The first radius-expansion X-ray burst from GX 3+1

During several observations in 1999 August with RXTE of the low-mass X-ray binary GX 3+1, we found a single short and strong X-ray burst. This is the first burst from GX 3+1 which clearly shows evidence for radius expansion o f the neutron-star photosphere during the thermo-nuclear runaway. We show t hat the cooling phase of the neutron star photosphere starts already just b efore the end of the contraction phase. Considering the fact that the radiu s expansion is due to the burst luminosity being at the Eddington luminosit y, assuming standard burst parameters and accounting for gravitational reds hift effects we derive a distance to the source of similar to 4.5 kpc, alth ough relaxing these assumptions may lead to uncertainties up to similar to 30%. By comparing the persistent flux with that observed at the peak of the burst we infer that near the time of the X-ray burst the persistent lumino sity of GX 3+1 is similar to 0.17 L-edd, confirming predictions from theore tical modeling of X-ray spectra of bright sources like GX 3+1.