GRO J0422+32: The Lowest Mass Black Hole?

Abstract

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 Av = 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 degrees corresponding to an upper limit on the mass of the compact object of 4.92 M_sun. Combining the models with the observed spectral energy distribution of the system, the most likely value for the orbital inclination angle is 45 +/- 2 degrees. This inclination angle corresponds to a primary black hole mass of 3.97 +/- 0.95 M_sun. 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_sun range.