arXiv subjects

K. A. Pounds

Publications and source records attributed to K. A. Pounds.

At least 19 recordsLinked to original sources

arXiv2013

The soft X-ray spectrum of the luminous narrow line Seyfert galaxy PG1211+143 - evidence for a second high velocity outflow component

An XMM-Newton observation of the luminous Seyfert galaxy PG1211+143 in 2001 revealed the first clear evidence for a highly ionised high speed wind (in a non-BAL AGN), with a velocity of v~0.09c based on the identification of blue-shifted absorption lines in both EPIC and RGS spectra. A subsequent analysis of EPIC spectra, including additional absorption lines, led to an upward revision of the wind speed to ~0.14c, while broad band modelling indicated the need for a second, partial covering absorber to account for continuum curvature and spectral variability. We show here, in a new analysis of the XMM-Newton RGS data, that this additional absorber is detected in the soft X-ray spectra, with the higher spectral resolution providing a much improved velocity constraint, with v~0.07c. Similar variability of the 0.07c and 0.14c outflow components suggest they are physically linked, and we speculate that occurs by the fast (primary) wind impacting on small clumps of higher density, slow moving matter close to the disc. We show that strong, velocity broadened soft X-ray emission features, located at the redshift of PG1211+143, indicate the extended scale of the ionised outflow.

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arXiv2013

The End of the Black Hole Dark Ages, and Warm Absorbers

We consider how the radiation pressure of an accreting supermassive hole (SMBH) affects the interstellar medium around it. Much of the gas originally surrounding the hole is swept into a shell with a characteristic radius somewhat larger than the black hole's radius of influence ($\sim$ 1-100 pc). The shell has a mass directly comparable to the ($M - σ$) mass the hole will eventually reach, and may have a complex topology. We suggest that outflows from the central supermassive black holes are halted by collisions with the shell, and that this is the origin of the warm absorber components frequently seen in AGN spectra. The shell may absorb and reradiate some of the black hole accretion luminosity at long wavelengths, implying both that the bolometric luminosities of some known AGN may have been underestimated, and that some accreting SMBH may have escaped detection entirely.

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arXiv2012

An extended XMM-Newton observation of the Seyfert galaxy NGC 4051 - III. FeK emission and absorption

An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 in 2009 detected a photo-ionized outflow with a complex absorption line velocity structure and a broad correlation of velocity with ionization parameter, shown in Pounds et al (2011) to be consistent with a highly ionized, high velocity wind running into the interstellar medium or previous ejecta, losing much of its kinetic energy in the resultant strong shock. In the present paper we examine the Fe K spectral region in more detail and find support for two distinct velocity components in the highly ionized absorber, with values corresponding to the putative fast wind (~ 0.12c) and the post-shock flow (v ~ 5000-7000 km/s). The Fe K absorption line structure is seen to vary on a orbit-to-orbit timescale, apparently responding to both a short term increase in ionizing flux and - perhaps more generally - to changes in the soft X-ray (and simultaneous UV) luminosity. The latter result is particularly interesting in providing independent support for the existence of shocked gas being cooled primarily by Compton scattering of accretion disc photons. The Fe K emission is represented by a narrow fluorescent line from near-neutral matter, with a weak red wing modelled here by a relativistic diskline. The narrow line flux is quasi-constant throughout the 45-day 2009 campaign, but is resolved, with a velocity width consistent with scattering from a component of the post-shock flow. Evidence for a P Cygni profile is seen in several individual orbit spectra for resonance transitions in both Fe XXV and Fe XXVI.

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arXiv2011

An extended XMM-Newton observation of the Seyfert galaxy NGC 4051. II. Soft X-ray emission from a limb-brightened shell of post-shock gas

An extended XMM-Newton observation of the Seyfert I galaxy NGC 4051 in 2009 revealed a complex absorption spectrum, with a wide range of outflow velocities and ionisation states.The main velocity and ionisation structure was interpreted in Paper I in terms of a decelerating, recombining flow resulting from the shocking of a still higher velocity wind colliding with the ISM or slower moving ejecta. The high sensitivity of the XMM-Newton observation also revealed a number of broad emission lines, all showing evidence of self-absorption near the line cores. The line profiles are found here to be consistent with emission from a limb-brightened shell of post-shock gas building up ahead of the contact discontinuity. While the broad emission lines remain quasi-constant as the continuum flux changes by an order of magnitude, recombination continua of several H- and He-like ions are found to vary in response to the continuum, providing an important key to scaling the ionised flow.

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arXiv2011

The rapid X-ray variability of NGC 4051

We present an analysis of the high frequency X-ray variability of NGC 4051 (M_BH ~ 1.7*10^6 M_sun) based on a series of XMM-Newton observations taken in 2009 with a total exposure of ~570 ks (EPIC pn). These data reveal the form of the power spectrum over frequencies from 10^-4 Hz, below the previously detected power spectral break, to > 10^-2 Hz, above the frequency of the innermost stable circular orbit (ISCO) around the black hole (nu_ISCO ~ 10^-3 - 10^-2 Hz, depending on the black hole spin parameter j). This is equivalent to probing frequencies >1 kHz in a stellar mass (M_BH ~ 10 Msun) black hole binary system. The power spectrum is a featureless power law over the region of the expected ISCO frequency, suggesting no strong enhancement or change in the variability at the fastest orbital period in the system. Despite the huge amplitude of the flux variations between the observations (peak-to-peak factor of >50) the power spectrum appears to be stationary in shape, and varies in amplitude at all observed frequencies following the previously established linear rms-flux relation. The rms-flux relation is offset in flux by a small and energy-dependent amount. The simplest interpretation of the offset is in terms of a very soft spectral component that is practically constant (compared to the primary source of variability). One possible origin for this emission is a circumnuclear shock energised by a radiatively driven outflow from the central regions, and emitting via inverse-Compton scattering of the central engine's optical-UV continuum (as inferred from a separate analysis of the energy spectrum). A comparison with the power spectrum of a long XMM-Newton observation taken in 2001 gives only weak evidence for non-stationarity in power spectral shape or amplitude. [Abridged]

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arXiv2010

An extended XMM-Newton observation of the Seyfert galaxy NGC 4051. I. Evidence for a shocked outflow

An extended XMM-Newton observation of the Seyfert 1 galaxy NGC 4051 has revealed a rich absorption line spectrum indicating the presence of a photoionized outflow with a wide range of velocities and ionization parameter. At low continuum fluxes an emission line spectrum is well defined with both narrow and broad components of several abundant metal ions. The absorption line velocity structure and a broad correlation of velocity and ionization parameter are consistent with an outflow scenario where a highly ionized, high velocity wind, perhaps launched during intermittent super-Eddington accretion, runs into the interstellar medium or previous ejecta, losing much of its kinetic energy in the resultant strong shock. We explore the possibility that a quasi-constant soft X-ray emission component may be evidence of this post-shock cooling. This revised view of AGN outflows is consistent with multiple minor Eddington accretion episodes creating a momentum-driven feedback linking black hole and host galaxy growth.

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arXiv2009

Quantifying the fast outflow in the luminous Seyfert galaxy PG1211+143

We report two new XMM-Newton observations of PG1211+143 in December 2007, again finding evidence of the fast outflow of highly ionised gas first detected in 2001. Stacking the new spectra with those from two earlier XMM-Newton observations reveals strong and broad emission lines of FeXXV and OVIII, indicating the fast outflow to be persistent and to have a large covering factor. This finding confirms a high mass rate for the ionised ouflow in PG1211+143 and provides the first direct measurement of a wide angle, sub-relativistic outflow from an AGN transporting mechanical energy with the potential to disrupt the growth of the host galaxy. We suggest PG1211+143 may be typical of an AGN in a rapid super-Eddington growth phase.

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arXiv2009

Suzaku Observation of a Hard Excess in 1H 0419-577: Detection of a Compton-Thick Partial-Covering Absorber

We present results from a 200 ks Suzaku observation of 1H 0419-577 taken during 2007 July. The source shows a strong excess of counts above 10 keV compared to the extrapolation of models based on previous data in the 0.5-10 keV band. The 'hard excess' in 1H 0419-577 can be explained by the presence of a Compton-thick partial-covering absorber that covers ~ 70% of the source. The Compton-thick gas likely originates from a radius inside of the optical BLR and may form part of a clumpy disk wind. The fluorescent Fe Ka luminosity measured by Suzaku is consistent with that expected from an equatorial disk wind.

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arXiv2009

The Growth of Supermassive Black Holes Across Cosmic Time

One of the main themes in extragalactic astronomy for the next decade will be the evolution of galaxies over cosmic time. Many future observatories, including JWST, ALMA, GMT, TMT and E-ELT will intensively observe starlight over a broad redshift range, out to the dawn of the modern Universe when the first galaxies formed. It has, however, become clear that the properties and evolution of galaxies are intimately linked to the growth of their central black holes. Understanding the formation of galaxies, and their subsequent evolution, will therefore be incomplete without similarly intensive observations of the accretion light from supermassive black holes (SMBH) in galactic nuclei. To make further progress, we need to chart the formation of typical SMBH at z>6, and their subsequent growth over cosmic time, which is most effectively achieved with X-ray observations. Recent technological developments in X-ray optics and instrumentation now bring this within our grasp, enabling capabilities fully matched to those expected from flagship observatories at longer wavelengths.

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arXiv2007

Comparison of high and low state X-ray spectra in the Type 1 QSO 2MASS 0918+2117

When observed by XMM-Newton in 2003 the type 1 QSO 2MASS 0918+2117 was found to in a low flux state, some ~4-5 times fainter than during an earlier Chandra observation. The 2-6 keV spectrum was unusually hard, with evidence for a reflection-dominated continuum, while a soft excess prevented confirmation of the anticipated low energy absorber. In a second XMM-Newton observation in 2005, the X-ray flux is found to have recovered, with a 2-10 keV continuum slope now typical of broad-line active galaxy, and clear evidence for low energy absorption. We find the preferred ionisation state of the absorbing gas to be low, consistent with the red nucleus and strong optical polarisation of 2MASS 0918+2117. A residual soft excess is of similar spectral form and flux to that seen in 2003.

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arXiv2006

The soft X-ray spectrum of PG1211+143

The narrow line QSO PG1211+143 has been a focus of recent attempts to understand the soft excess in AGN, while the 2001 XMM-Newton observation of this luminous AGN also provided evidence for a massive and energetic outflow. Here we consider a physical link between the energetic outflow and the variable soft excess.

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arXiv2006

Is the X-ray spectrum of the narrow emission line QSO PG1211+143 defined by its energetic outflow?

An XMM-Newton observation of the bright QSO PG1211+143 in 2001 revealed a blue-shifted absorption line spectrum indicative of a high velocity radial outflow of highly ionised gas. Unless highly collimated, the outflow mass rate was shown to be comparable to the accretion rate, with mechanical energy a significant fraction of the bolometric luminosity. Analysis of the full XMM-Newton data set now allows the wider effects of that energetic outflow to be explored. We find that absorption and re-emission of the primary continuum flux in the ionised outflow, together with a second, less strongly absorbed, continuum component can explain the strong `soft excess' in PG1211+143 without the extreme velocity `smearing' in conflict with observed absorption line widths. Previously unpublished data from a second XMM-Newton observation of PG1211+143 is shown to be consistent with the new spectral model, finding that the additional continuum component dominates the spectral variability. We speculate that this variable continuum component is powered by the high velocity outflow.

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arXiv2006

Confirming the high velocity outflow in PG 1211+143

An XMM-Newton observation of the bright QSO PG1211+143 was previously reported to show evidence for a massive, energetic outflow, with an outflow velocity of v~0.1c based on the identification of blue-shifted absorption lines detected in both EPIC and RGS spectra. Subsequently, an order-of-magnitude lower velocity has been claimed from an ion-by-ion model fit to the RGS data. We show here, in a re-analysis of the higher signal-to-noise EPIC data, that the high velocity is confirmed, with the resolution of additional absorption lines yielding a revised outflow velocity in the range ~0.13-0.15c. Confirmation of a massive and energetic outflow in a non-BAL AGN has important implications for metal enrichment of the IGM and for the feedback mechanism implied by the correlation of black hole and galactic bulge masses. We note the near-Eddington luminosity of PG1211+143 may be the critical factor in driving such an energetic outflow, a situation likely to be common in AGN at higher redshift.

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arXiv2005

XMM-NEWTON Observations of Red AGN

XMM-Newton spectra of five red, 2MASS AGN, selected from a sample observed by Chandra to be relatively X-ray bright and to cover a range of hardness ratios, confirm the presence of substantial absorbing material in three sources with optical classifications ranging from Type 1 to Type 2. A flat (hard), power law continuum is observed in the other two. The combination of X-ray absorption and broad optical emission lines suggests either a small (nuclear) absorber or a favored viewing angle so as to cover the X-ray source but not the broad emission line region (BELR). A soft excess is detected in all three Type 1 sources. We speculate that this may arise in an extended region of ionised gas, perhaps linked with the polarised (scattered) optical light present in these sources. The spectral complexity revealed by XMM-Newton emphasizes the limitations of the low S/N \chandra data. The new results strengthen our earlier conclusions that the observed X-ray continua of red AGN are unusually hard at energies >2 keV. Their observed spectra are consistent with contributing significantly to the missing hard/absorbed population of the Cosmic X-ray Background (CXRB) although their intrinsic power law slopes are typical of broad-line (Type 1) AGN (Gamma ~1.7-1.9). This suggests that the missing X-ray-absorbed CXRB population may include Type 1 AGN/QSOs in addition to the Type 2 AGN generally assumed.

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arXiv2005

X-ray absorption and re-emission from an ionised outflow in the Type 1 QSO 2MASS 234449+1221

We report on the analysis of a short XMM-Newton observation of the reddened Type 1 QSO 2MASS 234449+1221 first identified in the Two Micron All-Sky Survey. The underlying X-ray continuum is found to be typical of a broad-line active galaxy, with photon index Gamma ~ 1.9. Low energy absorption can be modelled by a column N_H ~ 10^22 cm^{-2} of moderately ionised gas or a smaller column of cold gas. Addition of a soft X-ray emission component significantly improves the fit in both cases. With the assumption that the soft X-ray flux represents emission from gas photoionised by the incident X-ray continuum, a comparison of the absorbed and emitted luminosities indicates a covering factor of ~ 8-17%. The unusual opportunity to simultaneously observe and quantify ionised absorption and emission in 2MASS 234449+1221 is due to the relatively large opacity (for a Type 1 AGN) of the absorbing gas, which depresses the normally strong continuum below ~ 1 keV. A comparison of the soft X-ray emission of 2MASS 234449+1221 with that of other Type 1 and Type 2 AGN suggests the existence of an inner turbulent extension to ionised outflows, not detected in current high resolution X-ray spectra.

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arXiv2005

An XMM-Newton observation of Mrk 3 - a Seyfert galaxy just over the edge

A 100ks XMM-Newton observation of the nearby Seyfert 2 galaxy Mrk 3 offers a unique opportunity to explore the complexity of its X-ray spectrum. We find the \~3-8 keV continuum to be dominated by reflection from cold matter, with fluorescent K-shell lines detected from Ni, Fe, Ca, Ar, S, Si and Mg. At higher energies an intrinsic power law continuum, with canonical Seyfert 1 photon index, is seen through a near-Compton-thick cold absorber. A soft excess below \~3 keV is found to be dominated by line emission from an outflow of `warm' gas, photo-ionised and photo-excited by the intrinsically strong X-ray continuum. Measured blue-shifts in the strong Fe K-alpha and OVII and VIII emission lines are discussed in terms of the properties of the putative molecular torus and ionised outflow.

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arXiv2004

A high velocity ionised outflow and XUV photosphere in the narrow emission line quasar PG1211+143

We report on the analysis of a ~60 ksec XMM observation of the bright, narrow emission line quasar PG 1211+143. Absorption lines are seen in both EPIC and RGS spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionised outflow velocity of ~24000 km s^-1. The highest energy lines require a column density of N_H ~ 5 x 10^23 cm^-2, at an ionisation parameter of log(xi) ~ 3.4. If the origin of this high velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius ~130 Schwarzschild radii, providing a natural explanation for the Big Blue Bump (and strong soft X-ray) emission in PG 1211+143.

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arXiv2004

Resolving the large scale spectral variability of the luminous Seyfert 1 galaxy 1H 0419-577: Evidence for a new emission component and absorption by cold dense matter

An XMM-Newton observation of the luminous Seyfert 1 galaxy 1H 0419-577 in September 2002, when the source was in an extreme low-flux state, found a very hard X-ray spectrum at 1-10 keV with a strong soft excess below ~1 keV. Comparison with an earlier XMM-Newton observation when 1H 0419-577 was `X-ray bright' indicated the dominant spectral variability was due to a steep power law or cool Comptonised thermal emission. Four further XMM-Newton observations, with 1H 0419-577 in intermediate flux states, now support that conclusion, while we also find the variable emission component in intermediate state difference spectra to be strongly modified by absorption in low ionisation matter. The variable `soft excess' then appears to be an artefact of absorption of the underlying continuum while the `core' soft emission can be attributed to recombination in an extended region of more highly ionised gas. We note the wider implications of finding substantial cold dense matter overlying (or embedded in) the X-ray continuum source in a luminous Seyfert 1 galaxy.

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