Hubble Space Telescope Discovery of an Optical Counterpart to the Supersoft X-Ray Source in the Globular Cluster M3*

We report the detection with the Hubble Space Telescope of an optical counterpart to the transient supersoft X-ray source 1E 1339.8+2837 in the globular cluster M3. The counterpart is found near the faint end of the subgiant branch in the V versus V - I color-magnitude diagram but is extremely bright in the U band. Variability is detected over a range of timescales, suggesting the presence of an accretion disk and perhaps also ellipsoidal variations of the subgiant secondary. The optical colors of the binary are similar to those of cataclysmic variables recently discovered in 47 Tuc and NGC 6397. We suggest that magnetically channeled accretion may explain the relatively low X-ray luminosity of this source's supersoft state.