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_solar. Combining the models with the observed spectral energy distribution of the system, the most likely value for the orbital inclination angle is 45^deg+/-2^deg. This inclination angle corresponds to a primary black hole mass of 3.97+/-0.95M_solar. 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_solar range.

This work was based on observations obtained with the Apache Point Observatory 3.5 m telescope, which is owned and operated by the Astrophysical Research Consortium, and the 3 m Shane telescope at Lick Observatory, which is a Multicampus Research Unit of the University of California.