Lance Miller

Abstract:

Context. We present the results from a combined study of the average X-ray spectral and timing properties of 14 nearby AGN. Aims. We investigate whether a "spectral-timing" AGN correlation exists, similar to the one observed in Cyg X-1, compare the two correlations, and constrain possible physical mechanisms responsible for the X-ray emission in compact, accreting objects. Methods. 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 the best-fit slopes to construct their sample distribution function, and we used the median of the distribution, and the mean of the best-fit slopes, which are above the 80th percentile of the distributions, to estimate the mean spectral slope of the objects. The latter estimate is more appropriate in the case when the energy spectra of the sources are significantly affected by absorption and/or reflection effects. We also used results from the literature to estimate the average spectral slope of the three remaining objects. Results. 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 correlation is similar to the spectral-timing correlation that has been observed in Cyg X-1, but not the same. Conclusions. 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 these systems. 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, and can also explain the difference between the spectral-timing correlations in Cyg X-1 and AGN, 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. © ESO 2009.

Abstract:

PDS 456 is a nearby (z = 0.184), luminous (L bol ∼ 10 47 erg s-1) type I quasar. A deep 190 ks Suzaku observation in 2007 February revealed the complex, broadband X-ray spectrum of PDS 456. The Suzaku spectrum exhibits highly statistically significant absorption features near 9 keV in the quasar rest frame. We show that the most plausible origin of the absorption is from blueshifted resonance (1s-2p) transitions of hydrogen-like iron (at 6.97 keV in the rest frame). This indicates that a highly ionized outflow may be present moving at near relativistic velocities ( ∼ 0.25c). A possible hard X-ray excess is detected above 15 keV with the Hard X-ray Detector (at 99.8% confidence), which may arise from high column density gas (N H > 1024 cm -2) partially covering the X-ray emission, or through strong Compton reflection. Here we propose that the iron K-shell absorption in PDS 456 is associated with a thick, possibly clumpy outflow, covering about 20% of 4π steradian solid angle. The outflow is likely launched from the inner accretion disk, within 15-100 gravitational radii of the black hole. The kinetic power of the outflow may be similar to the bolometric luminosity of PDS 456. Such a powerful wind could have a significant effect on the co-evolution of the host galaxy and its supermassive black hole, through feedback. © 2009. The American Astronomical Society. All rights reserved.

Abstract:

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 broad-line region and may form part of a clumpy disk wind. The fluorescent Fe Kα luminosity measured by Suzaku is consistent with that expected from an equatorial disk wind. © 2009. The American Astronomical Society.

Abstract:

We present the final spectroscopic QSO catalogue from the 2dF-SDSS LRG (luminous red galaxy) and QSO (2SLAQ) survey. This is a deep, 18 < g < 21.85 (extinction corrected), sample aimed at probing in detail the faint end of the broad line active galactic nuclei luminosity distribution at z ≲ 2.6. The candidate QSOs were selected from SDSS photometry and observed spectroscopically with the 2dF spectrograph on the Anglo-Australian Telescope. This sample covers an area of 191.9 deg2 and contains new spectra of 16 326 objects, of which 8764 are QSOs and 7623 are newly discovered [the remainder were previously identified by the 2dF QSO Redshift Survey (2QZ) and SDSS]. The full QSO sample (including objects previously observed in the SDSS and 2QZ surveys) contains 12 702 QSOs. The new 2SLAQ spectroscopic data set also contains 2343 Galactic stars, including 362 white dwarfs, and 2924 narrow emission-line galaxies with a median redshift of z = 0.22. We present detailed completeness estimates for the survey, based on modelling of QSO colours, including host-galaxy contributions. This calculation shows that at g ≃ 21.85 QSO colours are significantly affected by the presence of a host galaxy up to redshift z ∼ 1 in the SDSS ugriz bands. In particular, we see a significant reddening of the objects in g - i towards the fainter g-band magnitudes. This reddening is consistent with the QSO host galaxies being dominated by a stellar population of age at least 2-3 Gyr. The full catalogue, including completeness estimates, is available on-line at http://www.2slaq.info/ . © 2008 RAS.

Abstract:

Context. The Seyfert I galaxy MCG-6-30-15 shows one of the best examples of a broad "red wing" of emission in its X-ray spectrum at energies 2 ≤ E ≤ 6.4 keV, commonly interpreted as being caused by relativistically-blurred reflection close to the event horizon of the black hole. Aims. We aim to test an alternative model in which absorption creates the observed spectral shape, explains the puzzling lack of variability of the red wing and reduces the high reflection albedo, substantially greater than unity, that is otherwise inferred at energies E ≥ 20keV. Methods. We compiled all the available long-exposure, high-quality data for MCG-6-30-15: 522 ks of Chandra HETGS, 282ks of XMM-Newton PN/RGS and 253 ks of Suzaku XIS/PIN data. This is the first analysis of this full dataset. We investigated the spectral variability on timescales ≥ ks using principal components analysis and fitted spectral models to "flux state" and mean spectra over the energy range 0.5-45 keV (depending on detector). The absorber model was based on the zones previously identified in the high-resolution grating data. Joint fits were carried out to any data that were simultaneous. Results. Multiple absorbing zones covering a wide range of ionisation are required by the grating data, including a highly ionised outflowing zone. A variable partial-covering zone plus absorbed low-ionisation reflection, distant from the source, provides a complete description of the variable X-ray spectrum. A single model fits all the data. We conclude that these zones are responsible for the red wing, its apparent lack of variability, the absorption structure around the FeKα line, the soft-band "excess" and the high flux seen in the hard X-ray band. A relativistically-blurred Fe line is not required in this model. We suggest the partial covering zone is a clumpy wind from the accretion disk . © 2008 ESO.