Observations of the X-ray nova GRO J0422+32. I. Outburst and the decay to quiescence

We present optical photometry and spectroscopy and BATSE observations of the X-ray nova GRO J0422+32, obtained during outburst and its subsequent decay to quiescence. Although the X-ray and optical properties of GRO J0422+32 are broadly similar to those of other X-ray novae, it is unique in several respects. The unusually protracted decay to quiescence of the optical light curve has been punctuated by at least two minioutbursts of ∼4 mag. The BATSE and optical outbursts are each separated by ∼120 days. We find that the optical luminosity of GRO J0422+32 during the primary outburst is dominated by reprocessing of E > 10 keV X-rays. In contrast, the optical minioutbursts are most likely generated by an intrinsically bright disk rather than X-ray reprocessing: they do not appear to have any X-ray counterparts. Extremely broad (up to 6000 km s^-1 FWZI) absorption lines have also been observed during both primary outbursts and minioutbursts. During the second minioutburst, Hα and Hβ emission was observed superposed on red-shifted absorption features. We find that the interoutburst light curve of GRO J0422+32 may be inconsistent with an accretion disk instability as the origin of the minioutbursts. Finally, a transient 5.1/10.2 hr modulation, which may be related to the orbital period, has been observed during roughly half of our observations. However, confirmation of the orbital period must await observations in quiescence.