Optical and X-ray variability of supersoft X-ray sources

Abstract

We report new results of ROSAT observations and our optical multi-colour monitoring campaign of supersoft sources in the LMC. During this campaign, we observed an optical low-state of RX J0513.9-6951, lasting ≈40 days. The decrease of the optical flux, ΔV≈1 mag, and the accompanying reddening, Δ(B−V)≈0.1–0.2 mag and Δ(V - R) ∼ 0.1 mag, can be quantitatively described by variations in the irradiation of the accretion disk by the hot central star. In this simple model, the photospheric radius of a white dwarf with nuclear burning of accreted matter shrinks in response to a temporarily slightly reduced mass-transfer rate. The same model can account for the soft X-ray outburst found in the quasi-simultaneous monitoring with ROSAT.

We also report an improved X-ray position of the transient source RX J0550.0-7151 which excludes the suggested identification of this supersoft source with a bright foreground star. Our optical spectroscopy of all objects within the X-ray error circle is complete to V≲19.5 but has revealed no obvious counterpart.