Broadband short term variability of the quasar PDS 456

TL;DR

This study analyzes a 2013 Suzaku observation of quasar PDS 456, revealing short-term X-ray variability caused by variable absorption from dense, clumpy wind components moving at about 0.25c, and estimates the X-ray emitting region's size.

Contribution

It provides the first direct estimate of the X-ray emitting region's size in PDS 456 and links spectral variability to clumpy wind absorption at relativistic speeds.

Findings

Absorbers have outflow velocities ~0.25c.

X-ray emitting region size ≤ 20 Rg.

Spectral variability due to variable partial covering.

Abstract

We present a detailed analysis of a recent $500$ ks net exposure \textit{Suzaku} observation, carried out in 2013, of the nearby ($z=0.184$) luminous (L$_{\rm bol}\sim10^{47}$ erg s$^{-1}$) quasar PDS 456 in which the X-ray flux was unusually low. The short term X-ray spectral variability has been interpreted in terms of variable absorption and/or intrinsic continuum changes. In the former scenario, the spectral variability is due to variable covering factors of two regions of partially covering absorbers. We find that these absorbers are characterised by an outflow velocity comparable to that of the highly ionised wind, i.e. $\sim0.25$ c, at the $99.9\%$ $(3.26\sigma)$ confidence level. This suggests that the partially absorbing clouds may be the denser clumpy part of the inhomogeneous wind. Following an obscuration event we obtained a direct estimate of the size of the X-ray emitting region, to be not larger than $20~R_{\rm g}$ in PDS 456.