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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 468; no. 3; pp. 3663 - 3681
• Main Authors: Jin, Chichuan, Done, Chris, Ward, Martin
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01-07-2017
• Subjects: galaxies: nuclei; galaxies: active; accretion, accretion discs
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stx718

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 R
g away from the hot corona emitting hard X-rays, and is probably associated with a geometrically thick (‘puffed-up’) inner disc region.