Supersoft X-ray sources: Confronting the van den Heuvel paradigm

Abstract

The origin of the extremely soft (low temperature) X-rays produced in the supersoft X-ray sources (or SSS) has been a subject of debate for much of the 25 years since their discovery. The currently accepted paradigm is that they are Eddington-limited accreting white dwarfs in which the accreted material is undergoing steady nuclear burning, thereby achieving the high luminosities recorded from the brightest SSS in the Magellanic Clouds. Such a model requires an extremely high mass transfer rate from the donor, only achievable via thermally unstable mass transfer from a massive (>1 M ⊙) donor. An ideal SSS to probe this model in detail is the (only) recurrent, transient SSS, RX J0513.9-6951 which outbursts in X-rays every 100–150 days for a month or so, but is accompanied by a substantial (∼1 mag) optical decline. Using XMM we have obtained simultaneous soft X-ray, UV and optical observations through such an event, showing that as the X-ray flux declines the temperature declines as well (moving the peak emission into the UV) whereas the optical flux and (implied) white dwarf radius increase. Such an expansion of the white dwarf is evidenced by highly ionised outflowing gas in the high resolution X-ray spectra.