Discovery of the Infrared Counterpart of the Rapid Burster with JWST

The Rapid Burster (MXB 1730-335) is the only known neutron star that produces both Type I and Type II X-ray bursts, making it a key system for understanding accretion physics onto neutron stars. It lies in Liller 1, a dense, metal-rich stellar system whose crowded core and roughly 11 magnitudes of visual extinction have concealed the Rapid Burster's optical/infrared counterpart for nearly five decades.

Using JWST/NIRCam infrared observations of Liller 1, the authors report the first unambiguous detection of the Rapid Burster's optical/infrared counterpart. The source was caught in an X-ray active state.

The counterpart exhibits dramatic infrared variability, fading by approximately 2.5 magnitudes in JWST's F200W band (1.755-2.227 microns), with strong fluctuations on minute timescales. These observations provide an infrared anchor for the Rapid Burster and open the way for future spectroscopy and simultaneous infrared and X-ray monitoring to constrain the donor's spectral type and the system's accretion physics.