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° 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.
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Footnotes
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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.