Discovery of the Infrared Counterpart of the Rapid Burster with JWST

The Rapid Burster (MXB 1730 - 335) is the only known neutron star to produce both Type I and Type II X-ray bursts, making it a key system for understanding accretion physics onto neutron stars. It resides in Liller 1, a dense, metal-rich stellar system whose crowded core and about 11 magnitudes of visual extinction have concealed the Rapid Burster's optical/infrared (OIR) counterpart for nearly five decades. Here, we present infrared observations of Liller 1 obtained with JWST/NIRCam that, for the first time, reveal the unambiguous OIR counterpart to the Rapid Burster. Caught in an X-ray active state, the counterpart exhibits dramatic infrared variability, fading by about 2.5 magnitudes in JWST's F200W band (1.755 - 2.227 μ m) 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 IR and X-ray monitoring to constrain the donor's spectral type and the system's accretion physics.