We present an XMM-Newton observation of the bright , narrow-line , ultrasoft Seyfert 1 galaxy Ton S180 .
The 0.3–10 keV X-ray spectrum is steep and curved , showing a steep slope above 2.5 keV ( \Gamma \sim 2.3 ) and a smooth , featureless excess of emission at lower energies .
The spectrum can be adequately parameterised using a simple double power-law model .
The source is strongly variable over the course of the observation but shows only weak spectral variability , with the fractional variability amplitude remaining approximately constant over more than a decade in energy .
The curved continuum shape and weak spectral variability are discussed in terms of various physical models for the soft X-ray excess emission , including reflection off the surface of an ionised accretion disc , inverse-Compton scattering of soft disc photons by thermal electrons , and Comptonisation by electrons with a hybrid thermal/non-thermal distribution .
We emphasise the possibility that the strong soft excess may be produced by dissipation of accretion energy in the hot , upper atmosphere of the putative accretion disc .