CCD photometry of GRO J0422+32 during activity and quiescence*

Claude Chevalier and Sergio A. Ilovaisky

Observatoire de Haute-Provence (CNRS)
F-04870 St.Michel l'Observatoire, France

Received 28 July 1994, accepted 16 September 1994
Astronomy & Astrophysics

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