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 (L bol ∼ 10 47 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 broadband 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,low /cm −2 ) = 22.3 ± 0.1 and log(N H,high /cm −2 ) = 23.2 ± 0.1, with average line of sight covering factors of ∼ 80% (with typical ∼ 5% variations) and 60% (±10 − 15%), respectively. During 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 along the line of sight. The partial covering absorber prefers an outflow velocity of v pc = 0.25 +0.01 −0.05 c at c ? RAS arXiv:1602.04023v1 [astro-ph.HE] 12 Feb 2016 2 Matzeu et al.the > 99.9% confidence level over the case where v 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 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 − 20 R g , although the hard X-ray (> 2 keV) emission may originate from a more compact or patchy corona of hot electrons, which is typically ∼ 6 − 8 R g in size.

Discovery of Broad Soft X-Ray Absorption Lines From the Quasar Wind in PDS 456

“…In the 2001 sequence, which has a strongly absorbed spectrum, an additional neutral, partial covering absorption component (with ) was applied to the soft X-ray power law, because otherwise the photon index was found to be unusually hard with G~1. We note that a similar partial covering component has been found in some of the previous Suzaku observations of PDS 456, when the spectrum becomes unusually hard Reeves et al 2014;Matzeu et al 2016).…”

“…s, that is, weeks to months. It is also likely that some variability can occur on shorter timescales, as found during the low-flux 2013 Suzaku observations, where changes in the soft X-ray spectrum on timescales of 100 ks could be accounted for by variability of a partially covering absorber (Matzeu et al 2016).…”

“…Indeed, a prolonged absorption event may have occurred in PDS 456 months prior to the 2013 XMM-Newton observations, during an extended low-flux observation with Suzaku in 2013 February-March (see Gofford et al 2014;Matzeu et al 2016 for details of these observations). During the 2013 Suzaku observations, the soft X-ray flux was lower by about a factor of 10 compared to the relatively unobscured level observed in XMM-Newton OBS.…”

“…The denser clumps responsible for the soft X-ray absorption appear radially coincident with the expected location of the AGN broad emission line region, while they may also be responsible for the soft X-ray re-emission from the wind. The denser clouds may also be sufficiently compact (DR R 10 g ) to act as the putative partial covering X-ray absorber and may thus be responsible for the strong (order of magnitude) absorption variability seen at soft X-rays toward this quasar Matzeu et al 2016). Indeed, in AGNs in general, a complex topology of clouds surrounding the X-ray continuum source may be able to account for the general distribution of broadband X-ray spectral shapes observed from both the type I and type II AGN populations (Tatum et al 2013(Tatum et al , 2016.…”

NuSTAR View of the Black Hole Wind in the Galaxy Merger IRAS F11119+3257

“…Instead, already from the long Suzaku observation, the absorber column density was found to vary between N H ;(3-9)×10 24 cm −2 on a timescale as short as three days. This may suggest clumpiness in the wind, as already reported for other quasar disk winds (e.g., Matzeu et al 2016;Reeves et al 2016).…”