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1.
In this paper we present a fully relativistic approach to modelling both the continuum emission and the reflected fluorescent iron line from a primary X-ray source near a Kerr black hole. The X-ray source is located above an accretion disc orbiting around the black hole. The source is assumed to be a static point source located on an arbitrary position above the disc, on or off the axis of rotation. We carry out Monte Carlo simulations in order to estimate the iron line spectrum as well as its equivalent width. Because of the gravitational lensing effect, an enhancement of the iron line is expected when the primary source is located close to the central black hole. We find that for a source located on the axis of rotation the enhancement is relatively modest. An observer at inclination 30° would measure an equivalent width of ∼300 eV in the extreme case of a maximally rotating black hole and a source located at height 1.5 gravitational radii from the centre. This corresponds to an equivalent width enhancement factor of about 2 compared with the classical value where no lensing effect comes into play. However, when allowing the source to be located off the axis of rotation, much stronger enhancement can be obtained. In the extreme case of a maximally rotating black hole and a source located just above the approaching side of the disc, an observer at inclination 30° could measure an equivalent width as high as ∼1.5 keV (i.e., ∼10 times the classical value). We also find that observers located at high inclination angles observe a stronger line than observers at low inclination angles.  相似文献   

2.
We reanalyse the ASCA and BeppoSAX data of MCG–6-30-15, using a double-zone model for the iron line profile. In this model, the X-ray source is located around ≈10 Schwarzschild radii and the regions interior and exterior to the X-ray source produce the line emission. We find that this model fits the data with a similar reduced χ 2 to the standard single-zone model. Thus we show that the presence of a broad iron line feature does not necessarily require that the X-ray source be located close to the last stable orbit or in the disc rotation axis.
Within the framework of this model, the best-fitting inclination angle of the source     for the intermediate-intensity ASCA data set is compatible with that determined by earlier modelling of optical lines. The observed variability of the line profile with intensity can be explained as variations of the X-ray source size. That several active galactic nuclei with broad lines have the peak centroid near 6.4 keV can be explained under certain conditions.
We also show that the simultaneous broad-band observations of this source by BeppoSAX rule out the Comptonization model which was an alternative to the standard inner-disc one. We thereby strengthen the case that line broadening occurs as a result of the strong gravitational influence of a black hole.  相似文献   

3.
We investigate the properties of fluorescent iron lines that arise as a result of the illumination of a black hole accretion disc by an X-ray source located above the disc's surface. We study in detail the light-bending model of the variability of the lines, extending previous work on the subject. We indicate that the bending of photon trajectories to the equatorial plane (a distinct property of the Kerr metric) is the most feasible effect underlying the reduced variability of the lines observed in several objects. A model involving an X-ray source with a varying radial distance, located within a few central gravitational radii around a rapidly rotating black hole, close to the disc's surface, may explain both the elongated red wing of the line profile and the complex variability pattern observed in MCG–6-30-15 by XMM–Newton . We also point out that illumination by radiation that returns to the disc (following the previous reflection) contributes significantly to the formation of the line profile in some cases. As a result of this effect, the line profile always has a pronounced blue peak (which is not observed in the deep minimum state in MCG–6-30-15), unless the reflecting material is absent within the innermost 2–3 gravitational radii.  相似文献   

4.
Recent X-ray observations have shown evidence for exceptionally broad and skewed iron Kα emission lines from several accreting black hole systems. The lines are assumed to be due to fluorescence of the accretion disc illuminated by a surrounding corona and require a steep emissivity profile increasing into the innermost radius. This appears to question both standard accretion disc theory and the zero-torque assumption for the inner boundary condition, both of which predict a much less extreme profile. Instead it argues that a torque may be present due to magnetic coupling with matter in the plunging region or even to the spinning black hole itself. Discussion so far has centred on the torque acting on the disc. However, the crucial determinant of the iron line profile is the radial variation of the power radiated in the corona. Here we study the effects of different inner boundary conditions on the coronal emissivity and on the profiles of the observable Fe Kα lines. We argue that in the extreme case where a prominent highly redshifted component of the iron line is detected, requiring a steep emissivity profile in the innermost part and a flatter one outside, energy from the gas plunging into the black hole is being fed directly to the corona.  相似文献   

5.
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6.
We present a study of the spectral variability of the Seyfert I galaxy MCG–6-30-15 based on the two long XMM–Newton observations from 2000 and 2001. The X–ray spectrum and variability properties of the 2001 data have previously been well described with a two-component model consisting of a variable power-law and a much less variable reflection component, containing a broad relativistic iron line from the accretion disc around a rapidly rotating Kerr black hole. The lack of variability of the reflection component has been interpreted as an effect of strong gravitational light bending very close to the central black hole. Using an improved reflection model, we fit the two-component model to time-resolved spectra of both observations. Assuming that the photon index of the power law is constant, we reconfirm the old result and show that this does not depend on the time-scale of the analysis.  相似文献   

7.
The fluorescent iron K α emission-line profile provides an excellent probe of the innermost regions of active galactic nuclei. Fe  xxv and Fe  xxvi in diffuse plasma above the accretion disc can affect the X-ray spectrum by iron K α resonant absorption. This in turn can influence the interpretation of the data and the estimation of the accretion disc and black hole parameters. We embark on a fully relativistic computation of this effect and calculate the iron line profile in the framework of a specific model in which rotating, highly ionized and resonantly absorbing plasma occurs close to the black hole. This can explain the features seen in the iron K α line profile recently obtained by Nandra et al. for the type 1 Seyfert galaxy NGC 3516. We show that the redshift of this feature can be mainly gravitational in origin and accounted for without the need to invoke fast accretion of matter on to the black hole. New X-ray satellites such as XMM , ASTRO-E and Chandra provide excellent opportunities to test the model against high-quality observational data.  相似文献   

8.
We discuss constraints on black hole spin and spin-related astrophysics as derived from X-ray spectroscopy. After a brief discussion about the robustness with which X-ray spectroscopy can be used to probe strong gravity, we summarize how these techniques can constrain black hole spin. In particular, we highlight XMM-Newton studies of the Seyfert galaxy MCG-6-30-15 and the stellar-mass black hole GX 339-4. The broad X-ray iron line profile, together with reasonable and general astrophysical assumptions, allow a non-rotating black hole to be rejected in both of these sources. If we make the stronger assertion of no emission from within the innermost stable circular orbit, the MCG-6-30-15 data constrain the dimensionless spin parameter to be a > 0.93. Furthermore, these XMM-Newton data are already providing evidence for exotic spin-related astrophysics in the central regions of this object. We conclude with a discussion of the impact that Constellation-X will have on the study of strong gravity and black hole spin.  相似文献   

9.
We analysed simultaneous archival XMM–Newton and Rossi X-ray Timing Explorer observations of the X-ray binary and black hole candidate Swift J  1753.5−0127  . In a previous analysis of the same data, a soft thermal component was found in the X-ray spectrum, and the presence of an accretion disc extending close to the innermost stable circular orbit was proposed. This is in contrast with the standard picture in which the accretion disc is truncated at large radii in the low/hard state. We tested a number of spectral models and found that several of them fit the observed spectra without the need of a soft disc-like component. This result implies that the classical paradigm of a truncated accretion disc in the low/hard state cannot be ruled out by these data. We further discovered a broad iron emission line between 6 and 7 keV in these data. From fits to the line profile we found an inner disc radius that ranges between ∼6 and 16 gravitational radii, which can be in fact much larger, up to ∼250 gravitational radii, depending on the model used to fit the continuum and the line. We discuss the implications of these results in the context of a fully or partially truncated accretion disc.  相似文献   

10.
Spectra of Seyfert 1s are commonly modelled as emission from an X-ray-illuminated flat accretion disc orbiting a central black hole. This provides both reprocessed and direct components of the X-ray emission, as required by observations of individual objects, and possibly a fraction of the cosmological X-ray background. There is some observational motivation for us to at least consider the role that an effectively concave disc surface might play: (1) a reprocessed fraction ≳1/2 in some Seyferts and possibly in the X-ray background, and (2) the commonality of a sharp iron line peak for Seyferts at 6.4 keV despite a dependence of peak location on inclination angle for flat disc models. Here it is shown that a concave disc may not only provide a larger total fraction of reprocessed photons, but can also reprocess a much larger fraction of photons in its outer regions compared with a flat disc. This reduces the sensitivity of the 6.4-keV peak location to the inner disc inclination angle because the outer regions are less affected by Doppler and gravitational effects. If the X-ray source is isotropic, the reprocessed fraction is directly determined by the concavity. If the X-ray source is anisotropic, the location of iron line peak can still be determined by concavity but the total reflected fraction need not be as large as for the isotropic emitter case. The geometric calculations herein are applicable to general accretion disc systems illuminated from the centre.  相似文献   

11.
We present XMM-Newton European Photon Imaging Camera (EPIC) observations of the bright Seyfert 1 galaxy MCG–6-30-15, focusing on the broad Fe K α line at ∼6 keV and the associated reflection continuum, which is believed to originate from the inner accretion disc. We find these reflection features to be extremely broad and redshifted, indicating an origin in the very central regions of the accretion disc. It seems likely that we have caught this source in the 'deep minimum' state first observed by Iwasawa et al. The implied central concentration of X-ray illumination is difficult to understand in any pure accretion disc model. We suggest that we are witnessing the extraction and dissipation of rotational energy from a spinning black hole by magnetic fields connecting the black hole or plunging region to the disc.  相似文献   

12.
The broad X-ray iron line, detected in many active galactic nuclei, is likely to be produced by fluorescence from the X-ray-illuminated central parts of an accretion disc close to a supermassive black hole. The time-averaged shape of the line can be explained most naturally by a combination of special and general relativistic effects. Such line profiles contain information about the black hole spin and the accretion disc, as well as the geometry of the emitting region, and may help to test general relativity in the strong gravity regime. In this paper we embark on the computation of the temporal response of the line to the illuminating flux. Previous studies concentrated on the calculation of reverberation signatures from static sources illuminating the disc. In this paper we focus on the more physically justified case of flares located above the accretion disc and corotating with it. We compute the time-dependent iron line, taking into account all general relativistic effects, and show that its shape is of a very complex nature, and we also present light curves accompanying the iron line variability. We suggest that present and future X-ray satellites like XMM or Constellation-X may be capable of detecting features present in the computed reverberation maps.  相似文献   

13.
Details of the observations of a new (second) outburst of the X-ray transientMAXI J1836-194 discovered late in August 2011, a suspected black hole in a low-mass binary system, with the instruments of the SWIFT and INTEGRAL orbiting observatories are presented. The outburst was weaker than the first one; the source had a power-law spectrum in a wide X-ray (0.3–400 keV) energy range without any clear evidence for the presence of a soft (blackbody) component related to the emission from the outer accretion disk regions. This shows that the outburst was a “failed” one: the source did not pass through the sequence of spectral states characteristic of X-ray novae. The observed optical emission from the source whose variability was strongly correlated with its X-ray variability seems to have also been an extension of the power-law spectrum. Spectrum uniformity is, on the whole, unusual for other sources containing a black hole and raises the question about the nature of the emission from MAXI J1836-194 (disk or jet).  相似文献   

14.
We present a new explanation for the origin of the steep power-law(SPL) state of X-ray binaries.The power-law component of X-ray emission is the synchrotron radiation of relativistic electrons in highly magnetized compact spots orbiting near the inner stable circular orbit of a black hole.It has a hard spectrum that extends to above MeV energies,which is determined by the electron acceleration rate.These photons are then down-scattered by the surrounding plasma to form an observed steep spectrum.We discuss ...  相似文献   

15.
We describe a new approach to calculating photon trajectories and gravitational lensing effects in the strong gravitational field of the Kerr black hole. These techniques are applied to explore both the imaging and spectral properties of photons emitted from an accretion disc, which perform multiple orbits of the central mass before escaping to infinity. Viewed at large inclinations, these higher-order photons contribute ∼20 per cent of the total luminosity of the system for a Schwarzschild hole, while for an extreme Kerr black hole this fraction rises to ∼60 per cent. In more realistic models, these photons will be reabsorbed by the disc at large distances from the hole, but this returning radiation could provide a physical mechanism to resolve the discrepancy between the predicted and observed optical/ultraviolet colours in active galactic nuclei. Conversely, at low inclinations, higher-order images reintercept the disc plane close to the black hole, so need not be absorbed by the disc if this is within the plunging region. These photons form a bright ring carrying approximately 10 per cent of the total disc luminosity for a Schwarzschild black hole. The spatial separation between the inner edge of the disc and the ring is similar to the size of the event horizon. This is resolvable for supermassive black holes with proposed X-ray interferometery missions such as the Microarcsecond X-ray Imaging Mission (MAXIM), and so has the potential to provide an observational test of strong field gravity.  相似文献   

16.
Dynamics of oscillating relativistic tori around Kerr black holes   总被引：1，自引：0，他引：1  
We present a comprehensive numerical study of the dynamics of relativistic axisymmetric accretion tori with a power-law distribution of specific angular momentum orbiting in the background space–time of a Kerr black hole. By combining general relativistic hydrodynamics simulations with a linear perturbative approach we investigate the main dynamical properties of these objects over a large parameter space. The astrophysical implications of our results extend and improve two interesting results that have been recently reported in the literature. First, the induced quasi-periodic variation of the mass quadrupole moment makes relativistic tori of nuclear matter densities, as those formed during the last stages of binary neutron star mergers, promising sources of gravitational radiation, potentially detectable by interferometric instruments. Secondly, p-mode oscillations in relativistic tori of low rest-mass densities could be used to explain high-frequency quasi-periodic oscillations observed in X-ray binaries containing a black hole candidate under conditions more generic than those considered so far.  相似文献   

17.
We present a spectrophotometric study of short-term optical variability in the quiescent black hole X-ray transient V404 Cyg. This includes two nights of high-time-resolution H α spectroscopy with which we resolve much of the time variability, and a further six nights of archival spectroscopy with lower time resolution but higher spectral resolution. We find significant variability in most of the data considered, with both the H α line and the continuum often varying in a correlated way. This includes both dramatic flares lasting a few hours in which the line flux nearly doubles and lower-level flickering. The strongest flares involve development of asymmetry in the line profile, with the red wing usually strongest independent of orbital phase. It is unclear why this is the case, but we discuss several possible explanations. We consider the energetics of the flares and compare with plausible models including chromospheric activity on the companion star, local magnetic reconnection events within the disc and varying irradiation from close to the black hole. Based on the line profile changes during the flares, we conclude that the most likely origin for the variability is variable photoionization by the central source, although local flares within the disc cannot be ruled out.  相似文献   

18.
The current paradigm of high energy spectroscopy tells us that light emitted from a wide variety of objects has its origin close to the black hole event horizon. As such, these photons are subject to general relativistic effects such as light-bending, gravitational lensing and redshift, time-dilation, etc. These gravitational effects are well-understood from a theoretical standpoint and therefore, provide a natural mechanism to test the properties of strong gravitational fields. To this end, we have developed a new (semi-analytic) strong gravity code, capable of describing the contribution of photons that perform multiple orbits of the hole. We apply this code to a simple Keplerian accretion disk in order to understand the role played by the angular emissivity, black hole spin and higher order images in forming the line profile.  相似文献   

19.
We report the results of a systematic timing analysis of RXTE observations of GRS 1915+105 when the source was in its variability class θ, characterized by alternating soft and hard states on a time-scale of a few hundred seconds. The aim was to examine the high-frequency part of the power spectrum in order to confirm the hectohertz quasi-periodic oscillations (QPO) previously reported from observations from mixed variability behaviours. During the hard intervals (corresponding to state C in the classification of Belloni et al.), we find a significant QPO at a frequency of ∼170 Hz, although much broader (Q∼2) than previously reported. No other significant peak is observed at frequencies >30 Hz. A time-resolved spectral analysis of selected observations shows that the hard intervals from class θ show a stronger and steeper  (Γ= 2.8–3.0)  power-law component than hard intervals from other classes. We discuss these results in the framework of hectohertz QPOs reported from GRS 1915+105 and other black hole binaries.  相似文献   

20.
We report the discovery of emission features in the X-ray spectrum of GRO J1655–40 obtained with RXTE during the observation of 1997 February 26. We have fitted the features first by two Gaussian lines which in four spectra analysed have average energies of 5.85±0.08 and 7.32±0.13 keV, strongly suggestive that these are the red- and blueshifted wings of an iron disc line. These energies imply a velocity of ∼0.33 c . The blue wing is less bright than in the calculated profiles of disc lines near a black hole subject to Doppler boosting; however, known Fe absorption lines in GRO J1655–40 at energies between ∼7 and 8 keV can reduce the apparent brightness of the blue wing. Secondly, we have fitted the spectra using the disc line model of Laor based on a full relativistic treatment plus an absorption line, and show that good fits are obtained. This gives a rest-frame energy of the disc line between 6.4 and 6.8 keV, indicating that the line is iron K α emission probably of significantly ionized material. The Laor model shows that the line originates in a region of the accretion disc extending from ∼10 Schwarzschild radii from the black hole outwards. The line is direct evidence for the black hole nature of the compact object, and is the first discovery of a highly red- and blueshifted iron disc line in a Galactic source.  相似文献   

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