arXiv subjects
Publications and source records attributed to T. J. Turner.
A non-proprietary Mission Concept available for presentation to NASA, providing High Area, High Resolution Imaging Spectroscopy and Timing with Arcmin Angular Resolution Submitted in response to NASA 2011 RFI NNH11ZDA018L 'Concepts for the Next NASA X-ray Astronomy Mission'
X-ray spectra of AGN are complex. X-ray absorption and emission features trace gas covering a wide range of column densities and ionization states. High resolution spectra show the absorbing gas to be outflowing, perhaps in the form of an accretion disk wind. The absorbing complex shapes the form of the X-ray spectrum while X-ray reverberation and absorption changes explain the spectral and timing behaviour of AGN. We discuss recent progress, highlighting some new results and reviewing the implications that can be drawn from the data.
The Extreme Physics Explorer (EPE) is a concept timing/spectroscopy mission that would use micro-channel plate optics (MCPO) to provide 4 square meters effective area focused to ~1 arc-min onto an X-ray calorimeter. We describe science drivers for such a mission, possible designs for the large area MCPO needed for EPE, and the challenges of the large area MCPO design.
Reverberation from scattering material around the black hole in active galactic nuclei is expected to produce a characteristic signature in a Fourier analysis of the time delays between directly-viewed continuum emission and the scattered light. Narrow-line Seyfert 1 galaxies (NLS1) are highly variable at X-ray energies, and are ideal candidates for the detection of X-ray reverberation. We show new analysis of a small sample of NLS1 that clearly shows the expected time-delay signature, providing strong evidence for the existence of a high covering fraction of scattering and absorbing material a few tens to hundreds of gravitational radii from the black hole. We also show that an alternative interpretation of time delays in the NLS1 1H0707-495, as arising about one gravitational radius from the black hole, is strongly disfavoured in an analysis of the energy-dependence of the time delays.
We consider new Suzaku data for NGC 3516 taken during 2009, along with other recent X-ray observations of the source. The cumulative characteristics of NGC 3516 cannot be explained without invoking changes in the line-of-sight absorption. Contrary to many other well-studied Seyfert galaxies, NGC 3516 does not show a positive lag of hard X-ray photons relative to soft photons over the timescales sampled. In the context of reverberation models for the X-ray lags, the lack of such a signal in NGC 3516 is consistent with flux variations being dominated by absorption changes. The lack of any reverberation signal in such a highly variable source disfavors intrinsic continuum variability in this case. Instead, the colorless flux variations observed at high flux states for NGC 3516 are suggested to be a consequence of Compton-thick clumps of gas crossing the line-of-sight.
We present the analysis of a new broad-band X-ray spectrum (0.6-180.0 keV) of the radio-quiet quasar MR 2251-178 which uses data obtained with both Suzaku and the Swift/BAT. In accordance with previous observations, we find that the general continuum can be well described by a power-law with Γ=1.6 and an apparent soft-excess below 1 keV. Warm absorption is clearly present and absorption lines due to the Fe UTA, Fe L (Fe XXIII-XXIV), S XV and S XVI are detected below 3 keV. At higher energies, Fe K absorption from Fe XXV-XXVI is detected and a relatively weak (EW=25[+12,-8] eV) narrow Fe Kα emission line is observed at E=6.44\pm0.04 keV. The Fe Kα emission is well modelled by the presence of a mildly ionised (ξ\leq30) reflection component with a low reflection fraction (R<0.2). At least 5 ionised absorption components with 10^{20} \leq N_H \leq 10^{23} cm^{-2} and 0 \leq log(ξ)/erg cm s^{-1} \leq 4 are required to achieve an adequate spectral fit. Alternatively, we show that the continuum can also be fit if a Γ~2.0 power-law is absorbed by a column of N_H~10^{23} cm^{-2} which covers ~30% of the source flux. Independent of which continuum model is adopted, the Fe L and Fe XXV Heα lines are described by a single absorber outflowing with v_out~0.14 c. Such an outflow/disk wind is likely to be substantially clumped (b~10^{-3}) in order to not vastly exceed the likely accretion rate of the source.
We present the results of a deep 300 ks Chandra HETG observation of the highly variable narrow-line Seyfert Type 1 galaxy NGC 4051. The HETG spectrum reveals 28 significant soft X-ray ionised lines in either emission or absorption; primarily originating from H-like and He-like K-shell transitions of O, Ne, Mg and Si (including higher order lines and strong forbidden emission lines from O VII and Ne IX) plus high ionisation L-shell transitions from Fe XVII to Fe XXII and lower ionisation inner-shell lines (e.g. O VI). Modelling the data with XSTAR requires four distinct ionisation zones for the gas, all outflowing with velocities < 1000 km/s. A selection of the strongest emission/absorption lines appear to be resolved with FWHM of ~600 km/s. We also present the results from a quasi-simultaneous 350 ks Suzaku observation of NGC 4051 where the XIS spectrum reveals strong evidence for blueshifted absorption lines at ~6.8 and ~7.1 keV, consistent with previous findings. Modelling with XSTAR suggests that this is the signature of a highly ionised, high velocity outflow (log ξ~ 4.1; v ~ -0.02c) which potentially may have a significant effect on the host galaxy environment via feedback. Finally, we also simultaneously model the broad-band 2008 XIS+HXD Suzaku data with archival Suzaku data from 2005 when the source was observed to have entered an extended period of low flux in an attempt to analyse the cause of the long-term spectral variability. We find that we can account for this by allowing for large variations in the normalisation of the intrinsic power-law component which may be interpreted as being due to significant changes in the covering fraction of a Compton-thick partial-coverer obscuring the central continuum emission.
We present a study of reddening and absorption towards the Narrow Line Regions (NLR) in active galactic nuclei (AGN) selected from the Revised Shapley-Ames, 12mu, and Swift/Burst Alert Telescope samples. For the sources in host galaxies with inclinations of b/a > 0.5, we find that mean ratio of [O III] 5007A, from ground-based observations, and [O IV] 28.59mu, from Spitzer/Infrared Spectrograph observations, is a factor of 2 lower in Seyfert 2s than Seyfert 1s. The combination of low [O III]/[O IV] and [O III] 4363/5007 ratios in Seyfert 2s suggests more extinction of emission from the NLR than in Seyfert 1s. Similar column densities of dusty gas, NH ~ several X 10^21 cm^-2, can account for the suppression of both [O III] 5007A and [O III] 4363A, as compared to those observed in Seyfert 1s. Also, we find that the X-ray line OVII 22.1A is weaker in Seyfert 2s, consistent with absorption by the same gas that reddens the optical emission. Using a Hubble Space Telescope/Space Telescope Imaging Spectrograph slitless spectrum of the Seyfert 1 galaxy NGC 4151, we estimate that only ~ 30% of the [O III] 5007A comes from within 30 pc of the central source, which is insufficient to account for the low [O III]/[OIV] ratios in Seyfert 2s. If Seyfert 2 galaxies have similar intrinsic [OIII] spatial profiles, the external dusty gas must extend further out along the NLR, perhaps in the form of nuclear dust spirals that have been associated with fueling flows towards the AGN.
We focus on determining the underlying physical cause of a Seyfert galaxy's appearance as type a 1.8 or 1.9. Are these "intermediate" Seyfert types typical Seyfert 1 nuclei with reddened broad-line regions? Or are they objects with intrinsically weak continua and broad emission lines? We compare measurements of the optical reddening of the narrow and broad-line regions with each other and with the X-ray column derived from XMM-Newton 0.5-10 keV spectra to determine the presence and location of dust in the line of sight. We also searched the literature to see if the objects showed evidence for broad-line variability, and determined if the changes were consistent with a change in reddening or a change in the intrinsic ionizing continuum flux. We find that 10 of 19 objects previously classified as Seyfert 1.8/1.9s received this designation due to their low continuum flux. In four objects the classification was due to BLR reddening, either by the torus or dust structures in the vicinity of the NLR; in the remaining five objects there is not sufficient evidence to favor one scenario over the other. These findings imply that, in general, samples of 1.8/1.9s are not suitable for use in studies of the gas and dust in the central torus.
The narrow-line Seyfert 1 galaxy 1H0707-495 has previously been identified as showing time lags between flux variations in the soft- (0.3-1 keV) and medium-energy (1-4 keV) X-ray bands that oscillate between positive and negative values as a function of the frequency of the mode of variation. Here we measure and analyse the lags also between a harder X-ray band (4-7.5 keV) and the soft and medium bands, using existing XMM-Newton data, and demonstrate that the entire spectrum of lags, considering both the full energy range, 0.3-7.5 keV, and the full frequency range, 10^-5 < nu < 10^-2 Hz, are inconsistent with previous claims of arising as reverberation associated with the inner accretion disk. Instead we demonstrate that a simple reverberation model, in which scattering or reflection is present in all X-ray bands, explains the full set of lags without requiring any ad hoc explanation for the time lag sign changes. The range of time delays required to explain the observed lags extends up to about 1800 s in the hard band. The results are consistent with reverberation caused by scattering of X-rays passing through an absorbing medium whose opacity decreases with increasing energy and that partially-covers the source. A high covering factor of absorbing and scattering circumnuclear material is inferred.
We perform multi-dimensional radiative transfer simulations to compute spectra for a hydrodynamical simulation of a line-driven accretion disk wind from an active galactic nucleus. The synthetic spectra confirm expectations from parameterized models that a disk wind can imprint a wide variety of spectroscopic signatures including narrow absorption lines, broad emission lines and a Compton hump. The formation of these features is complex with contributions originating from many of the different structures present in the hydrodynamical simulation. In particular, spectral features are shaped both by gas in a successfully launched outflow and in complex flows where material is lifted out of the disk plane but ultimately falls back. We also confirm that the strong Fe Kalpha line can develop a weak, red-skewed line wing as a result of Compton scattering in the outflow. In addition, we demonstrate that X-ray radiation scattered and reprocessed in the flow has a pivotal part in both the spectrum formation and determining the ionization conditions in the wind. We find that scattered radiation is rather effective in ionizing gas which is shielded from direct irradiation from the central source. This effect likely makes the successful launching of a massive disk wind somewhat more challenging and should be considered in future wind simulations.
We present the broad-band 0.6-150 keV Suzaku and Swift BAT spectra of the low luminosity Seyfert galaxy, NGC 7213. The time-averaged continuum emission is well fitted by a single powerlaw of photon index Gamma = 1.75 and from consideration of the Fermi flux limit we constrain the high energy cutoff to be 350 keV < E < 25 MeV. Line emission from both near-neutral iron K_alpha at 6.39 keV and highly ionised iron, from Fe_(xxv) and Fe_(xxvi), is strongly detected in the Suzaku spectrum, further confirming the results of previous observations with Chandra and XMM-Newton. We find the centroid energies for the Fe_(xxv) and Fe_(xxvi) emission to be 6.60 keV and 6.95 keV respectively, with the latter appearing to be resolved in the Suzaku spectrum. We show that the Fe_(xxv) and Fe_(xxvi) emission can result from a highly photo-ionised plasma of column density N_(H) ~ 3 x 10^(23) cm^(-2). A Compton reflection component, e.g., originating from an optically-thick accretion disc or a Compton-thick torus, appears either very weak or absent in this AGN, subtending < 1 sr to the X-ray source, consistent with previous findings. Indeed the absence of either neutral or ionised Compton reflection coupled with the lack of any relativistic Fe K signatures in the spectrum suggests that an inner, optically-thick accretion disc is absent in this source. Instead, the accretion disc could be truncated with the inner regions perhaps replaced by a Compton-thin Radiatively Inefficient Accretion Flow. Thus, the Fe_(xxv) and Fe_(xxvi) emission could both originate in ionised material perhaps at the transition region between the hot, inner flow and the cold, truncated accretion disc on the order of 10^(3) - 10^(4) gravitational radii from the black hole. The origin for the unresolved neutral Fe K_alpha emission is then likely to be further out, perhaps originating in the optical BLR or a Compton-thin pc-scale torus.
Highly-ionized fast accretion-disk winds have been suggested as an explanation for a variety of observed absorption and emission features in the X-ray spectra of Active Galactic Nuclei. Simple estimates have suggested that these flows may be massive enough to carry away a significant fraction of the accretion energy and could be involved in creating the link between supermassive black holes and their host galaxies. However, testing these hypotheses, and quantifying the outflow signatures, requires high-quality theoretical spectra for comparison with observations. Here we describe extensions of our Monte Carlo radiative transfer code that allow us to generate realistic theoretical spectra for a much wider variety of disk wind models than possible in our previous work. In particular, we have expanded the range of atomic physics simulated by the code so that L- and M-shell ions can now be included. We have also substantially improved our treatment of both ionization and radiative heating such that we are now able to compute spectra for outflows containing far more diverse plasma conditions. We present example calculations that illustrate the variety of spectral features predicted by parametrized outflow models and demonstrate their applicability to the interpretation of data by comparison with observations of the bright quasar PG1211+143. We find that the major features in the observed 2 - 10 keV spectrum of this object can be well-reproduced by our spectra, confirming that it likely hosts a massive outflow.
A Suzaku X-ray observation of NGC 4051 taken during 2005 Nov reveals line emission at 5.44 keV in the rest-frame of the galaxy which does not have an obvious origin in known rest-frame atomic transitions. The improvement to the fit statistic when this line is accounted for establishes its reality at >99.9% confidence: we have also verified that the line is detected in the three XIS units independently. Comparison between the data and Monte Carlo simulations shows that the probability of the line being a statistical fluctuation is p < 3.3 x 10^-4. Consideration of three independent line detections in Suzaku data taken at different epochs yields a probability p< 3 x 10^-11 and thus conclusively demonstrates that it cannot be a statistical fluctuation in the data. The new line and a strong component of Fe Ka emission from neutral material are prominent when the source flux is low, during 2005. Spectra from 2008 show evidence for a line consistent with having the same flux and energy as that observed during 2005, but inconsistent with having a constant equivalent width against the observed continuum. The stability of the line flux and energy suggests that it may not arise in transient hotspots, as has been suggested for similar lines in other sources, but could arise from a special location in the reprocessor, such as the inner edge of the accretion disk. Alternatively, the line energy may be explained by spallation of Fe into Cr, as discussed in a companion paper.
Long-exposure Suzaku X-ray observations of the active galaxy NGC 4051 from 2005 and 2008 are analysed, in an attempt to reach a self-consistent understanding of the spectral variability on long timescales and at high time resolution. Principal components analysis and a maximum likelihood method of power spectrum analysis are used. In common with other type I AGN, the spectral variability is dominated by a varying-normalisation power-law component together with a quasi-steady, hard-spectrum offset component that contains Fe K atomic features. NGC 4051 displays a strong excess over a power-law at energies above 20 keV, some fraction of which also appears to vary with the power-law continuum. The fast-timescale power spectrum has a shape consistent with previous determinations, but significant differences are found between the low and high flux states of the source, demonstrating that the power spectrum is non-stationary. Frequency-dependent time lags between the hard and soft bands of up to 970+/-225s are measured. The existence of the observed lags excludes the possibility that the hard spectral component originates as reflection from the inner accretion disk. We instead show that the frequency- and energy-dependent time lags may be explained as reverberation, caused by reflection from a thick shell of material with maximum lags of about 10,000s. If the reflecting material surrounds the AGN, it extends to a distance about 1.5x10^14 cm, 600 gravitational radii, from the illuminating source and the global covering factor is C(g) >~ 0.4, confirming suggestions that type I AGN have high covering factors of absorbing and reflecting material. Given the spectral and timing similarities with other type I AGN, we infer that this structure is common in the type I population. [abridged]
We report on the highest to date signal-to-noise-ratio X-ray spectrum of the luminous quasar PDS 456, as obtained during two XMM-Newton orbits in September 2007. The present spectrum is considerably different from several previous X-ray spectra recorded for PDS 456 since 1998. The ultra-high-velocity outflow seen as recently as February 2007 is not detected in absorption. Conversely, a significant reflection component is detected. The reflection model suggests the reflecting medium may be outflowing at a velocity v/c = -0.06 +/- 0.02. The present spectrum is analyzed in the context of the previous ones in an attempt to understand all spectra within the framework of a single model. We examine whether an outflow with variable partial covering of the X-ray source along the line of sight that also reflects the source from other lines of sight can explain the dramatic variations in the broad-band spectral curvature of PDS 456. It is established that absorption plays a major role in shaping the spectrum of other epochs, while the 2007 XMM-Newton spectrum is dominated by reflection, and the coverage of the source by the putative outflow is small (< 20%).
We investigate conditions for and consequences of spallation in radio-quiet Seyfert galaxies. The work is motivated by the recent discovery of significant line emission at 5.44 keV in Suzaku data from NGC 4051. The energy of the new line suggests an identification as Cr I Ka emission, however the line is much stronger than would be expected from material with cosmic abundances, leading to a suggestion of enhancement owing to nuclear spallation of Fe by low energy cosmic rays from the active nucleus. We find that the highest abundance enhancements are likely to take place in gas out of the plane of the accretion disk and that timescales for spallation could be as short as a few years. The suggestion of a strong nuclear flux of cosmic rays in a radio-quiet Seyfert galaxy is of particular interest in light of the recent suggestion from Pierre Auger Observatory data that ultra-high-energy cosmic rays may originate in such sources.
We present the results from a combined study of the average X-ray spectral and timing properties of 14 nearby AGN. For 11 of the sources in the sample, we used all the available data from the RXTE archive, which were taken until the end of 2006. There are 7795 RXTE observations in total for these AGN, obtained over a period of ~7-11 years. We extracted their 3-20 keV spectra and fitted them with a simple power-law model, modified by the presence of a Gaussian line (at 6.4 keV) and cold absorption, when necessary. We used these best-fit slopes to estimate the mean spectral slope for each object, while we used results from the literature to estimate the average spectral slope of the three objects without archival, monitorin RXTE data. Our results show that the AGN average spectral slopes are not correlated either with the black hole mass or the characteristic frequencies that were detected in the power spectra.They are positively correlated, though, with the characteristic frequency when normalised to the sources black hole mass. This is similar to the spectral-timing correlation that has been observed in Cyg X-1, but not the same.The AGN spectral-timing correlation can be explained if we assume that the accretion rate determines both the average spectral slope and the characteristic time scales in AGN. The spectrum should steepen and the characteristic frequency should increase, proportionally, with increasing accretion rate. We also provide a quantitative expression between spectral slope and accretion rate. Thermal Comptonisation models are broadly consistent with our result, but only if the ratio of the soft photons' luminosity to the power injected to the hot corona is proportionally related to the accretion rate.