Short term X-ray spectral variability of the quasar PDS 456 observed in a low flux state
Date Issued
2016
Author(s)
Gabriele A. Matzeu
•
James N. Reeves
•
•
•
Michele T Costa
•
•
Jason Gofford
Abstract
We present an analysis of the 2013 Suzaku campaign on the nearby luminous
quasar PDS 456, covering a total duration of ~1 Ms and a net exposure of 455
ks. During these observations, the X-ray flux was suppressed by a factor of >10
in the soft X-ray band when compared to other epochs. We investigated the
broadband continuum by constructing a spectral energy distribution, making use
of the optical/UV photometry and hard X-ray spectra from the later
XMM-Newton/NuSTAR campaign in 2014. The high energy part of this low flux state
cannot be accounted for by 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 source. Two absorption layers
are required, of column density $\log (N_{\rm{H,low}}/{\rm
cm^{-2}})=22.3\pm0.1$ and $\log (N_{\rm{H,high}}/{\rm cm^{-2}})=23.2\pm0.1$,
with average covering factors of ~80% (with typical 5% variations) and 60%
($\pm$10-15%), respectively. In these observations PDS 456 displays significant
short term X-ray spectral variability, on timescales of ~100 ks, which can be
accounted for by variable covering of the absorbing gas. The partial covering
absorber prefers an outflow velocity of $v_{\rm pc} = 0.25^{+0.01}_{-0.05}c$ at
the >99.9% confidence level over the case where $v_{\rm pc}=0$. This is
consistent with the velocity of the highly ionised 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 we estimate the size-scale of the X-ray source
from its variability. The radial extent of the X-ray emitter is found to be of
the order ~15-20 $R_{\rm g}$, although the hard X-ray (>2 keV) emission may
originate from a more compact or patchy corona of hot electrons, which is ~6-8
$R_{\rm g}$ in size.
Volume
458
Issue
2
Start page
1311
Issn Identifier
0035-8711
Ads BibCode
2016MNRAS.458.1311M
Rights
open.access
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