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      Analysis of X-ray spectral variability and black hole mass determination of the NLS1 galaxy Mrk 766

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          Abstract

          We present an XMM-Newton time-resolved spectral analysis of the NLS1 galaxy Mrk 766. We analyse eight available observations of the EPIC-pn camera taken between May 2000 and June 2005 to investigate the X-ray spectral variability as produced by changes in the mass accretion rate. The 0.2-10 keV spectra are extracted in time bins longer than 3 ks to accurately trace the variations of the best fit parameters of our adopted Comptonisation spectral model. We test a bulk-motion Comptonisation (BMC) model which is in general applicable to any physical system powered by accretion onto a compact object, and assumes that soft seed photons are efficiently up-scattered via inverse Compton scattering in a hot and dense electron corona. The Comptonised spectrum has a characteristic power-law shape, whose slope was found to increase for large values of the normalisation of the seed component, that is proportional to the mass accretion rate (in Eddington units). Our baseline spectral model also includes a warm absorber lying on the line of sight and radiation reprocessing from the accretion disk or from outflowing matter in proximity of the central compact object. Our study reveals that the normalisation-slope correlation, observed in Galactic Black Hole sources (GBHs), also holds for Mrk 766: variations of the photon index in the range Gamma~1.9-2.4 are indeed likely to be related to the variations of m-dot, as observed in X-ray binary systems. We finally applied a scaling technique based on the observed correlation to estimate the BH mass in Mrk 766. This technique is commonly and successfully applied to measure masses of GBHs, and this is the first time it is applied in detail to estimate the BH mass in an AGN. We obtain a value of M_{BH}=1.26^{+1.00}_{-0.77}x10^6 M_{sun} that is in very good agreement with that estimated by the reverberation mapping

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          Dynamics of Line-Driven Disk Winds in Active Galactic Nuclei II: Effects of Disk Radiation

          We explore consequences of a radiation driven disk wind model for mass outflows from active galactic nuclei (AGN). We performed axisymmetric time-dependent hydrodynamic calculations using the same computational technique as Proga, Stone and Kallman (2000). We test the robustness of radiation launching and acceleration of the wind for relatively unfavorable conditions. In particular, we take into account the central engine radiation as a source of ionizing photons but neglect its contribution to the radiation force. Additionally, we account for the attenuation of the X-ray radiation by computing the X-ray optical depth in the radial direction assuming that only electron scattering contributes to the opacity. Our new simulations confirm the main result from our previous work: the disk atmosphere can 'shield' itself from external X-rays so that the local disk radiation can launch gas off the disk photosphere. We also find that the local disk force suffices to accelerate the disk wind to high velocities in the radial direction. This is true provided the wind does not change significantly the geometry of the disk radiation by continuum scattering and absorption processes; we discuss plausibility of this requirement. Synthetic profiles of a typical resonance ultraviolet line predicted by our models are consistent with observations of broad absorption line (BAL) QSOs.
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            Discovery of a relation between black hole mass and soft X-ray time lags in active galactic nuclei

            We carried out a systematic analysis of time lags between X-ray energy bands in a large sample (32 sources) of unabsorbed, radio quiet active galactic nuclei (AGN), observed by XMM-Newton. The analysis of X-ray lags (up to the highest/shortest frequencies/time-scales), is performed in the Fourier-frequency domain, between energy bands where the soft excess (soft band) and the primary power law (hard band) dominate the emission. We report a total of 15 out of 32 sources displaying a high frequency soft lag in their light curves. All 15 are at a significance level exceeding 97 per cent and 11 are at a level exceeding 99 per cent. Of these soft lags, 7 have not been previously reported in the literature, thus this work significantly increases the number of known sources with a soft/negative lag. The characteristic time-scales of the soft/negative lag are relatively short (with typical frequencies and amplitudes of \nu\sim 0.07-4 \times 10^{-3} Hz and \tau\sim 10-600 s, respectively), and show a highly significant (\gsim 4\sigma) correlation with the black hole mass. The measured correlations indicate that soft lags are systematically shifted to lower frequencies and higher absolute amplitudes as the mass of the source increases. To first approximation, all the sources in the sample are consistent with having similar mass-scaled lag properties. These results strongly suggest the existence of a mass-scaling law for the soft/negative lag, that holds for AGN spanning a large range of masses (about 2.5 orders of magnitude), thus supporting the idea that soft lags originate in the innermost regions of AGN and are powerful tools for testing their physics and geometry.
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              X-ray Spectral Formation in a Converging Fluid Flow: Spherical Accretion into Black Holes

              We study Compton upscattering of low-frequency photons in a converging flow of thermal plasma. The photons escape diffusively and electron scattering is the dominant source of opacity. We solve numerically and approximately analytically the equation of radiative transfer in the case of spherical, steady state accretion into black holes. Unlike previous work on this subject, we consider the inner boundary at a finite radius and this has a significant effect on the emergent spectrum. It is shown that the bulk motion of the converging flow is more efficient in upscattering photons than thermal Comptonization, provided that the electron temperature in the flow is of order a few keV or less. In this case, the spectrum observed at infinity consists of a soft component coming from those input photons which escaped after a few scatterings without any significant energy change and of a power law which extends to high energies and is made of those photons which underwent significant upscattering. The luminosity of the power law is relatively small compared to that of the soft component. The more reflective the inner boundary is, the flatter the power-law spectrum becomes. The spectral energy power-law index for black-hole accretion is always higher than 1 and it is approximately 1.5 for high accretion rates. This result tempts us to say that bulk motion Comptonization might be the mechanism behind the power-law spectra seen in black-hole X-ray sources.
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                Author and article information

                Journal
                06 November 2013
                Article
                10.1051/0004-6361/201321904
                1311.1376
                f2e3f1e1-9f76-462e-9644-d5490dd01ed8

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                History
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                26 pages, 7 figures, 4 tables to be published in Astronomy and Astrophysics
                astro-ph.HE

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