Super-Eddington QSO RX J0439.6-5311 - I. Origin of the soft X-ray excess and structure of the inner accretion flow

Abstract

We report the results from a recent 133 ks XMM-Newton observation of a highly super- Eddington narrow-line Type-1 quasi-stellar object RX J0439.6-5311. This source has one of the steepest active galactic nuclei hard X-ray slopes, in addition to a prominent and smooth soft X-ray excess. Strong variations are found throughout the 0.3-10 keV energy range on all time-scales covered by the observation, with the soft excess mainly showing low-frequency (LF) variations below 0.1 mHz while the hard X-rays show stronger variability at higher frequencies. We perform a full set of spectral-timing analysis on the X-ray data, including a simultaneous modelling of the time-averaged spectra, frequency-dependent root-mean-square and covariance spectra, lag-frequency and lag-energy spectra. Especially, we find a significant time-lag signal in the LF band, which indicates that the soft X-rays lead the hard by ~4 ks, with a broad continuum-like profile in the lag spectrum. Our analysis strongly supports the model where the soft X-ray excess is dominated by a separate low temperature, optically thick Comptonization component rather than relativistic reflection or a jet. This soft X-ray emitting region is several tens or hundreds of Rg away from the hot corona emitting hard X-rays, and is probably associated with a geometrically thick ('puffed-up') inner disc region.