Short-term X-ray spectral variability of the quasar PDS 456 observed in a low-flux state

We present a detailed analysis of a recent, 2013 Suzaku campaign on the nearby (z = 0.184) luminous (L-bol similar to 10(47) erg s(-1)) quasar PDS 456. This consisted of three observations, covering a total duration of similar to 1 Ms and a net exposure of 455 ks. During these observations, the X-ray flux was unusually low, suppressed by a factor of > 10 in the soft X-ray band when compared to previous observations. We investigated the broad-band continuum by constructing a spectral energy distribution (SED), making use of the optical/UV photometry and hard X-ray spectra from the later simultaneous XMM-Newton and NuSTAR campaign in 2014. The high-energy part of this low-flux SED cannot be accounted for by physically self-consistent accretion disc and corona models without attenuation by absorbing gas, which partially covers a substantial fraction of the line of sight towards the X-ray continuum. At least two layers of absorbing gas are required, of column density log (N-H,N-low/cm(-2)) = 22.3 +/- A 0.1 and log (N-H,N-high/cm(-2)) = 23.2 +/- A 0.1, with average line-of-sight covering factors of similar to 80 per cent (with typical similar to 5 per cent variations) and 60 per cent (+/- 10-15 per cent), respectively. During these observations PDS 456 displays significant short-term X-ray spectral variability, on time-scales of similar to 100 ks, which can be accounted for by variable covering of the absorbing gas along the line of sight. The partial covering absorber prefers an outflow velocity of at the > 99.9 per cent confidence level over the case where v(pc) = 0. This is consistent with the velocity of the highly ionized outflow responsible for the blueshifted iron K absorption profile. We therefore suggest that the partial covering clouds could be the denser, or clumpy part of an inhomogeneous accretion disc wind. Finally estimates are placed upon the size-scale of the X-ray emission region from the source variability. The radial extent of the X-ray emitter is found to be of the order similar to 15-20R(g), although the hard X-ray (> 2 keV) emission may originate from a more compact or patchy corona of hot electrons, which is typically similar to 6-8R(g) in size.