arXiv subjects

Ken Pounds

Publications and source records attributed to Ken Pounds.

At least 19 recordsLinked to original sources

arXiv2023

Low-redshift absorption in the Seyfert galaxy PG 1211+143 -- a distant inflow maintaining off-plane accretion or the gravitational redshift of matter orbiting the SMBH?

The detection of a high velocity (~ 0.3c) inflow of highly ionized matter during an extended XMM-Newton observation of the luminous Seyfert galaxy PG 1211+143 in 2014 provided the first direct evidence of a short-lived accretion event, and an explanation for the powerful winds (UFOs) now recognised as a common property of many luminous Seyfert galaxies. Although the ultra-fast inflow - observed at a redshift of 0.483 - was detected in only one of seven spacecraft orbits, weaker (lower column) but more persistent absorption is seen - at a redshift of 0.123 - in the high exposure, soft X-ray (RGS) spectra summed over all 7 individual spacecraft orbits. Similar stacking of the higher energy (pn camera) spectra reveals underlying absorption at a redshift of 0.148. Interpreted, conventionally, as a Doppler redshift, the RGS observation indicates a line-of-sight inflow velocity v ~ 0.038c and (free-fall) radial location at 1400 R_g, with the higher redshift and ionization in pn camera spectra perhaps detecting that inflow closer to the black hole. A very different explanation would be absorption in matter subject to the strong gravity close to the SMBH, an interpretation supported by the launch of a new UFO in the final spacecraft orbit.

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arXiv2023

Observing the launch of an Eddington wind in the luminous Seyfert galaxy PG1211+143

The luminous narrow line Seyfert galaxy PG1211+143 was the first non-BAL AGN to reveal a powerful ionized wind, based on early observations with ESA's XMM-Newton X-ray Observatory. Subsequent observations, mainly with XMM-Newton and the Japanese Suzaku Observatory, found such winds to be a common feature of luminous AGN. Typical outflow velocities of v ~ 0.1c and flow momenta mv ~ L_Edd /c are consistent with winds being launched by continuum driving from a disc when the local mass accretion rate is super-Eddington. Here we report the launch of a new, ultra-fast outflow component in PG1211+143, near the end of a 5-week XMM-Newton observing campaign, and discuss its origin in an ultra-fast {\it inflow} detected some 3 weeks earlier. We note that the inflow lasted for at least 3 days and delivered at least 10 Earth mass of fresh material into the innermost region of the source. While this mass by itself is insufficient to cause a complete inner disc restructuring - a prediction supported by lack of change in simultaneous UV fluxes - we suggest that a ring of matter at R ~ 20 R_g, located via its gravitational redshift (Pounds and Page 2024), was subsequently accreted, leading to the launch of a new outflow at a velocity of v ~ 0.27c.

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arXiv2022

Are ultrafast inflows in AGN truly rare -- or just much harder to see?

Short-term variability and multiple velocity components in the powerful highly ionized wind of the archetypal UFO PG 1211+143 are indicative of inner disc instabilities or short-lived accretion events. The detection of a high velocity (~ 0.3c) inflow of highly ionized matter, located at 20 R_g, offered the first direct observational support for the latter scenario, where matter approaching at a high inclination to the black hole spin plane may result in warping and tearing of the inner accretion disc, with subsequent inter-ring collisions producing shocks, loss of rotational support and rapid mass infall. Simultaneous soft x-ray spectra reveal a lower velocity (~ 0.1c) inflow of less ionized matter, identified as 'upstream' at 200 R_g, with a line of sight through matter converging on the supermassive black hole. We discuss here why ultrafast ionized winds are relatively common in luminous Seyfert galaxies, while detection of the 0.3c inflow in PG 1211+143 remains a rare example.

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arXiv2021

Possible evidence for shock-cooling in the accretion flow of the luminous Seyfert galaxy PG1211+143

Short-term variability and multiple velocity components in the powerful highly ionized wind of the archetypal UFO PG1211+143 are indicative of inner disc instabilities or short-lived accretion events. The recent detection of a high velocity inflow offered the first direct observational support for the latter scenario, where matter approaching at a high inclination to the black hole spin plane may result in warping and tearing of the inner accretion disc, with subsequent inter-ring collisions producing shocks, loss of rotational support and rapid mass infall. Here we identify a variable continuum component in the same data set, well-modelled by a hot thermal Comptonised spectrum that could represent cooling radiation from the shocked gas.

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arXiv2018

An ultra-fast inflow in the luminous Seyfert PG1211+143

Blueshifted absorption lines in the X-ray spectra of AGN show that ultra-fast outflows with typical velocities $v \sim 0.1c$ are a common feature of these luminous objects. Such powerful AGN winds offer an explanation of the observed M-$σ$ relation linking the mass of the supermassive black hole and the velocity dispersion in the galaxy's stellar bulge. An extended XMM-Newton study of the luminous Seyfert galaxy PG1211+143 recently revealed a variable multi-velocity wind. Here we report the detection of a short-lived, ultra-fast inflow during the same observation. Previous reports of inflows used single absorption lines with uncertain identifications, but this new result identifies an array of resonance absorption lines of highly ionised Fe, Ca, Ar, S and Si, sharing a common redshift when compared with a grid of realistic photoionization spectra. The redshifted absorption arises in a column of highly ionized matter close to the black hole, with a line-of-sight velocity, $v \sim 0.3c$, inconsistent with the standard picture of a plane circular accretion disc. This may represent the first direct evidence for chaotic accretion in AGN, where accretion discs are generally misaligned to the black hole spin. For sufficient inclinations, the Lense-Thirring effect can break the discs into discrete rings, which then precess, collide and shock, causing near free-fall of gas towards the black hole. The observed accretion rate for the reported infall is comparable to the hard X-ray luminosity in PG1211+143, suggesting that direct infall may be a significant contributor to inner disc accretion.

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arXiv2018

X-ray time lags in PG 1211+143

We investigate the X-ray time lags of a recent ~630ks XMM-Newton observation of PG 1211+143. We find well-correlated variations across the XMM-Newton EPIC bandpass, with the first detection of a hard lag in this source with a mean time delay of up to ~3ks at the lowest frequencies. We find that the energy-dependence of the low-frequency hard lag scales approximately linearly with log(E) when averaged over all orbits, consistent with the propagating fluctuations model. However, we find that the low-frequency lag behaviour becomes more complex on timescales longer than a single orbit, suggestive of additional modes of variability. We also detect a high-frequency soft lag at ~10^{-4}Hz with the magnitude of the delay peaking at <0.8ks, consistent with previous observations, which we discuss in terms of small-scale reverberation.

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arXiv2018

The variable, fast soft X-ray wind in PG 1211+143

The analysis of a series of seven observations of the nearby ($z=0.0809$) QSO, PG 1211+143, taken with the Reflection Grating Spectrometer (RGS) on-board XMM-Newton in 2014, are presented. The high resolution soft X-ray spectrum, with a total exposure exceeding 600 ks, shows a series of blue-shifted absorption lines, from the He and H-like transitions of N, O and Ne, as well as from L-shell Fe. The strongest absorption lines are all systematically blue-shifted by $-0.06c$, originating in two absorption zones, from low and high ionization gas. Both zones are variable on timescales of days, with the variations in absorber opacity effectively explained by either column density changes or by the absorber ionization responding directly to the continuum flux. We find that the soft X-ray absorbers probably exist in a two-phase wind, at a radial distance of $10^{17}-10^{18}$ cm from the black hole, with the lower ionization gas as denser clumps embedded within a higher ionization outflow. The overall mass outflow rate of the soft X-ray wind may be as high as $2{\rm M}_{\odot}$ yr$^{-1}$, close to the Eddington rate for PG 1211+143 and similar to that previously deduced from the Fe K absorption.

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arXiv2017

Exploring accretion disc physics and black hole growth with regular monitoring of ultrafast AGN winds

15 years of XMM-Newton observations have established that ultra-fast, highly ionized winds are common in radio-quiet AGN. A simple theory of Eddington-limited accretion correctly predicts the typical velocity (~0.1c) and high ionization of such winds, with observed flow energy capable of ejecting star-forming gas. With a recent extended XMM-Newton observation of the archetypal UFO, PG1211+143, revealing a more complex flow pattern, we suggest that targetted observations over the next decade offer unique potential for probing the inner accretion disc structure and SMBH growth.

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arXiv2016

Testing relativistic reflection and resolving outflows in PG 1211+143 with XMM-Newton and NuSTAR

We analyze the broad-band X-ray spectrum (0.3-50 keV) of the luminous Seyfert 1 / quasar PG 1211+143 - the archetypal source for high-velocity X-ray outflows - using near-simultaneous XMM-Newton and NuSTAR observations. We compare pure relativistic reflection models with a model including the strong imprint of photoionized emission and absorption from a high-velocity wind (Pounds16a,16b), finding a spectral fit that extrapolates well over the higher photon energies covered by NuSTAR. Inclusion of the high S/N XMM-Newton spectrum provides much tighter constraints on the model parameters, with a much harder photon index / lower reflection fraction compared to that from the NuSTAR data alone. We show that pure relativistic reflection models are not able to account for the spectral complexity of PG 1211+143 and that wind absorption models are strongly required to match the data in both the soft X-ray and Fe K spectral regions. In confirming the significance of previously reported ionized absorption features, the new analysis provides a further demonstration of the power of combining the high throughput and resolution of long-look XMM-Newton observations with the unprecedented spectral coverage of NuSTAR.

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arXiv2016

Imprints of a high velocity wind on the soft x-ray spectrum of PG 1211+143

An extended XMM-Newton observation of the luminous narrow line Seyfert galaxy PG 1211+143 in 2014 has revealed a more complex high velocity wind, with components distinguished in velocity, ionization level, and column density. Here we report soft x-ray emission and absorption features from the ionized outflow, finding counterparts of both high velocity components, v ~ 0.129c and v ~ 0.066c, recently identified in the highly ionized Fe K absorption spectrum. The lower ionization of the co-moving soft x-ray absorbers imply a distribution of higher density clouds embedded in the main outflow, while much higher column densities for the same flow component in the hard x-ray spectra suggest differing sight lines to the continuum x-ray source.

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arXiv2016

Detection of a second high velocity component in the highly ionized wind from PG 1211+143

An extended XMM-Newton observation of the luminous narrow line Seyfert galaxy PG 1211+143 in 2014 has revealed a more complex highly ionized, high velocity outflow. The detection of previously unresolved spectral structure in Fe K absorption finds a second outflow velocity component of the highly ionized wind, with an outflow velocity of v~0.066+/-0.003c, in addition to a still higher velocity outflow of v~0.129+/-0.002c consistent with that first seen in 2001. We note that chaotic accretion, consisting of many prograde and retrograde events, offers an intriguing explanation of the dual velocity wind. In that context the persisting outflow velocities could relate to physically distinct orientations of the inner accretion flow, with prograde accretion yielding a higher launch velocity than retrograde accretion in a ratio close to that observed.

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arXiv2015

X-ray timing analysis of the quasar PG 1211+143

We report on a timing analysis of a new ~630ks XMM-Newton observation of the quasar, PG 1211+143. We find a well-defined X-ray power spectrum with a well-detected bend at ~7e-5 Hz, consistent with the established bend-timescale--black-hole-mass correlation for luminous, accreting black holes. We find the linear rms-flux relation commonly observed in accreting black hole systems and investigate the energy-dependence of the rms. The fractional rms is roughly constant with energy on short timescales (< 1 day; within observations) whereas there is enhanced soft band variability on long timescales (between observations typically spaced by a few days). Additionally, we also report on the optical--UV variability using the OM on-board XMM-Newton and a ~2-month-long overlapping monitoring programme with Swift. We find that, although there is little UV variability within observations (<1 day), UV variations of a few per cent exist on time-scales of ~days--weeks.

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arXiv2015

Powerful Outflows and Feedback from Active Galactic Nuclei

Active Galactic Nuclei (AGN) represent the growth phases of the supermassive black holes in the center of almost every galaxy. Powerful, highly ionized winds, with velocities $\sim 0.1- 0.2c$ are a common feature in X--ray spectra of luminous AGN, offering a plausible physical origin for the well known connections between the hole and properties of its host. Observability constraints suggest that the winds must be episodic, and detectable only for a few percent of their lifetimes. The most powerful wind feedback, establishing the $M -σ$ relation, is probably not directly observable at all. The $M - σ$ relation signals a global change in the nature of AGN feedback. At black hole masses below $M-σ$ feedback is confined to the immediate vicinity of the hole. At the $M-σ$ mass it becomes much more energetic and widespread, and can drive away much of the bulge gas as a fast molecular outflow.

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arXiv2013

X-ray observations of powerful AGN outflows - implications for feedback

Highly ionised winds with velocities ~ 0.1-0.2c were first detected in X-ray spectra of non-BAL AGN a decade ago. Subsequent observations and archival searches have shown such winds to be a common feature of luminous AGN, increasing the belief that powerful ionised winds have a wider importance in galaxy feedback models. Paradoxically, for the best-quantified high velocity outflow (the luminous Seyfert PG1211+143) the wind appears too powerful to be compatible with the observed stellar bulge and black hole masses, suggesting the energy coupling of wind to bulge gas must be inefficient. A recent XMM-Newton observation of the narrow line Seyfert NGC 4051 offers an explanation of this apparent paradox, finding evidence for the fast ionised wind to lose most of its kinetic energy after shocking against the ISM. Importantly, the wind momentum is maintained through such a shock, supporting the view that a momentum-driven flow provides the critical link between black hole and stellar bulge masses implied by the observed M-sigma relationship.

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arXiv2013

Searching for Black Holes in Space - the key role of X-ray observations

Although General Relativity had provided the physical basis of black holes, evidence for their existence had to await the Space Era when X-ray observations first directed the attention of astronomers to the unusual binary stars Cygnus X-1 and A0620-00. Subsequently, a number of faint Ariel 5 and Uhuru sources, mainly at high Galactic latitude, were found to lie close to bright Seyfert galaxies,suggesting the nuclear activity in AGN might also be driven by accretion in the strong gravity of a black hole. Detection of rapid X-ray variability with EXOSAT later confirmed that the accreting object in AGN was almost certainly a supermassive black hole.

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arXiv2013

The shocked outflow in NGC 4051 - momentum-driven feedback, UFO's and warm absorbers

An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 in 2009 revealed an unusually rich absorption spectrum with outflow velocities, in both RGS and EPIC spectra, up to ~ 9000 km/s (Pounds and Vaughan 2011). Evidence was again seen for a fast ionised wind with velocity ~ 0.12c (Tombesi 2010, Pounds and Vaughan 2012). Detailed modelling with the XSTAR photoionisation code now confirms the general correlation of velocity and ionisation predicted by mass conservation in a Compton-cooled shocked wind (King 2010). We attribute the strong column density gradient in the model to the addition of strong two-body cooling in the later stages of the flow, causing the ionisation (and velocity) to fall more quickly, and confining the lower ionisation gas to a narrower region. The column density and recombination timescale of the highly ionised flow component, seen mainly in Fe K lines, determine the primary shell thickness which, when compared with the theoretical Compton cooling length, determines a shock radius of ~ 10^17 cm. Variable radiative recombination continua (RRC) provide a key to scaling the lower ionisation gas, with the RRC flux then allowing a consistency check on the overall flow geometry. We conclude that the 2009 observation of NGC 4051 gives strong support to the idea that a fast, highly ionised wind, launched from the vicinity of the supermassive black hole, will lose much of its mechanical energy after shocking against the ISM at a sufficiently small radius for strong Compton cooling. However, the total flow momentum will be conserved, retaining the potential for a powerful AGN wind to support momentum-driven feedback (King 2003; 2005). We speculate that the `warm absorber' components often seen in AGN spectra result from accumulation of shocked wind and ejected ISM.

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arXiv2008

On why the Iron K-shell absorption in AGN is not a signature of the local Warm/Hot Intergalactic Medium

We present a comparison between the 2001 XMM-Newton and 2005 Suzaku observations of the quasar, PG1211+143 at z=0.0809. Variability is observed in the 7 keV iron K-shell absorption line (at 7.6 keV in the quasar frame), which is significantly weaker in 2005 than during the 2001 XMM-Newton observation. From a recombination timescale of <4 years, this implies an absorber density n>0.004 particles/cm3, while the absorber column is 5e22<N_H <1 1e24 particles/cm2. Thus the sizescale of the absorber is too compact (pc scale) and the surface brightness of the dense gas too high (by 9-10 orders of magnitude) to arise from local hot gas, such as the local bubble, group or Warm/Hot Intergalactic Medium (WHIM), as suggested by McKernan et al. (2004, 2005). Instead the iron K-shell absorption must be associated with an AGN outflow with mildly relativistic velocities. Finally we show that the the association of the absorption in PG1211+143 with local hot gas is simply a coincidence, the comparison between the recession and iron K absorber outflow velocities in other AGN does not reveal a one to one kinematic correlation.

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arXiv2005

Exploring X-ray emission and absorption in AGN with XMM-Newton EPIC

Broad band X-ray spectra of high statistical quality obtained with the EPIC instruments on XMM-Newton are being used to address key issues in AGN Physics, including the evidence for strong gravity, massive and energetic outflows and the effects of reflection and absorption in cold dense matter. A progress report on these topics is presented by reference to high quality EPIC spectra of 5 bright Seyfert galaxies covering a wide range in luminosity.

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