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Journal ArticleDOI

Origin of the X-ray disc-reflection steep radial emissivity

TL;DR: In this paper, the authors explore the physically motivated conditions that give rise to the observed steep disc-reflection emissivity profiles and obtain very steep reflection profiles with q ∼ 4−5.
Abstract: Context. X-ray reflection off the accretion disc surrounding a black hole, together with the associated broad iron Kα line, has been widely used to constrain the innermost accretion-flow geometry and black hole spin. Some recent measurements have revealed steep reflection emissivity profiles in a number of active galactic nuclei and X-ray binaries. Aims. We explore the physically motivated conditions that give rise to the observed steep disc-reflection emissivity profiles. Methods. We perform aset of simulations based on the configuration of a possible future high-resolution X-raymission. Computations are carried out for typical X-ray bright Seyfert-1 galaxies. Results. We find that steep emissivity profiles with q ∼ 4−5 (where the emissivity is � (r) ∝ r −q ) are produced considering either i) a lamp-post scenario where a primary compact X-ray source is located close to the black hole, or ii) the radial dependence of the disc ionisation state. If both effects are taken into account, emissivity profiles as steep as q ∼ 7 can be obtained from X-ray spectra modelled via conventional reflection models. We also highlight the role of the reflection angular emissivity: the radial emissivity index q is overestimated when the standard limb-darkening law is used to describe the data. Conclusions. Very steep emissivity profiles with q ≥ 7 are naturally obtained by applying reflection models that take into account the radial profile ξ(r) of the disc ionisation induced by a compact X-ray source located close to the central black hole.

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Citations
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Journal ArticleDOI
TL;DR: In this paper, a detailed, general relativistic approach to model this irradiation for different geometries of the primary X-ray source is presented, including the standard point source on the rotational axis as well as more jet-like sources, which are radially elongated and accelerating.
Abstract: X-ray irradiation of the accretion disc leads to strong reflection features, which are then broadened and distorted by relativistic effects. We present a detailed, general relativistic approach to model this irradiation for different geometries of the primary X-ray source. These geometries include the standard point source on the rotational axis as well as more jet-like sources, which are radially elongated and accelerating. Incorporating this code in the RELLINE model for relativistic line emission, the line shape for any configuration can be predicted. We study how different irradiation geometries affect the determination of the spin of the black hole. Broad emission lines are produced only for compact irradiating sources situated close to the black hole. This is the only case where the black hole spin can be unambiguously determined. In all other cases the line shape is narrower, which could either be explained by a low spin or an elongated source. We conclude that for those cases and independent of the quality of the data, no unique solution for the spin exists and therefore only a lower limit of the spin value can be given

363 citations

Journal ArticleDOI
TL;DR: A review of the state of the art in this research field can be found in this paper, which describes the possible approaches to test the Kerr metric with current and future observational facilities and discusses current constraints.
Abstract: Astrophysical black hole candidates are thought to be the Kerr black holes of general relativity, but there is not yet direct observational evidence that the spacetime geometry around these objects is described by the Kerr solution. The study of the properties of the electromagnetic radiation emitted by gas or stars orbiting these objects can potentially test the Kerr black hole hypothesis. This paper reviews the state of the art of this research field, describing the possible approaches to test the Kerr metric with current and future observational facilities and discussing current constraints.

274 citations

Journal ArticleDOI
TL;DR: In this paper, the authors present 3-50keV NuSTAR observations of the active galactic nuclei Mrk 335 in a very low flux state and find that the spectra can be well fitted with relativistic reflection, and that the lowest flux state spectrum is described by reflection alone, suggesting the effects of extreme light bending occurring within ∼2 gravitational radii (RG) of the event horizon.
Abstract: We present 3–50keV NuSTAR observations of the active galactic nuclei Mrk 335 in a very low flux state. The spectrum is dominated by very strong features at the energies of the iron line at 5–7keV and Compton hump from 10–30keV. The source is variable during the observation, withthevariabilityconcentratedatlowenergies,whichsuggestingeitherarelativisticreflection oravariableabsorptionscenario.Inthiswork,wefocusonthereflectioninterpretation,making use of new relativistic reflection models that self consistently calculate the reflection fraction, relativistic blurring and angle-dependent reflection spectrum for different coronal heights to model the spectra. We find that the spectra can be well fitted with relativistic reflection, and that the lowest flux state spectrum is described by reflection alone, suggesting the effects of extreme light-bending occurring within ∼2 gravitational radii (RG) of the event horizon. The reflection fraction decreases sharply with increasing flux, consistent with a point source moving up to above 10 RG as the source brightens. We constrain the spin parameter to greater than 0.9 at the 3σ confidence level. By adding a spin-dependent upper limit on the reflection fraction to our models, we demonstrate that this can be a powerful way of constraining the spin parameter, particularly in reflection dominated states. We also calculate a detailed emissivity profile for the iron line, and find that it closely matches theoretical predictions for a compact source within a few RG of the black hole.

144 citations

Journal ArticleDOI
TL;DR: In this paper, a simple relativistically-blurred X-ray reflection model was used to determine the spin and the inner radius of the disc in accreting black holes, and it was shown that robust determination of disc truncation requires that the location of the coronal source is quasi-static and at a height and radius less than the truncation radius.
Abstract: We discuss the application of simple relativistically-blurred X-ray reflection models to the determination of the spin and the inner radius of the disc in accreting black holes. Observationally, the nature of the corona is uncertain a priori, but a robust determination of the inner disk radius can be made when the disc emissivity index is tightly constrained. When the inner disc is well illuminated, the black hole spin can also be determined. Using reflection modelling derived from ray tracing, we show that robust determination of disc truncation requires that the location of the coronal source is quasi-static and at a height and radius less than the truncation radius of the disc. Robust spin measurements require that at least part of the corona lies less than about 10 gravitational radii above the black hole in order that the innermost regions, including the innermost stable circular orbit, are well illuminated. The width of the blurring kernel (e.g., the iron line) has a strong dependence on coronal height. These limitations may be particularly applicable at low Eddington fractions (e.g. the low/hard state, and low-luminosity AGN) where the height of the corona may be relatively large, or outflowing, and tied to jet production.

109 citations

Journal ArticleDOI
TL;DR: In this article, a self-consistent reflection continuum was developed for the Type-C quasi-periodic oscillations (QPOs) of the inner flow of the accretion disc, giving rise to a blueshifted/redshifted iron line.
Abstract: Accreting stellar mass black holes (BHs) routinely exhibit Type-C quasi-periodic oscillations (QPOs). These are often interpreted as Lense–Thirring precession of the inner accretion flow, a relativistic effect whereby the spin of the BH distorts the surrounding space–time, inducing nodal precession. The best evidence for the precession model is the recent discovery, using a long joint XMM–Newton and NuSTAR observation of H 1743−322, that the centroid energy of the iron florescence line changes systematically with QPO phase. This was interpreted as the inner flow illuminating different azimuths of the accretion disc as it precesses, giving rise to a blueshifted/redshifted iron line when the approaching/receding disc material is illuminated. Here, we develop a physical model for this interpretation, including a self-consistent reflection continuum, and fit this to the same H 1743−322 data. We use an analytic function to parametrize the asymmetric illumination pattern on the disc surface that would result from inner flow precession, and find that the data are well described if two bright patches rotate about the disc surface. This model is preferred to alternatives considering an oscillating disc ionization parameter, disc inner radius and radial emissivity profile. We find that the reflection fraction varies with QPO phase (3.5σ), adding to the now formidable body of evidence that Type-C QPOs are a geometric effect. This is the first example of tomographic QPO modelling, initiating a powerful new technique that utilizes QPOs in order to map the dynamics of accreting material close to the BH.

73 citations

References
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Journal ArticleDOI
TL;DR: In this article, the authors examined the disc-jet connection in stellar mass and supermassive black holes by investigating the properties of their compact emission in the X-ray and radio bands, and found that the radio luminosity is correlated with both M and L X, at a highly significant level.
Abstract: We examine the disc-jet connection in stellar mass and supermassive black holes by investigating the properties of their compact emission in the X-ray and radio bands. We compile a sample of ∼100 active galactic nuclei with measured masses, 5-GHz core emission, and 2-10 keV luminosities, together with eight galactic black holes with a total of ∼50 simultaneous observations in the radio and X-ray bands. Using this sample, we study the correlations between the radio (L R ) and the X-ray (L X ) luminosity and the black hole mass (M). We find that the radio luminosity is correlated with both M and L X , at a highly significant level. In particular, we show that the sources define a 'Fundamental Plane' in the three-dimensional (log L R , log L X , log M) space, given by log L R = (0.60 + 0 . 1 1 -0.11) log L X + (0.78 + 0 . 1 1 -0.09) log M + 7.33 + 4 . 0 5 -4.07, with a substantial scatter of σ R = 0.88. We compare our results to the theoretical relations between radio flux, black hole mass, and accretion rate derived by Heinz & Sunyaev. Such relations depend only on the assumed accretion model and on the observed radio spectral index. Therefore, we are able to show that the X-ray emission from black holes accreting at less than a few per cent of the Eddington rate is unlikely to be produced by radiatively efficient accretion, and is marginally consistent with optically thin synchrotron emission from the jet. On the other hand, models for radiatively inefficient accretion flows seem to agree well with the data.

1,235 citations

Journal ArticleDOI
TL;DR: In this article, the observed spectra consist of both direct radiation from the primary X-ray source and reflection from the surrounding accretion flow, and features imprinted in it by photoabsorption, iron fluorescence and Compton scattering.
Abstract: Weak, broad emission lines due to low-ionisation stages of iron, and other spectral features, have recently been observed in the X-ray spectra of Active Galactic Nuclei and in some Galatic X-ray binaries. These features are due to X-ray irradiation of relatively cold, dense gas very close to the central compact object. The observed spectra consist of both direct radiation from the primary X-ray source and reflection' from the surrounding accretion flow. The reflected spectrum has features imprinted in it by photoabsorption, iron fluorescence and Compton scattering. The strength, shape and broadening of this reflected spectrum is a diagnostic of the geometry, ionisation state and iron abundance of the accretion flow, and through timing studies, may yield the mass of the central object.

1,150 citations

Journal ArticleDOI
TL;DR: In this article, a two-phase accretion disk model with thermal Comptonization is presented, with emphasis on thermal Comptonisation as the basic mechanism for producing the X-ray power-law continuum of Seyfert galaxies within the framework of the inner emission region.
Abstract: Attention is given to a two-phase accretion disk model, with emphasis on thermal Comptonization as the basic mechanism for producing the X-ray power-law continuum of Seyfert galaxies within the framework of the new picture of the inner emission region. A substantial fraction, f, of the gravitational power is assumed to be dissipated via buoyancy and reconnection of the magnetic fields in a hot tenuous 'corona' surrounding the disk's main body. Coupled thermal balance equations for the two phases yield the temperature of the hot phase and the slope of the Comptonized component self-consistently as a function of f and tau, the optical depth of the hot phase. It is found that for f approximately equal to 1 and tau less than 1, the temperature of the hot phase adjusts so as to maintain alpha approximately equal to 1. For small tau the temperature of the hot phase is sufficient for the production of electron-positron pairs to be important. Pair production contributes to the optical depth and limits the maximum temperature allowed.

951 citations