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Determining black hole spin from broad iron lines is potentially the best approach in active galactic nuclei. However this measurement is rather subtle and although a number of spin values have been published in recent years, the affect of systematic errors remains largely unquantified. We explore systematic effects on the measured spin parameter, illustrating these effects using all available Suzaku and XMM-Newton data on the Seyfert 1 galaxy Fairall 9. We explore effects resulting from different data analysis techniques, and imperfect spectral models. We find that systematic errors dominate over statistical errors and uncertainties in the underlying accretion disk Physics. We identify a particularly important degeneracy that exists between narrow emission features in the iron K band and the blue wing of the broad line - CCD spectra do not have sufficient spectral resolution to separate these two features. Moreover, we find that the spin parameter can be sensitive to modeling of the soft part of the spectrum, i.e. the modeling of the soft excess. We investigate the extent to which time-variability of the spectrum (i.e. multi-epoch fitting) can be used to resolve these issues. Including time variability studies, we can conclude that the soft excess in Fairall 9 does not originate from only blurred reflection. Instead an additional component is required to permit consistent modeling of the entire spectrum over time.

In addition to providing vital clues as to the formation and evolution of black holes, the spin of black holes may be an important energy source in the Universe. Over the past couple of years, tremendous progress has been made in the realm of observational measurements of spin. In particular, detailed characterization and modeling of X-ray spectral features emitted from the inner regions of black hole accretion disks have allowed us to probe the location of the innermost stable circular orbit and hence the spin. In this contribution, I will describe the Suzaku AGN Spin Survey, an on-going Cycle 4--6 Suzaku Key Project that aims to probe five nearby AGN with sufficient depth that strong gravity effects and spin can be robustly assessed. Of the three objects for which we currently have our deep datasets, we can constrain spin in two (NGC3783, a>0.9; Fairall 9, $a=0.45\pm 0.15$) whereas complexities with the warm absorber prevent us from reporting results for NGC3516. We conclude with a brief discussion of spin-dependent selection biases in flux-limited surveys., 8 pages. To appear in the proceedings of the conference "Suzaku 2011, Exploring the X-ray Universe: Suzaku and Beyond"

The variable warm absorber seen with {\em ASCA} in the X-ray spectrum of MCG$-$6-30-15 shows complex time behaviour in which the optical depth of OVIII anticorrelates with the flux whereas that of OVII is unchanging. The explanation in terms of a two zone absorber has since been challenged by {\em BeppoSAX} observations. These present a more complicated behaviour for the OVIII edge. We demonstrate here that the presence of a third, intermediate, zone can explain all the observations. In practice, warm absorbers are likely to be extended, multi-zone regions of which only part causes directly observable absorption edges at any given time., Comment: 5 pages, 6 figures, accepted for publication in MNRAS

Spectroscopy of the broad iron iron with ASCA and BeppoSAX has up opened the innermost regions of accreting black hole systems to detailed study. In this contribution, I discuss how observations with future X-ray missions will extend these studies and all us to observationally address issues which are currently only in the realm of the theorists. In particular, high-throughput spectroscopy with XMM and, eventually, Constellation-X will allow the full diagnostic power of iron line variability to be realized. Instabilities of the inner accretion flow, the geometry of the variable X-ray source, and the black hole mass and spin will all be open to study. Eventually, X-ray interferometry will allow direct imaging of the black hole region in nearby active galaxies, thereby providing the ultimate probe of black hole astrophysics., 9 pages, 5 postscript figures, invited review at "X-ray Astronomy 1999", Bologna, Sept 1999

We discuss the results of three-dimensional magnetohydrodynamic simulations, using a pseudo-Newtonian potential, of thin disk (h/r ~ 0.1) accretion onto black holes. We find (i) that magnetic stresses persist within the marginally stable orbit, and (ii) that the importance of those stresses for the dynamics of the flow depends upon the strength of magnetic fields in the disk outside the last stable orbit. Strong disk magnetic fields (alpha > 0.1) lead to a gross violation of the zero-torque boundary condition at the last stable orbit, while weaker fields (alpha ~ 0.01) produce results more akin to traditional models for thin disk accretion onto black holes. Fluctuations in the magnetic field strength in the disk could lead to changes in the radiative efficiency of the flow on short timescales., 6 pages, to appear in proceedings, 20th Texas Symposium on Relativistic Astrophysics, eds J.C. Wheeler and H. Martel

We analyze a 75 ksec observation of the active galaxy Centaurus A carried out with RXTE in 1998 August. The joint PCA/HEXTE broad band X-ray spectrum can be well described by a heavily absorbed (NH∼7×1022 cm−2) power law with a photon index of Γ=1.8. We present the interpretation of this spectrum in terms of Monte Carlo computations. No significant temporal variability was found during this observation.