0 v 1 2 7 A ug 2 00 3 GRO J 0422 + 32 : The Lowest Mass Black Hole ? 1 Dawn

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 corresponding to an upper limit on the mass of the compact object of 4.92M⊙. 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. Subject headings: binaries: close− stars: black holes− stars: individual (J0422+32) − stars: low mass − stars: variables: other This work was based on observations obtained with the Apache Point Observatory 3.5-meter telescope, which is owned and operated by the Astrophysical Research Consortium, and the 3-meter Shane telescope at Lick Observatory, which is a Multi-campus Research Unit of the University of California.