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Ponti G.,University of Southampton | Papadakis I.,University of Crete | Papadakis I.,Foundation for Research and Technology | Bianchi S.,Third University of Rome | And 4 more authors.
Astronomy and Astrophysics | Year: 2012

Context. We report on the results of the first XMM-Newton systematic "excess variance" study of all the radio quiet, X-ray un-obscured AGN. The entire sample consist of 161 sources observed by XMM-Newton for more than 10 ks in pointed observations, which is the largest sample used so far to study AGN X-ray variability on time scales less than a day. Aims. Recently it has been suggested that the same engine might be at work in the core of every black hole (BH) accreting object. In this hypothesis, the same variability should be observed in all AGN, once rescaled by the M BH (M BH) and accretion rate (ṁ). Methods. We systematically compute the excess variance for all AGN, on different time-scales (10, 20, 40 and 80 ks) and in different energy bands (0.3-0.7, 0.7-2 and 2-10 keV). Results. We observe a highly significant and tight (∼0.7 dex) correlation between σ 2 rms and M BH. The subsample of reverberation mapped AGN shows an even smaller scatter (only a factor of 2-3) comparable to the one induced by the M BH uncertainties. This implies that X-ray variability can be used as an accurate tool to measure M BH and this method is more accurate than the ones based on single epoch optical spectra. This allows us to measure M BH for 65 AGN and estimate lower limits for the remaining 96 AGN. On the other hand, the σ 2 rms vs. accretion rate dependence is weaker than expected based on the PSD break frequency scaling. This strongly suggests that both the PSD high frequency break and the normalisation depend on accretion rate in such a way that they almost completely counterbalance each other (PSD amp ∞ ṁ -0.8). A highly significant correlation between σ 2 rms and 2-10 keV spectral index is observed. The highly significant correlations between σ 2 rms and both the L Bol and the FWHM Hβ are consistent with being just by-products of the σ 2 rms vs. M BH relation. The soft and medium σ 2 rms is very well correlated with the hard σ 2 rms, with no deviations from a linear one to one correlation. This suggests that the additional soft components (i.e. soft excess, warm absorber) add a minor contribution to the total variability. Once the variability is rescaled for M BH and ṁ, no significant difference between narrow-line and broad-line Seyfert 1 is observed. Conclusions. The results are in agreement with a picture where, to first approximation, all local AGN have the same variability properties once rescaled for M BH and ṁ. © 2012 ESO. Source


Emmanoulopoulos D.,University of Southampton | Papadakis I.E.,University of Crete | Papadakis I.E.,Foundation for Research and Technology | Dovciak M.,Czech Republic Astronomical Institute | McHardy I.M.,University of Southampton
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present the first systematic physical modelling of the time-lag spectra between the soft (0.3-1 keV) and the hard (1.5-4 keV) X-ray energy bands, as a function of Fourier frequency, in a sample of 12 active galactic nuclei which have been observed by XMM-Newton. We concentrate particularly on the negative X-ray time-lags (typically seen above 10-4 Hz), i.e. soft-band variations lag the hard-band variations, and we assume that they are produced by reprocessing and reflection by the accretion disc within a lamp-post X-ray source geometry. We also assume that the response of the accretion disc, in the soft X-ray bands, is adequately described by the response in the neutral Fe Kα line at 6.4 keV for which we use fully general relativistic ray-tracing simulations to determine its time evolution. These response functions, and thus the corresponding time-lag spectra, yield much more realistic results than the commonly used, but erroneous, top-hat models. Additionally, we parametrize the positive part of the time-lag spectra (typically seen below 10-4 Hz) by a power law. We find that the bestfitting black hole (BH) masses, M, agree quite well with those derived by other methods, thus providing us with a new tool for BH mass determination. We find no evidence for any correlation between M and the BH spin parameter, α, the viewing angle, θ, or the height of the X-ray source above the disc, h. Also on average, the X-ray source lies only around 3.7 gravitational radii above the accretion disc and θ is distributed uniformly between 20° and 60°. Finally, there is a tentative indication that the distribution of α may be bimodal above and below 0.62. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Fragos T.,Harvard - Smithsonian Center for Astrophysics | Lehmer B.D.,Johns Hopkins University | Lehmer B.D.,NASA | Naoz S.,Harvard - Smithsonian Center for Astrophysics | And 4 more authors.
Astrophysical Journal Letters | Year: 2013

X-ray photons, because of their long mean-free paths, can easily escape the galactic environments where they are produced, and interact at long distances with the intergalactic medium, potentially having a significant contribution to the heating and reionization of the early universe. The two most important sources of X-ray photons in the universe are active galactic nuclei (AGNs) and X-ray binaries (XRBs). In this Letter we use results from detailed, large scale population synthesis simulations to study the energy feedback of XRBs, from the first galaxies (z ∼ 20) until today. We estimate that X-ray emission from XRBs dominates over AGN at z ≳ 6-8. The shape of the spectral energy distribution of the emission from XRBs shows little change with redshift, in contrast to its normalization which evolves by ∼4 orders of magnitude, primarily due to the evolution of the cosmic star-formation rate. However, the metallicity and the mean stellar age of a given XRB population affect significantly its X-ray output. Specifically, the X-ray luminosity from high-mass XRBs per unit of star-formation rate varies an order of magnitude going from solar metallicity to less than 10% solar, and the X-ray luminosity from low-mass XRBs per unit of stellar mass peaks at an age of ∼300 Myr and then decreases gradually at later times, showing little variation for mean stellar ages ≳ 3 Gyr. Finally, we provide analytical and tabulated prescriptions for the energy output of XRBs, that can be directly incorporated in cosmological simulations. © 2013. The American Astronomical Society. All rights reserved. Source


Emmanoulopoulos D.,University of Southampton | McHardy I.M.,University of Southampton | Papadakis I.E.,University of Crete | Papadakis I.E.,Foundation for Research and Technology
Monthly Notices of the Royal Astronomical Society | Year: 2013

The production of artificial light curves with known statistical and variability properties is of great importance in astrophysics. Consolidating the confidence levels during cross-correlation studies, understanding the artefacts induced by sampling irregularities, establishing detection limits for future observatories are just some of the applications of simulated data sets. Currently, the widely used methodology of amplitude and phase randomization is able to produce artificial light curves which have a given underlying power spectral density (PSD) but which are strictly Gaussian distributed. This restriction is a significant limitation, since the majority of the light curves, e.g. active galactic nuclei, X-ray binaries, gamma-ray bursts, show strong deviations from Gaussianity exhibiting 'burst-like' events in their light curves yielding longtailed probability density functions (PDFs). In this study, we propose a simple method which is able to precisely reproduce light curves which match both the PSD and the PDF of either an observed light curve or a theoretical model. The PDF can be representative of either the parent distribution or the actual distribution of the observed data, depending on the study to be conducted for a given source. The final artificial light curves contain all of the statistical and variability properties of the observed source or theoretical model, i.e. the same PDF and PSD, respectively. Within the framework of Reproducible Research, the code and the illustrative example used in this paper are both made publicly available in the form of an interactive MATHEMATICA notebook. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source


Gliozzi M.,George Mason University | Papadakis I.E.,University of Crete | Papadakis I.E.,Foundation for Research and Technology | Grupe D.,Pennsylvania State University | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2013

PKS 0558-504 is a highly-variable, X-ray-bright, radio-loud, narrow-line Seyfert 1 galaxy with super-Eddington accretion rate and extended jets that do not dominate the emission beyond the radio band. Therefore, this source represents an ideal laboratory to shed some light on the central engine in highly accreting systems and specifically on the link between accretion disc and corona. Here we present the results from a 1.5-year monitoring with Swift-XRT and Swift-UVOT. The simultaneous coverage at several wavelengths confirms that PKS 0558-504 is highly variable in any band from the optical to ultraviolet (UV) and X-rays, with the latter showing the largest amplitude changes but with the UV emission dominating the radiative output. A cross-correlation analysis reveals a tight link between the emission in the optical and UV bands and provides suggestive evidence in favour of a scenario where the variability originates in the outer part of the accretion flow and propagates inwards before triggering the activity of the X-ray-emitting corona. Finally, a positive correlation between the soft-X-ray flux and the hard photon index suggests that in PKS 0558-504 the seed photons are provided to the corona by the soft-excess component © 2013. Source

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