arXiv subjects

Martin Ward

Publications and source records attributed to Martin Ward.

At least 37 records · Page 2Linked to original sources

arXiv2020

State-of-the-art AGN SEDs for Photoionization Models: BLR Predictions Confront the Observations

The great power offered by photoionization models of Active Galactic Nuclei (AGN) emission-line regions has long been mitigated by the fact that very little is known about the spectral energy distribution (SED) between the Lyman limit, where intervening absorption becomes a problem, and 0.3 keV, where soft x-ray observations become possible. The emission lines themselves can, to some degree, be used to probe the SED, but only in the broadest terms. This paper employs a new generation of theoretical SEDs which are internally self-consistent, energy-conserving, and tested against observations, to infer properties of the emission-line regions. The SEDs are given as a function of the Eddington ratio, allowing emission-line correlations to be investigated on a fundamental basis. We apply the simplest possible tests, based on the foundations of photoionization theory, to investigate the implications for the geometry of the emission-line region. The SEDs become more far-ultraviolet bright as the Eddington ratio increases, so the equivalent widths of recombination lines should also become larger, an effect which we quantify. The observed lack of correlation between Eddington ratio and equivalent width shows that the cloud covering factor must decrease as Eddington ratio increases. This would be consistent with recent models proposing that the broad-line region is a failed dusty wind off the accretion disc.

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arXiv2017

Determining the torus covering factors for a sample of type 1 AGN in the local Universe

In the unified scheme of active galactic nuclei, a dusty torus absorbs and then reprocesses a fraction of the intrinsic luminosity which is emitted at longer wavelengths. Thus, subject to radiative transfer corrections, the fraction of the sky covered by the torus as seen from the central source (known as the covering factor $f_c$) can be estimated from the ratio of the infrared to the bolometric luminosities of the source as $f_c=L_{\rm torus}/L_{\rm Bol}$. However, the uncertainty in determining $L_{\rm Bol}$ has made the estimation of covering factors by this technique difficult, especially for AGN in the local Universe where the peak of the observed SEDs lies in the UV (ultraviolet). Here, we determine the covering factors of an X-ray/optically selected sample of 51 type~1 AGN. The bolometric luminosities of these sources are derived using a self-consistent, energy-conserving model that estimates the contribution in the unobservable far-UV region, using multi-frequency data obtained from SDSS, \textit{XMM-Newton}, \textit{WISE}, 2MASS and UKIDSS. We derive a mean value of $f_c\sim$0.30 with a dispersion of 0.17. Sample correlations, combined with simulations, show that $f_c$ is more strongly anti-correlated with $λ_{\rm Edd}$ than with $L_{\rm Bol}$. This points to large-scale torus geometry changes associated with the Eddington-dependent accretion flow, rather than a receding torus, with its inner sublimation radius determined solely by heating from the central source. Furthermore, we do not see any significant change in the distribution of $f_c$ for sub-samples of radio-loud sources or Narrow Line Seyfert~1 galaxies (NLS1s), though these sub-samples are small.

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arXiv2017

Super-Eddington QSO RX J0439.6-5311. II. Multi-wavelength Constraints on the Global Structure of the Accretion Flow

We present a detailed multi-wavelength study of an unobscured, highly super-Eddington Type-1 QSO RX J0439.6-5311. We combine the latest XMM-Newton observation with all archival data from infrared to hard X-rays. The optical spectrum is very similar to that of 1H 0707-495 in having extremely weak [O {\sc iii}] and strong Fe {\sc ii} emission lines, although the black hole mass is probably slightly higher at $5-10 \times10^{6}~\rm M_{\odot}$. The broadband SED is uniquely well-defined due to the extremely low Galactic and intrinsic absorption, so the bolometric luminosity is tightly constrained. The optical/UV accretion disc continuum is seen down to 900 Å, showing that there is a standard thin disc structure down to $R \ge$ 190-380 $R_{\rm g}$ and determining the mass accretion rate through the outer disc. This predicts a much higher bolometric luminosity than observed, indicating that there must be strong wind and/or advective energy losses from the inner disc, as expected for a highly super-Eddington accretion flow. Significant outflows are detected in both the NLR and BLR emission lines, confirming the presence of a wind. We propose a global picture for the structure of a super-Eddington accretion flow where the inner disc puffs up, shielding much of the potential NLR material, and show how inclination angle with respect to this and the wind can explain very different X-ray properties of RX J0439.6-5311 and 1H 0707-495. Therefore, this source provides strong supporting evidence that `simple' and `complex' super-Eddington NLS1s can be unified within the same accretion flow scenario but with different inclination angles. We also propose that these extreme NLS1s could be the low-redshift analogs of weak emission-line quasars (WLQs).

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arXiv2017

Super-Eddington QSO RX J0439.6-5311. I. Origin of the Soft X-ray Excess and Structure of the Inner Accretion Flow

We report the results from a recent 133 ks XMM-Newton observation of a highly super-Eddington narrow-line Type-1 QSO RX J0439.6-5311. This source has one of the steepest AGN hard X-ray slopes, in addition to a prominent and smooth soft X-ray excess. Strong variations are found throughout the 0.3 to 10 keV energy range on all time-scales covered by the observation, with the soft excess mainly showing low frequency variations below 0.1 mHz while the hard X-rays show stronger variability at higher frequencies. We perform a full set of spectral-timing analysis on the X-ray data, including a simultaneous modelling of the time-average spectra, frequency-dependent RMS and covariance spectra, lag-frequency and lag-energy spectra. Especially, we find a significant time-lag signal in the low frequency band, which indicates that the soft X-rays lead the hard by $\sim$4 ks, with a broad continuum-like profile in the lag spectrum. Our analysis strongly supports the model where the soft X-ray excess is dominated by a separate low temperature, optically thick Comptonisation component rather than relativistic reflection or a jet. This soft X-ray emitting region is several tens or hundreds of $R_{g}$ away from the hot corona emitting hard X-rays, and is probably associated with a geometrically thick (`puffed-up') inner disc region.

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arXiv2016

The habitability of the Universe through 13 billion years of cosmic time

The field of astrobiology has made tremendous progress in modelling galactic-scale habitable zones which offer a stable environment for life to form and evolve in complexity. Recently, this idea has been extended to cosmological scales by studies modelling the habitability of the local Universe in its entirety (e.g. Dayal et al. 2015; Li & Zhang 2015). However, all of these studies have solely focused on estimating the potentially detrimental effects of either Type II supernovae (SNII) or Gamma Ray Bursts (GRBs), ignoring the contributions from Type Ia supernovae (SNIa) and active galactic nuclei (AGN). In this study we follow two different approaches, based on (i) the amplitude of deleterious radiation and (ii) the total planet-hosting volume irradiated by deleterious radiation. We simultaneously track the contributions from the key astrophysical sources (SNII, SNIa, AGN and GRBs) for the entire Universe, for both scenarios, to determine its habitability through 13.8 billion years of cosmic time. We find that SNII dominate the total radiation budget and the volume irradiated by deleterious radiation at any cosmic epoch closely followed by SNIa (that contribute half as much as SNII), with GRBs and AGN making up a negligible portion (<1%). Secondly, as a result of the total mass in stars (or the total number of planets) slowly building-up with time and the total deleterious radiation density, and volume affected, falling-off after the first 3 billion years, we find that the Universe has steadily increased in habitability through cosmic time. We find that, depending on the exact model assumptions, the Universe is 2.5 to 20 times more habitable today compared to when life first appeared on the Earth 4 billion years ago. We find that this increase in habitability will persist until the final stars die out over the next hundreds of billions of years.

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arXiv2015

The Best Constraints on A Super-Eddington Accretion Flow: XMM-Newton Observations of An Intermediate-mass Black Hole

RX J1140.1+0307 (hereafter RX1140) is a Narrow Line Seyfert 1 (NLS1) with one of the lowest black hole masses known in an AGN (M ${\le} 10^6$ M$_{\odot}$). We show results from two new {\it XMM-Newton} observations, showing soft 2-10~keV spectra, a strong excess at lower energies, and fast X-ray variability as is typical of this class. The soft excess can be equally well fit by either low temperature Comptonisation or highly smeared, ionised reflection models, but we use a covariance analysis of the fast X-ray variability as well as lag and coherence spectra to show that the low temperature Comptonisation model gives a better description of the break in variability properties between soft and hard X-rays. Both models also require an additional component at the softest energies, as expected from the accretion disc. However, this inner disc spectrum does not join smoothly onto the variable optical and far UV emission (which should be produced in the outer disc) unless the mass is underestimated by an order of magnitude. The variable optical and far UV emission instead suggests that $L/L_{Edd}\sim 10$ through the outer disc, in which case advection and/or wind losses are required to explain the observed broadband spectral energy distribution. However, the similarity of the X-ray properties of RX1140 to other simple NLS1 such as PG 1244+026, RE J1034+396 and RX~J0136 means it is likely that these are also super-Eddington sources. This means their spectral energy distribution cannot be used to determine black hole spin despite appearing to be disc dominated. It also means that the accretion geometry close to the black hole is unlikely to be a flat disc as assumed in the new X-ray reverberation mapping techniques.

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arXiv2013

A Long XMM-Newton Observation of An Extreme Narrow Line Seyfert 1: PG 1244+026

We explore the origin of the strong soft X-ray excess in Narrow Line Seyfert 1 galaxies using spectral-timing information from a 120ks {\it XMM-Newton} observation of PG 1244+026. Spectral fitting alone cannot distinguish between a true additional soft X-ray continuum component and strongly relativistically smeared reflection, but both models also require a separate soft blackbody component. This is most likely intrinsic emission from the disc extending into the lowest energy X-ray bandpass. The {\it RMS} spectra on short timescales (200-5000s) contain both (non-disk) soft excess and power law emission. However, the spectrum of the variability on these timescales correlated with the 4-10 keV lightcurve contains only the power law. Together these show that there is fast variability of the soft excess which is independent of the 4-10 keV variability. This is inconsistent with a single reflection component making the soft X-ray excess as this necessarily produces correlated variability in the 4-10 keV bandpass. Instead, the {\it RMS} and covariance spectra are consistent with an additional cool Comptonisation component which does not contribute to the spectrum above 2 keV.

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arXiv2013

Modeling the Nuclear Infrared Spectral Energy Distribution of Type II Active Galactic Nuclei

We present results from model fitting to the Spectral Energy Distribution (SED) of a homogeneous sample of Seyfert II galaxies drawn from the $12μ$m Galaxy Sample. Imaging and nuclear flux measurements are presented in an accompanying paper (Videla et al., 2013). Here we add IRS Spitzer observations to further constrain the SEDs after careful subtraction of a starburst component. We use the library of CLUMPY torus models from Nenkova et al.~(2008ab) and also test the two-phase models recently produced by Stalevski et al.~(2012). We find that photometric and spectroscopic observations in the mid-IR (>5mu) are crucial to properly constrain the best-fit torus models. About half of our sources show clear near-IR excess of their SEDs above the best fit models. This problem can be less severe when using the Stalevski et al.~(2012) models. It is not clear what is the nature of this emission since best fitted black body temperatures are very high (~1700-2500 K) and the Type II classification of our sources would correspond to a small probability to peer directly into the hottest regions of the torus. Crucially, the derived torus parameters when using CLUMPY models are quite robust,, independently of whether the sources require an additional black body component or not. Our findings suggest that tori are characterized by N_0 > 5, sigma > 40, tau < 25, i > 40 degrees, Y < 50 and A_v^los ~ 100-300. From these we can determine that typical torus sizes and masses of 0.1-5.0 pc and 10^{4-6} M_\odot. We find tentative evidence that those nuclei with a detected Hidden Broad Line Regions are characterized by lower levels of extinction than those without one. Finally, we find no correlation between the torus properties and the presence of circumnuclear or more global star-formation.

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arXiv2013

Nuclear Infrared Spectral Energy Distribution of Type II Active Galactic Nuclei

We present near and mid--IR observations of a sample of Seyfert II galaxies drawn from the $12μ$m Galaxy Sample. The sample was observed in the J, H, K, L, M and N bands. Galaxy Surface Brightness Profiles are modeled using nuclear, bulge, bar (when necessary) and disk components. To check the reliability of our findings the procedure was tested using {\em Spitzer\/} observations of M\,31. Nuclear Spectral Energy Distributions (SEDs) are determined for 34 objects, and optical spectra are presented for 38, including analysis of their stellar populations using the STARLIGHT spectral synthesis code. Emission line diagnostic-diagrams are used to discriminate between genuine AGN and HII nuclei. Combining our observations with those found in the literature, we have a total of 40 SEDs. It is found that about 40\%\ of the SEDs are characterized by an upturn in the near-IR, which we have quantified as a NIR slope $α< 1$ for an SED characterized as $λf_λ \propto λ^α$. Three objects with an HII nucleus and two Seyfert nuclei with strong contamination from a circumnuclear starburst, also show an upturn. For genuine AGN this component could be explained as emission from the accretion disk, a jet, or from a very hot dust component leaking from the central region through a clumpy obscuring structure. The presence of a very compact nuclear starburst as the origin for this NIR excess emission is not favored by our spectroscopic data for these objects.

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arXiv2012

A combined Optical and X-ray Spectra Study for Type 1 AGN. III. Broadband SED Properties

In this third paper in a series of three, we present a detailed study of the AGN broadband SED based on a nearby unobscured Type 1 AGN sample. We perform a systematic cross-correlation study of the following key parameters: $Γ_{2-10keV}$, $L_{2-10keV}$, $L_{bol}$, $L_{bol}/L_{Edd}$, $κ_{2-10keV}$, $κ_{5100A}$, FWHM$_{Hβ}$, M$_{BH}$, $α_{ox}$, $α_{X}$ and $α_{UV}$, and identify various strong correlations among these parameters. The principal component analysis (PCA) is performed on the correlation matrix of the above parameters, which shows that the three physical parameters, i.e. black hole mass, mass accretion rate and Eddington ratio, drive the majority of the correlations. This is consistent with PCA results found from previous optical spectral studies. We produce various mean SEDs classified by each of the key parameters. Most parameters, except L$_{bol}$, show similar systematic changes in the mean SEDs such that the temperature at which the disc peaks is correlated with the ratio of power in the disc versus the Comptonised components and the hard X-ray spectral index. This underlying change in SED shape shows that AGN do exhibit intrinsically different spectral states. This is superficially similar to the SED differences in BHB seen as $λ_{Edd}$ increases, but the analogy does not hold in detail. Only objects with the highest $λ_{Edd}$ appear to correspond to a BHB spectral state (the disc dominated high/soft state). The AGN with typical mass accretion rates have spectra which do not match well with any state observed in BHB. We speculate that this could be due to the presence of a powerful UV line driven disc wind, which complicates simple mass scaling between stellar and supermassive black holes.

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arXiv2011

Intrinsic disc emission and the Soft X-ray Excess in AGN

(Abridged) Narrow Line Seyfert 1 (NLS1) galaxies have low mass black holes and mass accretion rates close to (or exceeding) Eddington, so a standard blackbody accretion disc should peak in the EUV. However, the lack of true absorption opacity in the disc means that the emission is better approximated by a colour temperature corrected blackbody, and this colour temperature correction is large enough ($\sim 2.4$) that the bare disc emission from a zero spin black hole can extend into the soft X-ray bandpass. Part of the soft X-ray excess seen in these objects must be intrinsic emission from the disc unless the vertical structure is very different to that predicted. However, the soft excess is much broader than predicted by a bare disc spectrum, indicating some Compton upscattering by cool, optically thick material. We associate this with the disc itself, so it must ultimately be powered by mass accretion. We build an energetically self consistent model assuming that the emission thermalises at large radii, but that at smaller radii the gravitational energy is split between powering optically thick Comptonised disc emission (forming the soft X-ray excess) and an optically thin corona above the disc (forming the tail to higher energies). We show examples of this model fit to the extreme NLS1 REJ1034+396, and to the much lower Eddington fraction Broad Line Seyfert 1 PG1048+231. We use these to guide our fits and interpretations of three template spectra made from co-adding multiple sources to track out a sequence of AGN spectra as a function of $L/L_{Edd}$. The new model is publically available within the {\sc xspec} spectral fitting package.

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arXiv2011

The X-ray emission of local luminous infrared galaxies

We study the X-ray emission of a representative sample of 27 local luminous infrared galaxies (LIRGs). The median IR luminosity of our sample is log L_IR/L_sun = 11.2, thus the low-luminosity end of the LIRG class is well represented. We used new XMM-Newton data as well as Chandra and XMM-Newton archive data. The soft X-ray (0.5-2 keV) emission of most of the galaxies (>80%), including LIRGs hosting a Seyfert 2 nucleus, is dominated by star-formation related processes. These LIRGs follow the star-formation rate (SFR) versus soft X-ray luminosity correlation observed in local starbursts. We find that ~15% of the non-Seyfert LIRGs (3 out of 20) have an excess hard X-ray emission relative to that expected from star-formation that might indicate the presence of an obscured AGN. The rest of the non-Seyfert LIRGs follow the SFR versus hard X-ray (2-10 keV) luminosity correlation of local starbursts. The non-detection of the 6.4 keV Fe K alpha emission line in the non-Seyfert LIRGs allows us to put an upper limit to the bolometric luminosity of an obscured AGN, L_bol <1043 erg s-1 . That is, in these galaxies, if they hosted a low luminosity AGN, its contribution to total luminosity would be less than 10%. Finally we estimate that the AGN contribution to the total luminosity for our sample of local LIRGs is between 7% and 10%.

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arXiv2010

An X-ray Study of Local Infrared Bright Galaxies

We are carrying out detailed study of the X-ray and infrared (IR) properties of a sample of local (d < 70 Mpc) luminous infrared galaxies (LIRGs) using XMM-Newton and Spitzer (imaging and spectroscopy). The main goal is to study the extreme processes of star formation and/or active galactic nuclei (AGN) taking place in this cosmologically important class of galaxies. In this proceedings we present the preliminary results obtained from the analysis of the XMM-Newton X-ray images and the X-ray spectral modeling.

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arXiv2010

Searching for Compton-thick active galactic nuclei at z~0.1

Using a suite of X-ray, mid-IR and optical active galactic nuclei (AGN) luminosity indicators, we search for Compton-thick (CT) AGNs with intrinsic L_X>10^42erg/s at z~0.03-0.2, a region of parameter space which is currently poorly constrained by deep narrow-field and high-energy (E>10keV) all-sky X-ray surveys. We have used the widest XMM-Newton survey (the serendipitous source catalogue) to select a representative sub-sample (14; ~10%) of the 147 X-ray undetected candidate CT AGNs in the Sloan Digital Sky Survey (SDSS) with f_X/f_[OIII]<1; the 147 sources account for ~50% of the overall Type-2 AGN population in the SDSS-XMM overlap region. We use mid-IR spectral decomposition analyses and emission-line diagnostics, determined from pointed Spitzer-IRS spectroscopic observations of these candidate CT AGNs, to estimate the intrinsic AGN emission (predicted L_X,2-10keV (0.2-30)x10^42erg/s). On the basis of the optical [OIII], mid-IR [OIV] and 6um AGN continuum luminosities we conservatively find that the X-ray emission in at least 6/14 (>43%) of our sample appear to be obscured by CT material with N_H>1.5x10^24cm^-2. Under the reasonable assumption that our 14 AGNs are representative of the overall X-ray undetected AGN population in the SDSS-XMM parent sample, we find that >20% of the optical Type-2 AGN population are likely to be obscured by CT material. This implies a space-density of log(Phi) >-4.9Mpc^-3 for CT AGNs with L_X>10^42erg/s at z~0.1, which we suggest may be consistent with that predicted by X-ray background synthesis models. Furthermore, using the 6um continuum luminosity to infer the intrinsic AGN luminosity and the stellar velocity dispersion to estimate M_BH, we find that the most conservatively identified CT AGNs in this sample may harbour some of the most rapidly growing black holes (median M_BH~3x10^7M_o) in the nearby Universe, with a median Eddington ratio of ~0.2.

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arXiv2008

RE J1034+396: The origin of the soft X-ray excess and QPO

The X-ray quasi-periodic oscillation (QPO) seen in RE J1034+396 is so far unique amongst AGN. Here we look at the another unique feature of RE J1034+396, namely its huge soft X-ray excess, to see if this is related in any way to the detection of the QPO. We show that all potential models considered for the soft energy excess can fit the 0.3-10 keV X-ray spectrum, but that the energy dependence of the rapid variability (which is dominated by the QPO) strongly supports a spectral decomposition where the soft excess is from low temperature Comptonization of the disc emission and remains mostly constant, while the rapid variability is produced by the power law tail changing in normalization. The presence of the QPO in the tail rather than in the disc is a common feature in black hole binaries, but low temperature Comptonization of the disc spectrum is not generally seen in these systems. The main exception to this is GRS 1915+105, the only black hole binary which routinely shows super-Eddington luminosities. We speculate that super-Eddington accretion rates lead to a change in disc structure, and that this also triggers the X-ray QPO.

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arXiv2008

A ~1-hour X-ray periodicity in an active galaxy RE J1034+396

Active galactic nuclei and quasars are thought to be scaled up versions of Galactic black hole binaries, powered by accretion onto supermassive black holes with masses of 10^6-10^9 M_Sun, as opposed to the ~10 M_Sun in binaries. One example of the similarities between these two types of systems is the characteristic rapid X-ray variability seen from the accretion flow. The power spectrum of this variability in black hole binaries consists of a broad, band-limited noise with multiple quasi-periodic oscillations superimposed, where power is concentrated over a narrow range of frequencies. Although the broad noise component has been observed in many active galactic nuclei, there are no significant detections of quasi-periodic oscillations. Here we report the discovery of a ~1h X-ray periodicity in a bright active galaxy RE J1034+396. The signal is highly statistically significant (at the 5.6 sigma level) and very coherent, with quality factor Q > 16. This reinforces the link between stellar and supermassive black holes, emphasizing the universal properties of accretion onto objects with very different masses. The X-ray modulation arises from the direct vicinity of the black hole, so this provides a new tool for studying active galactic nuclei.

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arXiv2003

A Massive X-Ray Outflow from the Quasar PDS 456

We report on XMM-Newton spectroscopic observations of the luminous, radio-quiet quasar PDS 456. The hard X-ray spectrum of PDS 456 shows a deep absorption trough (constituting 50% of the continuum) at energies above 7 keV in the quasar rest frame, which can be attributed to a series of blue-shifted K-shell absorption edges due to highly ionized iron. The higher resolution soft X-ray RGS spectrum exhibits a broad absorption line feature near 1 keV, which can be modeled by a blend of L-shell transitions from highly ionized iron (Fe XVII-XXIV). An extreme outflow velocity of 50000 km/s is required to model the K and L shell iron absorption present in the XMM-Newton data. Overall, a large column density ($N_{H}=5\times10^{23}$cm$^{-2}$) of highly ionized gas (logxi=2.5) is required in PDS 456. A high mass outflow rate of 10 solar masses per year (assuming a conservative outflow covering factor of 0.1 steradian) is derived, which is of the same order as the overall mass accretion rate in PDS 456. The kinetic energy of the outflow represents a substantial fraction (10%) of the quasar energy budget, whilst the large column and outflow velocity place PDS 456 towards the extreme end of the broad absorption line quasar population.

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arXiv2000

1-5 micron imaging of 3CRR galaxies: The K-z relation and the geometry of the torus

It has been claimed by Taylor et al. that the low-redshift end of the K-z relation for radio galaxies is too bright by about half a magnitude due to contributions from the obscured quasar nuclei. Such a result has major implications for the use of the K-band Hubble diagram in understanding the cosmological evolution of radio galaxies. In this paper we present 1-5 micron imaging data of a nearly-complete sample of low-redshift radio galaxies; this approach allows us to accurately determine the strengths of any unresolved nuclear components in the galaxies. We detect nuclear sources in five targets, whose broad-band colours are consistent with reddened quasar spectra. In all five cases the ratio of the inferred intrinsic near-infrared luminosity to the narrow-line luminosity is typical of quasars. We find a correlation between the inferred nuclear extinction and core-to-lobe ratio, which places constraints on the geometry of the torus. We find evidence for a shift of the K-z relation to fainter magnitudes, but by a much smaller amount (~0.1 mag) than Taylor et al. determined. Under the assumption that the nuclear sources in radio galaxies have the same intrinsic near-infrared spectra as quasars, our multi-wavelength images allow us to limit any possible shift to less than 0.3 magnitudes.

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