Gobetti 93/3 40129 Bologna, Italy
³ Department of Physics, University of Alberta, Edmonton, AB T6G 2G7, Canada
⁴ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁵ Eureka Scientific, Inc., 2452 Delmer Street, Oakland, CA 94602, USA
⁶ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
⁷ IRAP, CNRS, 9 avenue du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
⁸ Université de Toulouse, CNES, UPS-OMP, 31028 Toulouse, France

^★★ Corresponding author: cristina.pallanca3@unibo.it

Received: 3 July 2025
Accepted: 7 September 2025

Abstract

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.