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  • 1
    Electronic Resource
    Electronic Resource
    Active galactic nuclei as scaled-up Galactic black holes (2006)
    [s.l.] : Nature Publishing Group
    Nature 444 (2006), S. 730-732 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] A long-standing question is whether active galactic nuclei (AGN) vary like Galactic black hole systems when appropriately scaled up by mass. If so, we can then determine how AGN should behave on cosmological timescales by studying the brighter and much faster varying Galactic systems. As ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/nature05389
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  • 2
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    A quasi-periodic modulation of the iron line centroid energy in the black hole binary H1743-322 (2016)
    Oxford University Press
    Details
    Publication Date: 2016-07-13
    Description: Accreting stellar-mass black holes often show a ‘Type-C’ quasi-periodic oscillation (QPO) in their X-ray flux and an iron emission line in their X-ray spectrum. The iron line is generated through continuum photons reflecting off the accretion disc, and its shape is distorted by relativistic motion of the orbiting plasma and the gravitational pull of the black hole. The physical origin of the QPO has long been debated, but is often attributed to Lense–Thirring precession, a General Relativistic effect causing the inner flow to precess as the spinning black hole twists up the surrounding space–time. This predicts a characteristic rocking of the iron line between red- and blueshift as the receding and approaching sides of the disc are respectively illuminated. Here we report on XMM–Newton and NuSTAR observations of the black hole binary H1743–322 in which the line energy varies systematically over the ~4 s QPO cycle (3.70 significance), as predicted. This provides strong evidence that the QPO is produced by Lense–Thirring precession, constituting the first detection of this effect in the strong gravitation regime. There are however elements of our results harder to explain, with one section of data behaving differently than all the others. Our result enables the future application of tomographic techniques to map the inner regions of black hole accretion discs.
    Print ISSN: 0035-8711
    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 3
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    False periodicities in quasar time-domain surveys (2016)
    Oxford University Press
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    Publication Date: 2016-07-17
    Description: There have recently been several reports of apparently periodic variations in the light curves of quasars, e.g. PG 1302–102 by Graham et al. Any quasar showing periodic oscillations in brightness would be a strong candidate to be a close binary supermassive black hole and, in turn, a candidate for gravitational wave studies. However, normal quasars – powered by accretion on to a single, supermassive black hole – usually show stochastic variability over a wide range of time-scales. It is therefore important to carefully assess the methods for identifying periodic candidates from among a population dominated by stochastic variability. Using a Bayesian analysis of the light curve of PG 1302–102, we find that a simple stochastic process is preferred over a sinusoidal variation. We then discuss some of the problems one encounters when searching for rare, strictly periodic signals among a large number of irregularly sampled, stochastic time series, and use simulations of quasar light curves to illustrate these points. From a few thousand simulations of steep spectrum (‘red noise’) stochastic processes, we find many simulations that display few-cycle periodicity like that seen in PG 1302–102. We emphasize the importance of calibrating the false positive rate when the number of targets in a search is very large.
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    Electronic ISSN: 1365-2966
    Topics: Physics
    Published by Oxford University Press
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  • 4
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    A global look at X-ray time lags in Seyfert galaxies (2016)
    Oxford University Press
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    Publication Date: 2016-08-04
    Description: X-ray reverberation, where light-travel time delays map out the compact geometry around the inner accretion flow in supermassive black holes, has been discovered in several of the brightest, most variable and well-known Seyfert galaxies. In this work, we expand the study of X-ray reverberation to all Seyfert galaxies in the XMM–Newton archive above a nominal rms variability and exposure level (a total of 43 sources). Approximately 50 per cent of sources exhibit iron K reverberation, in that the broad iron K emission line responds to rapid variability in the continuum. We also find that on long time-scales, the hard band emission lags behind the soft band emission in 85 per cent of sources. This ‘low-frequency hard lag’ is likely associated with the coronal emission, and so this result suggests that most sources with X-ray variability show intrinsic variability from the nuclear region. We update the known iron K lag amplitude versus black hole mass relation, and find evidence that the height or extent of the coronal source (as inferred by the reverberation time delay) increases with mass accretion rate.
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    Electronic ISSN: 1365-2966
    Topics: Physics
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  • 5
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    Detection of the first infra-red quasi-periodic oscillation in a black hole X-ray binary (2016)
    Oxford University Press
    Details
    Publication Date: 2016-06-23
    Description: We present the analysis of fast variability of Very Large Telescope/ISAAC (Infrared Spectrometer And Array Camera) (infra-red), XMM–Newton /OM (optical) and EPIC-pn (X-ray), and RXTE /PCA (X-ray) observations of the black hole X-ray binary GX 339–4 in a rising hard state of its outburst in 2010. We report the first detection of a quasi-periodic oscillation (QPO) in the infra-red band (IR) of a black hole X-ray binary. The QPO is detected at 0.08 Hz in the IR as well as two optical bands ( U and V ). Interestingly, these QPOs are at half the X-ray QPO frequency at 0.16 Hz, which is classified as the type-C QPO; a weak sub-harmonic close to the IR and optical QPO frequency is also detected in X-rays. The band-limited sub-second time-scale variability is strongly correlated in IR/X-ray bands, with X-rays leading the IR by over 120 ms. This short time delay, shape of the cross-correlation function and spectral energy distribution strongly indicate that this band-limited variable IR emission is the synchrotron emission from the jet. A jet origin for the IR QPO is strongly favoured, but cannot be definitively established with the current data. The spectral energy distribution indicates a thermal disc origin for the bulk of the optical emission, but the origin of the optical QPO is unclear. We discuss our findings in the context of the existing models proposed to explain the origin of variability.
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    Topics: Physics
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  • 6
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    The flux-dependent X-ray time lags in NGC 4051 (2013)
    Oxford University Press
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    Publication Date: 2013-09-26
    Description: We present an analysis of the X-ray time lags for the highly variable Seyfert 1 galaxy NGC 4051, based on a series of XMM–Newton observations taken in 2009. We investigate the Fourier frequency dependent time lags in the light curves between the 0.3–1.0 keV and 2.0–5.0 keV energy bands as a function of source flux, including simultaneous modelling of the resulting lag-frequency spectra. We find the shape of the lag-frequency spectra to vary significantly and systematically with source flux. We model the lag-frequency spectra using simple transfer functions, and find that two time lag components are required, one in each energy band. The simplest acceptable fits have only the relative contribution of the lagged component in the hard band varying with flux level, which can be associated with changes in the energy spectrum. We discuss the interpretation of these results in terms of the currently popular models for X-ray time lags.
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    Topics: Physics
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  • 7
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    Probing the origin of quasi-periodic oscillations: the short-time-scale evolution of phase lags in GRS 1915+105 (2016)
    Oxford University Press
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    Publication Date: 2016-04-07
    Description: We present a model-independent analysis of the short-time-scale energy dependence of low-frequency quasi-periodic oscillations (QPOs) in the X-ray flux of GRS 1915+105. The QPO frequency in this source has previously been observed to depend on photon energy, with the frequency increasing with energy for observations with a high (2 Hz) QPO frequency, and decreasing with energy for observations with a low (2 Hz) QPO frequency. As this observed energy dependence is currently unexplained, we investigate if it is intrinsic to the QPO mechanism by tracking phase lags on (sub)second time-scales. We find that the phase lag between two broad energy bands systematically increases for 5–10 QPO cycles, after which the QPO becomes decoherent, the phase lag resets and the pattern repeats. This shows that the band with the higher QPO frequency is running away from the other band on short time-scales, providing strong evidence that the energy dependence of the QPO frequency is intrinsic. We also find that the faster the QPO decoheres, the faster the phase lag increases, suggesting that the intrinsic frequency difference contributes to the decoherence of the QPO. We interpret our results within a simple geometric QPO model, where different radii in the inner accretion flow experience Lense–Thirring precession at different frequencies, causing the decoherence of the oscillation. By varying the spectral shape of the inner accretion flow as a function of radius, we are able to qualitatively explain the energy-dependent behaviour of both QPO frequency and phase lag.
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    Topics: Physics
    Published by Oxford University Press
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  • 8
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    The remarkable timing properties of a 'hypersoft' state in GRO J1655-40 (2015)
    Oxford University Press
    Details
    Publication Date: 2015-05-30
    Description: We report the identification and study of an unusual soft state of the black hole low-mass X-ray binary GRO J1655-40, observed during its 2005 outburst by the Rossi X-ray Timing Explorer . Chandra X-ray grating observations have revealed a high-mass-outflow accretion disc wind in this state, and we show that the broad-band X-ray spectrum is remarkably similar to that observed in the so-called hypersoft state of the high-mass X-ray binary Cyg X-3, which possesses a strong stellar wind from a Wolf–Rayet secondary. The power-spectral density (PSD) of GRO J1655-40 shows a bending power-law shape, similar to that of canonical soft states albeit with larger fractional rms. However, the characteristic bend frequency of the PSD is strongly correlated with the X-ray flux, such that the bend frequency increases by two decades for less than a factor 2 increase in flux. The strong evolution of PSD bend frequency for very little change in flux or X-ray spectral shape seems to rule out the suppression of high-frequency variability by scattering in the wind as the origin of the PSD bend. Instead, we suggest that the PSD shape is intrinsic to the variability process and may be linked to the evolution of the scaleheight in a slim disc. An alternative possibility is that variability is introduced by variable absorption and scattering in the wind. We further argue that the hypersoft state in GRO J1655-40 and Cyg X-3 is associated with accretion close to or above the Eddington limit.
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    Topics: Physics
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  • 9
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    Phase-resolved spectroscopy of Type B quasi-periodic oscillations in GX 339-4 (2016)
    Oxford University Press
    Details
    Publication Date: 2016-06-17
    Description: We present a new spectral-timing technique for phase-resolved spectroscopy and apply it to the low-frequency Type B quasi-periodic oscillation (QPO) from the black hole X-ray binary GX 339-4. We show that on the QPO time-scale the spectrum changes not only in normalization, but also in spectral shape. Using several different spectral models which parametrize the blackbody and power-law components seen in the time-averaged spectrum, we find that both components are required to vary, although the fractional rms amplitude of blackbody emission is small, ~1.4 per cent compared to ~25 per cent for the power-law emission. However, the blackbody variation leads the power-law variation by ~0.3 in relative phase (~110°), giving a significant break in the Fourier lag-energy spectrum that our phase-resolved spectral models are able to reproduce. Our results support a geometric interpretation for the QPO variations where the blackbody variation and its phase relation to the power-law are explained by quasi-periodic heating of the approaching and receding sides of the disc by a precessing Comptonizing region. The small amplitude of blackbody variations suggests that the Comptonizing region producing the QPO has a relatively large scaleheight, and may be linked to the base of the jet, as has previously been suggested to explain the binary orbit inclination-dependence of Type B QPO amplitudes.
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    Topics: Physics
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  • 10
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    Long-term monitoring of the archetype Seyfert galaxy MCG-6-30-15: X-ray, optical and near-IR variability of the corona, disc and torus (2015)
    Oxford University Press
    Details
    Publication Date: 2015-09-24
    Description: We present long-term monitoring of MCG-6-30-15 in X-rays, optical and near-IR wavelengths, collected over 5 yr of monitoring. We determine the power spectrum density of all the observed bands and show that after taking into account the host contamination similar power is observed in the optical and near-IR bands. There is evidence for a correlation between the light curves of the X-ray photon flux and the optical B band, but it is not possible to determine a lag with certainty, with the most likely value being around 0 d. Strong correlation is seen between the optical and near-IR bands. Cross-correlation analysis shows some complex probability distributions and lags that range from 10 to 20 d, with the near-IR following the optical variations. Filtering the light curves in frequency space shows that the strongest correlations are those corresponding to the shortest time-scales. We discuss the nature of the X-ray variability and conclude that this is intrinsic and cannot be accounted for by absorption episodes due to material intervening in the line of sight. It is also found that the lags agree with the relation    4/3 , as expected for an optically thick geometrically thin accretion disc, although for a larger disc than that predicted by the estimated black hole mass and accretion rate in MCG-6-30-15. The cross-correlation analysis suggests that the torus is located at ~20 light-days from the central source and at most at ~50 light-days from the central region. This implies an active galactic nucleus bolometric luminosity of ~3 x 10 43 erg s –1  cm –2 .
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    Topics: Physics
    Published by Oxford University Press
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