Received 28 July 1994, accepted 16 September 1994
Astronomy & Astrophysics
Abstract
We present CCD photometry of the optical counterpart of the
X-ray transient GRO J0422+32 obtained with the 1.2-m telescope at
Haute-Provence on 79 nights from September 1992 through March 1994.
During this interval the source underwent several outbursts. One
double event of large amplitude (7 mag in V), lasting 250 days,
coincided with the major X-ray outburst and its secondary maximum, and
had an e-folding time three times longer than in X-rays, while two
other outbursts were shorter-lived (1 month), lower amplitude (5.5 mag
in V) events with subsequent smaller after-events. Our BVI data are
consistent with a power-law energy distribution (f~l**(-a)) over the
4000-9000 A range whose index a varies from 2.8 to 2.0 as the source
spectrum changes from being disk-dominated to one where the companion
becomes progressively visible, first in I, then in V. Adopting a
visual absorption of Av = 1.25 mag, and assuming a = 2 during
quiescence, we derive for the companion a probable spectral type of
M0V at ~2 kpc, with the disk contributing ~40% of the total light in
V. Time analysis of the high state data for January, February and
December 1993 shows a dominant period of 5.1 hours. The amplitude and
shape of this light curve are highly variable with several occurences
of nights with no detectable modulation. Analysis of low state data
for February and March 1994 reveals an identical period but with an
even more complex and changing light curve, and although the disk
contribution is still not negligeable in this state, part of the
variability and complexity of the light curve may be due to the
companion star itself
Key Words: Accretion: accretion disks - binaries: close - X-rays: transients