The 8-Day Modulation in V616 Mon (A0620-00)

Abstract

Photoelectric photometry of V616 Mon, the optical counterpart of the bright transient X-ray source A0620-00, obtained at the Haute Provence Observatory in October 1975 and at the European Southern Observatory in March 1976 shows evidence of a low-amplitude 8-day intensity modulation. Walter and Duerbeck (1976) have reported a 4-day periodicity soon after outburst. Interpreting their results in terms of a modulation having twice that period, and thus showing two maxima and two minima per cycle, and combining them with our data, we obtain a period of 7.80 ± 0.03 days. This modulation agrees in period and probably in phase with that detected in X-rays in January 1976 (Matilsky et al. 1976). Its amplitude increases with wavelength in March but no such effect is detectable in October. We note a similar lack of correlated color variability in the Duerbeck and Walter data. A recurring eclipse-like feature, which may be phase-locked to the 7.8-day cycle (Robertson et al. 1976), is superposed onto one cycle of the modulation in March. Significant intra-night variability is usually present, amounting to as much as 0.17 mag in two hours. Such variability, also detected at u.v. wavelengths (Wu et al. 1976) may be due to an underlying 1-day periodicity.