Four outburst cycles of A 0538 - 66: evidence for a rapidly evolving envelope around the primary

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Abstract
Continuous multicolour photometry is presented of the 16.6 day recurrent X-ray transient A 0538 – 66 over four consecutive outbursts in late 1981. The magnitudes and colours measured between outbursts are significantly different from previous observations during quiescence, indicating continued activity throughout these cycles. The onset of the outbursts can be as much as ∼ 1.3 days prior to maximum, and during the rises there are complex variations of both V and U–B. The decays are smooth, the longest lasting for ∼ 6 days. Evidence is presented for an extended envelope around the primary which evolves rapidly from cycle to cycle. The neutron star (in a highly eccentric orbit) interacts with this envelope at periastron, and the variability of the rise-times and peaks of the outbursts is governed by the varying size and density of this envelope.

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