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

Abstract

We present a detailed analysis of a recent, 2013 Suzaku campaign on the nearby (z = 0.184) luminous (Lbol ~ 1047 erg s-1) quasar PDS 456. This consisted of three observations, covering a total duration of ~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 (NH,low/cm-2) = 22.3 ± 0.1 and log (NH,high/cm-2) = 23.2 ± 0.1, with average line-of-sight covering factors of ~80 per cent (with typical ~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 ~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 vpc = 0.25+0.01-0.05 c at the >99.9 per cent confidence level over the case where vpc = 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 ~15-20Rg, although the hard X-ray (>2 keV) emission may originate from a more compact or patchy corona of hot electrons, which is typically ~6-8Rg in size.