Remarkable Spectral Variability of PDS 456
  We report on the highest to date signal-to-noise-ratio X-ray spectrum of the
luminous quasar PDS 456, as obtained during two XMM-Newton orbits in September
2007. The present spectrum is considerably different from several previous
X-ray spectra recorded for PDS 456 since 1998. The ultra-high-velocity outflow
seen as recently as February 2007 is not detected in absorption. Conversely, a
significant reflection component is detected. The reflection model suggests the
reflecting medium may be outflowing at a velocity v/c = -0.06 +/- 0.02. The
present spectrum is analyzed in the context of the previous ones in an attempt
to understand all spectra within the framework of a single model. We examine
whether an outflow with variable partial covering of the X-ray source along the
line of sight that also reflects the source from other lines of sight can
explain the dramatic variations in the broad-band spectral curvature of PDS
456. It is established that absorption plays a major role in shaping the
spectrum of other epochs, while the 2007 XMM-Newton spectrum is dominated by
reflection, and the coverage of the source by the putative outflow is small (<
20%).
