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J. N. Reeves
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We present quasi-simultaneous ASCA and RXTE observations of the most luminous known AGN in the local (z<0.3) Universe, the recently discovered quasar PDS 456. Multiwavelength observations have been conducted which show that PDS 456 has a bolometric luminosity of 10^47 erg/s peaking in the UV part of the spectrum. In the X-ray band the 2-10 keV (rest-frame) luminosity is 10^45 erg/s. The broad-band X-ray spectrum obtained with ASCA and RXTE contains considerable complexity. The most striking feature observed is a very deep, ionised iron K edge, observed at 8.7 keV in the quasar rest-frame. We find that these features are consistent with reprocessing from highly ionised matter, probably the inner accretion disk. PDS 456 appeared to show a strong (factor of 2.1) outburst in just 17ksec, although non-intrinsic sources cannot be completely ruled out. If confirmed, this would be an unusual event for such a high-luminosity source, with the light-crossing-time corresponding to 2 Schwarzschild radii. The implication would be that flaring occurs within the very central regions, or else that PDS 456 is a `super-Eddington or relativistically beamed system. Overall we conclude on the basis of the extreme blue/UV luminosity, the rapid X-ray variability and from the imprint of highly ionised material on the X-ray spectrum, that PDS 456 is a quasar with an unusually high accretion rate.
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X-ray and multi-wavelength observations of the most luminous known local (z<0.3) AGN, the recently discovered radio-quiet quasar PDS 456, are presented. The spectral energy distribution shows that PDS 456 has a bolometric luminosity of 1e47 erg/s, peaking in the UV. The X-ray spectrum obtained by ASCA and RXTE shows considerable complexity. The most striking feature observed is a deep, highly-ionised, iron K edge (8.7 keV, rest-frame), originating via reprocessing from highly ionised material, possibly the inner accretion disk. PDS 456 was found to be remarkably variable for its luminosity; in one flare the X-ray flux doubled in just about 15 ksec. If confirmed this would be an unprecedented event in a high-luminosity source, with a light-crossing time corresponding to about 2RS. The implications are that either flaring occurs within the very central regions, or else that PDS 456 is a super-Eddington or relativistically beamed system.
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 $99.9%$ $(3.26sigma)$ 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.
The evolution of galaxies is connected to the growth of supermassive black holes in their centers. During the quasar phase, a huge luminosity is released as matter falls onto the black hole, and radiation-driven winds can transfer most of this energy back to the host galaxy. Over five different epochs, we detected the signatures of a nearly spherical stream of highly ionized gas in the broadband X-ray spectra of the luminous quasar PDS 456. This persistent wind is expelled at relativistic speeds from the inner accretion disk, and its wide aperture suggests an effective coupling with the ambient gas. The outflows kinetic power larger than 10^46 ergs per second is enough to provide the feedback required by models of black hole and host galaxy co-evolution.
When a black hole accretes close to the Eddington limit, the astrophysical jet is often accompanied by radiatively driven, wide-aperture and mildly relativistic winds. Powerful winds can produce significant non-thermal radio emission via shocks. Among the nearby critical accretion quasars, PDS 456 has a very massive black hole (about one billion solar masses), shows a significant star-forming activity (about seventy solar masses per year) and hosts exceptionally energetic X-ray winds (power up to twenty per cent of the Eddington luminosity). To probe the radio activity in this extreme accretion and feedback system, we performed very-long-baseline interferometric (VLBI) observations of PDS 456 at 1.66 GHz with the European VLBI Network (EVN) and the enhanced Multi-Element Remotely Linked Interferometry Network (e-MERLIN). We find a rarely-seen complex radio-emitting nucleus consisting of a collimated jet and an extended non-thermal radio emission region. The diffuse emission region has a size of about 360 pc and a radio luminosity about three times higher than the nearby extreme starburst galaxy Arp 220. The powerful nuclear radio activity could result from either a relic jet with a peculiar geometry (nearly along the line of sight) or more likely from diffuse shocks formed naturally by the existing high-speed winds impacting on high-density star-forming regions.
New Swift monitoring observations of the variable, radio-quiet quasar, PDS 456, are presented. A bright X-ray flare was captured in September 2018, the flux increasing by a factor of 4 and with a doubling time-scale of 2 days. From the light crossing argument, the coronal size is inferred to be about 30 gravitational radii for a black hole mass of $10^{9} {rm M}_{odot}$ and the total flare energy exceeds $10^{51}$ erg. A hardening of the X-ray emission accompanied the flare, with the photon index decreasing from $Gamma=2.2$ to $Gamma=1.7$ and back again. The flare is produced in the X-ray corona, the lack of any optical or UV variability being consistent with a constant accretion rate. Simultaneous XMM-Newton and NuSTAR observations were performed, $1-3$ days after the flare peak and during the decline phase. These caught PDS 456 in a bright, bare state, where no disc wind absorption features are apparent. The hard X-ray spectrum shows a high energy roll-over, with an e-folding energy of $E_{rm fold}=51^{+11}_{-8}$ keV. The deduced coronal temperature, of $kT=13$ keV, is one of the coolest measured in any AGN and PDS 456 lies well below the predicted pair annihilation line in X-ray corona. The spectral variability, becoming softer when fainter following the flare, is consistent with models of cooling X-ray coronae. Alternatively, an increase in a non-thermal component could contribute towards the hard X-ray flare spectrum.
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