The mass of the black hole in the low-inclination LMXB transient system GRO J0422+32 (= Nova Persei 1992)

We present the first infrared light curve of GRO J0422 + 32, and use it along with R-band data and published I- and Cousins R-c-band data to constrain the inclination of the system to be between 10 degrees and 26 degrees, implying that the black hole mass is greater than similar to 15M.. Remodelling the data with a 30 per cent accretion dise contamination level changes the result little, the new limits being 13 degrees-31 degrees for the inclination, and greater than similar to 9 M. for the mass. Our results are the lowest inclination and, consequently, the highest mass claims yet made for this system. Previous observations of dips seen during outburst are suggestive of a partial eclipse. Here we show that an eclipse interpretation of the so-called orbital dips is inconsistent with other data; the 'dips' are more likely to be random luminosity variations of a disc in outburst. We also show, for the first time, how an equation by King et al. allows a lower limit for the compact object mass in the soft X-ray transients (SXTs) to be independently estimated by use of the orbital period and spectral type only. This allows the spectroscopist to identify immediately the strong black hole candidates amongst SXTs with orbital periods less than or similar to 2 d. These new mass estimates are in agreement with the ellipsoidally determined mass estimates, and indicate that GRO J0422 + 32 contains a black hole with a mass greater than or similar to 28 M..