Abstract
We systematically analyse all publicly available XMM-Newton spectra of radio-quiet PG quasars. The soft X-ray excess in these objects is well modelled by an additional, cool, Compton ...scattering region. However, the remarkably constant temperature derived for this component over the whole sample requires a puzzling fine tuning of the parameters. Instead, we propose that the soft excess is an artifact of strong, relativistically smeared, partially ionized absorption. The strong jump in opacity at ∼0.7 keV from O vii, O viii and iron can lead to an apparent soft excess below this energy, which is trivially constant since it depends on atomic processes. This can have a dramatic effect on the derived spectrum, which has implications for fitting the relativistic smearing of the reflected iron line emission from the disc.
The detection of an extremely broad iron line in XMM–Newton MOS data from the low/hard state of the black hole binary GX339−4 is the only piece of evidence which unambiguously conflicts with the ...otherwise extremely successful truncated disc interpretation of this state. However, it also conflicts with some aspect of observational data for all other alternative geometries of the low/hard state, including jet models, making it very difficult to understand. We re-analyse these data and show that they are strongly affected by pile-up even with extensive centroid removal as the source is ∼200 times brighter than the recommended maximum count rate. Instead, we extract the simultaneous PN timing-mode data which should not be affected by pile-up. These show a line which is significantly narrower than in the MOS data. Thus these data are easily consistent with a truncated disc, and indeed, strongly support such an interpretation.
Abstract
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
torus/L
Bol. However, the uncertainty in determining L
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 spectral energy distributions 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 multifrequency data obtained from SDSS, XMM–Newton, WISE, 2MASS and UKIDSS. We derive a mean value of f
c ∼ 0.30 with a dispersion of 0.17. Sample correlations, combined with simulations, show that f
c is more strongly anticorrelated with λEdd than with L
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.
Complex narrow-line Seyfert 1s (NLS1s), such as 1H 0707−495, differ from simple NLS1s like PG 1244+026 by showing stronger broad spectral features at Fe K and larger amplitude flux variability. These ...are correlated: the strongest Fe K features are seen during deep dips in the light curves of complex NLS1s. There are two competing explanations for these features, one where a compact X-ray source on the spin axis of a highly spinning black hole approaches the horizon and the consequent strong relativistic effects focus the intrinsic flux on to the inner edge of a thin disc, giving a dim, reflection-dominated spectrum. The other is that the deep dips are caused by complex absorption by clumps close to the hard X-ray source. The reflection-dominated model is able to reproduce the very short 30 s soft lag from reverberation seen in the complex NLS1 1H 0707−495. However, it does not explain the characteristic switch to hard lags on longer time-scales. Instead, a full model of propagating fluctuations coupled to reverberation can explain the switch in the simple NLS1 PG 1244+026 using a low spin black hole. However, PG 1244+026 has a longer reverberation lag of ∼200 s. Here we extend the successful propagation–reverberation model for the simple NLS1 PG 1244+026 to include the effect of absorption from clumps in a turbulent region above the disc. The resulting occultations of the inner accretion flow can introduce additional hard lags when relativistic effects are taken into account. This dilutes the soft lag from reverberation and shifts it to higher frequencies, making a smooth transition between the 200 s lags seen in simple NLS1s to the 30 s lags in complex NLS1s. These two classes of NLS1 could then be determined by inclination angle with respect to a clumpy, probably turbulent, failed wind structure on the disc.
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 106-109 , as opposed to the ...∼10 in binaries (here is the solar mass). 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 noise with multiple quasi-periodic oscillations superimposed on it. Although the broad noise component has been observed in many active galactic nuclei, there have hitherto been no significant detections of quasi-periodic oscillations. Here we report the discovery of an ∼1-hour X-ray periodicity in a bright active galaxy, RE J1034+396. The signal is highly statistically significant (at the 5.6 level) and very coherent, with quality factor Q > 16. The X-ray modulation arises from the direct vicinity of the black hole.
We extract the spectra of the strong low-frequency quasi-periodic oscillation (QPO) and its harmonic during the rising phase of an outburst in the black hole binary XTE J1550-564. We compare these ...frequency-resolved spectra to the time-averaged spectrum and the spectrum of the rapid (<0.1 s) variability. The spectrum of the time-averaged emission can be described by a disc, a Compton upscattered tail and its reflection. The QPO spectrum is very similar to the spectrum of the most rapid variability, implying it arises in the innermost regions of the flow. It contains little detectable disc, and its Compton spectrum is generally harder and shows less reflection than in the time-averaged emission. The harmonic likewise contains little detectable disc component, but has a Compton spectrum which is systematically softer than the QPO, softer even than the Compton tail in the time-averaged emission. We interpret these results in the context of the truncated disc model, where the inner disc is replaced by a hot flow. The QPO can arise in this picture from vertical (Lense-Thirring) precession of the entire hot inner flow, and its harmonic can be produced by the angular dependence of Compton scattering within the hot flow. We extend these models to include stratification of the hot flow, so that it is softer (lower optical depth) at larger radii closer to the truncated disc, and harder (higher optical depth) in the innermost parts of the flow where the rapid variability is produced. The different optical depth with radius gives rise to different angular dependence of the Comptonized emission, weighting the fundamental to the inner parts of the hot flow, and the harmonic to the outer. This is the first model which can explain both the spectrum of the QPO, and its harmonic, in a self consistent geometry.
The X-ray spectra of black hole binaries (BHB) in the low/hard state (LHS) first harden as the flux decreases, then soften. This change in behaviour has been variously attributed to either the X-rays ...switching from being produced in the flow to being dominated by the jet, or to the flow switching seed photons from the disc to self-generated seed photons from cyclo-synchrotron. Here, we build a simple truncated disc, hot inner flow, plus standard conical synchrotron jet model to explore what this predicts for the X-ray emission mechanism as a function of mass accretion rate.
We find that the change in X-ray spectral index can be quantitatively (not just qualitatively) explained by the seed photon switch in the hot flow, i.e. this supports models where the X-rays are always produced by the hot flow. By contrast, standard conical jet models are as radiatively inefficient as the hot flow so there is no transition in X-ray production mechanism with ṁ. Including the effects of electron cooling allows the jet X-rays to drop more slowly with accretion rate and hence overtake the X-rays from the hot flow; however, this produces a corresponding change in the radio-X-ray correlation, which is not observed. We argue that the unbroken radio-X-ray correlation down to quiescence rules out the jet transition model as an explanation for the trend in X-ray spectral index.
Our favoured model is then a truncated disc with an inner, hot, radiatively inefficient flow which always dominates the hard X-rays, coupled to a conical synchrotron jet which produces the radio emission. However, even this has issues at low
as the low optical depth and high temperature of the flow means that the Compton spectrum is not well approximated by a power law. This shows the need for a more sophisticated model for the electron distribution in the hot flow.
We extract the spectra of the fastest variability (above 10 Hz) from the black hole XTE J1550−564 during a transition from hard to soft state on the rise to outburst. We confirm previous results that ...the rapid variability contains no significant disc component despite this being strongly present in the total spectrum of the softer observations. We model ionized reflection significantly better than previous work, and show that this is also suppressed in the rapid variability spectrum compared to the total emission. This is consistent with the fast variability having its origin in a hot inner flow close to the black hole rather than in the accretion disc or in a corona above it. However, the rapid variability spectrum is not simply the same as the total Comptonized emission. It is always significantly harder, by an amount which increases as the spectrum softens during the outburst. This adds to evidence from time lags that the Comptonization region is inhomogeneous, with harder spectra produced closest to the black hole, the same region which produces the fastest variability.
One of the major unsolved questions concerning the understanding of the active galactic nucleus population is the origin of the dichotomy between radio-quiet and radio-loud quasars. The most ...promising explanation is provided by the spin paradigm, which suggests that radio-loud quasars have a higher black hole spin. However, the measurement of black hole spin remains extremely challenging. We here aim at comparing the mean radiative efficiencies of carefully matched samples of radio-loud and radio-quiet Sloan Digital Sky Survey (SDSS) quasars at 0.3 < z < 0.8 . We use the O iii luminosity as an indirect average tracer of the ionizing continuum in the extreme-UV regime where the differences in the spectral energy distribution (SED) due to black hole spin are most pronounced. We find that the radio-loud sample shows an enhancement in O iii line strength by a factor of at least 1.5 compared to a radio-quiet sample matched in redshift, black hole mass, and optical continuum luminosity or accretion rate. We argue that this enhancement is caused by differences in the SED, suggesting higher average bolometric luminosities at fixed accretion rate in the radio-loud population. This suggests that the radio-loud quasar population has on average systematically higher radiative efficiencies and therefore higher black hole spin than the radio-quiet population, providing observational support for the black hole spin paradigm.
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