Observations of GRO J0422 + 32. III. A low-inclination black hole X-ray nova

We present the results of our campaign of photometry of the X-ray nova GRO J0422 + 32 in quiescence, the purpose of which was to confirm the proposed orbital period of ∼ 5.1 hr, and to search for the ellipsoidal variations which are a prerequisite for the mass determination of the primary. We find evidence for a weak ellipsoidal modulation corresponding to an orbital period of 5.1 hr, with a semiamplitude of ∼0.03 mag - the lowest known of any quiescent X-ray nova. With the assumption that the I-band contamination by the accretion disk is no greater than that observed in other quiescent X-ray novae, we estimate that the orbital inclination must be ≤45°. Furthermore, a ZAMS secondary implies an upper limit on the distance of ∼2.2 kpc, and a quiescent accretion rate ∼3% that observed in A0620-00. The inclination estimate implies a mass of the compact object ≥3.4 M_⊙. ROSAT observations constrain the quiescent X-ray luminosity to be less than 2 × 10³² ergs s^-1 (for a distance of 2 kpc). We speculate that the low quiescent accretion rate through the disk, inferred from the optical observations, may well be related to the large outburst amplitude of GRO J0422 + 32.