GRO J0422+32: The Lowest Mass Black Hole?

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 ± 0.95 M ☉ . 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.