2003ApJ...599.1254G

We have obtained optical and infrared photometry of the soft X-ray transient GRO J0422+32. From this photometry, we find a secondary star spectral type of M1, and an extinction of A_V=0.74±0.09. We present the first observed infrared (J-, H-, and K-band) ellipsoidal variations, and model them with WD98, a recent version of the Wilson-Devinney light curve-modeling code. Assuming no significant contamination of the infrared light curves, we find a lower limit to the inclination angle of 43° corresponding to an upper limit on the mass of the compact object of 4.92 M_☉. Combining the models with the observed spectral energy distribution of the system, the most likely value for the orbital inclination angle is 45°±2°. This inclination angle corresponds to a primary black hole mass of 3.97±0.95M_☉. Thus, we contend that J0422+32 contains the lowest mass stellar black hole reported, and the first to have a measured mass that falls in the 3-5 M_☉range.