共查询到20条相似文献，搜索用时 625 毫秒
1.
A.C. Fabian 《Astronomische Nachrichten》2008,329(2):155-161
Iron line emission is common in the X‐ray spectra of accreting black holes. When the line emission is broad or variable then it is likely to originate from close to the black hole. X‐ray irradiation of the accretion flow by the power‐law X‐ray continuum produces the X‐ray ‘reflection’ spectrum which includes the iron line. The shape and variability of the iron lines and reflection can be used as a diagnostic of the radius, velocity and nature of the flow. The inner radius of the dense flow corresponds to the innermost stable circular orbit and thus can be used to determine the spin of the black hole. Studies of broad iron lines and reflection spectra offer much promise for understanding how the inner parts of accretion flows (and outflows) around black holes operate. There remains great potential for XMM‐Newton to continue to make significant progress in this work. The need for high quality spectra and thus for long exposure times is paramount. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
2.
We present Fe Kα line profiles from and images of relativistic discs with finite thickness around a rotating black hole using a novel code. The line is thought to be produced by iron fluorescence of a relatively cold X-ray-illuminated material in the innermost parts of the accretion disc and provides an excellent diagnostic of accretion flows in the vicinity of black holes. Previous studies have concentrated on the case of a thin, Keplerian accretion disc. This disc must become thicker and sub-Keplerian with increasing accretion rates. These can affect the line profiles and in turn can influence the estimation of the accretion disc and black hole parameters from the observed line profiles. We here embark on, for the first time, a fully relativistic computation which offers key insights into the effects of geometrical thickness and the sub-Keplerian orbital velocity on the line profiles. We include all relativistic effects such as frame-dragging, Doppler boost, time dilation, gravitational redshift and light bending. We find that the separation and the relative height between the blue and red peaks of the line profile diminish as the thickness of the disc increases. This code is also well suited to produce accretion disc images. We calculate the redshift and flux images of the accretion disc and find that the observed image of the disc strongly depends on the inclination angle. The self-shadowing effect appears remarkable for a high inclination angle, and leads to the black hole shadow being in this case, completely hidden by the disc itself. 相似文献
3.
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
4.
A. Martocchia C V. Karas G. Matt 《Monthly notices of the Royal Astronomical Society》2000,312(4):817-826
We performed detailed calculations of the relativistic effects acting on both the reflection continuum and the iron line from accretion discs around rotating black holes. Fully relativistic transfer of both illuminating and reprocessed photons has been considered in Kerr space–time. We calculated overall spectra, line profiles and integral quantities, and present their dependences on the black hole angular momentum. We show that the observed EW of the lines is substantially enlarged when the black hole rotates rapidly and/or the source of illumination is near above the hole. Therefore, such calculations provide a way to distinguish between different models of the central source. 相似文献
5.
Patrick Deegan Sergei Nayakshin 《Monthly notices of the Royal Astronomical Society》2007,377(2):897-904
Due to dynamical friction stellar mass black holes and neutron stars are expected to form high-density cusps in the inner parsec of our Galaxy. These compact remnants, expected to number around 20 000, may be accreting cold dense gas present there, and give rise to potentially observable X-ray emission. Here we build a simple but detailed time-dependent model of such emission. The possibility that these accretion flows are radiatively inefficient is taken into account and brings in some uncertainty in the conclusions. Despite this uncertainty, we find that at least several X-ray sources of this nature should be detectable with Chandra at any one time. Turning this issue around, we also ask a question of what current observational constraints might be telling us about the total number of compact remnants. In our 'best guess' model, a cusp of ∼40 000 remnants overpredicts the number of discrete sources and the total X-ray luminosity of the inner parsec, and is hence ruled out. In the most radiatively inefficient scenario that we consider, the radiative efficiency is set to be as small as ɛ= 10−5 . In this rather unlikely scenario, a cusp of ∼40 000 black holes would be allowed by the data, but several individual sources should still be visible. Future observations of the distribution and orbits of the cold ionized gas in the inner parsec of our Galaxy will put tighter constraints on the cusp of compact remnants. 相似文献
6.
Sheng-Miao Wu Ting-Gui Wang Xiao-Bo Dong 《Monthly notices of the Royal Astronomical Society》2008,389(1):213-222
Double-peaked broad emission lines in active galactic nuclei are generally considered to be formed in an accretion disc. In this paper, we compute the profiles of reprocessing emission lines from a relativistic, warped accretion disc around a black hole in order to explore the possibility that certain asymmetries in the double-peaked emission-line profile which cannot be explained by a circular Keplerian disc may be induced by disc warping. The disc warping also provides a solution for the energy budget in the emission-line region because it increases the solid angle of the outer disc portion subtended to the inner portion of the disc. We adopted a parametrized disc geometry and a central point-like source of ionizing radiation to capture the main characteristics of the emission-line profile from such discs. We find that the ratio between the blue and red peaks of the line profiles becoming less than unity can be naturally predicted by a twisted warped disc, and a third peak can be produced in some cases. We show that disc warping can reproduce the main features of multipeaked line profiles of four active galactic nuclei from the Sloan Digital Sky Survey. 相似文献
7.
A. C. Fabian 《Astrophysics and Space Science》2005,300(1-3):97-105
Several AGN and black hole X-ray binaries show a clear very broad iron line, which is strong evidence that the black holes
are rapidly spinning. Detailed analysis of these objects shows that the emission line is not significantly affected by absorption
and that the source variability is principally due to variation in amplitude of a power-law. Underlying this is a much less
variable, relativistically-smeared, reflection-dominated, component which carries the imprint of strong gravity at a few gravitational
radii. The strong gravitational light bending in these regions then explains the power-law variability as due to changes in
height of the primary X-ray source above the disc. The reflection component, in particular its variability and the profile
of the iron line, enables us to study the innermost regions around an accreting, spinning, black hole. 相似文献
8.
The galactic black hole binary systems give an observational template showing how the accretion flow changes as a function
of increasing mass accretion rate, or L/LEdd. These data can be synthesised with theoretical models of the accretion flow to give a coherent picture of accretion in strong
gravity, in which the major hard-soft spectral transition is triggered by a change in the nature and geometry of the inner
accretion flow from a hot, optically thin plasma to a cool, optically thick accretion disc. However, a straightforward application
of these models to AGN gives clear discrepancies in overall spectral shape. Either the underlying accretion model is wrong,
despite its success in describing the Galactic systems and/or there is additional physics which breaks the simple scaling
from stellar to supermassive black holes. 相似文献
9.
P. Kaaret 《Astronomische Nachrichten》2008,329(2):202-205
I discuss open questions regarding accreting black holes in binary systems, focusing on two questions: can we measure the spins of black holes and what is the nature of the ultraluminous X‐ray sources. XMM‐Newton could make significant strides in answering these questions and others over the next ten years. I suggest two observational programs designed to help increase our understanding of accreting black holes. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
10.
J. M. Miller 《Astronomische Nachrichten》2006,327(10):997-1003
In this contribution, I briefly review recent progress in detecting and measuring the properties of relativistic iron lines observed in stellar‐mass black hole systems, and the aspects of these lines that are most relevant to studies of similar lines in Seyfert‐1 AGN. In particular, the lines observed in stellar‐mass black holes are not complicated by complex low‐energy absorption or partial‐covering of the central engine, and strong lines are largely independent of the model used to fit the underlying broad‐band continuum flux. Indeed, relativistic iron lines are the most robust diagnostic of black hole spin that is presently available to observers, with specific advantages over the systematics–plagued disk continuum. If accretion onto stellar‐mass black holes simply scales with mass, then the widespread nature of lines in stellar‐mass black holes may indicate that lines should be common in Seyfert‐1 AGN, though perhaps harder to detect. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
11.
Sergei Nayakshin Rashid Sunyaev 《Monthly notices of the Royal Astronomical Society》2007,377(4):1647-1651
Compact remnants – stellar mass black holes and neutron stars formed in the inner few parsec of galactic centres are predicted to sink into the central parsec due to dynamical friction on low-mass stars, forming a high concentration cusp. Same physical region may also contain very high-density molecular clouds and accretion discs that are needed to fuel supermassive black hole (SMBH) activity. Here we estimate gas capture rates on to the cusp of stellar remnants, and the resulting X-ray luminosity, as a function of the accretion disc mass. At low disc masses, most compact objects are too dim to be observable, whereas in the high disc case most of them are accreting at their Eddington rates. We find that for low accretion disc masses, compact remnant cusps may be more luminous than the central SMBHs. This 'diffuse' emission may be of importance for local moderately bright active galactic nuclei (AGNs), especially low-luminosity AGNs. We also briefly discuss how this expected emission can be used to put constraints on the black hole cusp near our Galactic Centre. 相似文献
12.
Khee-Gan Lee Kinwah Wu Steven V. Fuerst Graziella Branduardi-Raymonl Oliver Crowley 《中国天文和天体物理学报》2009,9(4)
We have calculated the relativistic reflection component of the X-ray spectra of accretion disks in active galactic nuclei (AGN). Our calculations have shown that the spectra can be significantly modified by the motion of the accretion flow, and the gravity and rotation of the central black hole. The absorption edges in the spectra suffer severe en- ergy shifts and smearing, and the degree of distortion depends on the system parameters, in particular, the inner radius of the accretion disk and the disk viewing inclination angles. The effects are significant. Fluorescent X-ray emission lines from the inner accretion disk could be a powerful diagnostic of space-time distortion and dynamical relativistic effects near the event horizons of accreting black holes. However, improper treatment of the re- flection component in fitting the X-ray continuum could give rise to spurious line-like features. These features mimic the true fluorescent emission lines and may mask their relativistic signatures. Fully relativistic models for reflection continua together with the emission lines are needed in order to extract black-hole parameters from the AGN X-ray spectra. 相似文献
13.
Bárbara T. Ferreira Gordon I. Ogilvie 《Monthly notices of the Royal Astronomical Society》2008,386(4):2297-2310
According to one model, high-frequency quasi-periodic oscillations (QPOs) can be identified with inertial waves, trapped in the inner regions of accretion discs around black holes due to relativistic effects. In order to be detected, their amplitudes need to reach large enough values via some excitation mechanism. We work out in detail a non-linear coupling mechanism suggested by Kato, in which a global warping or eccentricity of the disc has a fundamental role. These large-scale deformations combine with trapped modes to generate 'intermediate' waves of negative energy that are damped as they approach either their corotation resonance or the inner edge of the disc, resulting in amplification of the trapped waves. We determine the growth rates of the inertial modes, as well as their dependence on the spin of the black hole and the properties of the disc. Our results indicate that this coupling mechanism can provide an efficient excitation of trapped inertial waves, provided the global deformations reach the inner part of the disc with non-negligible amplitude. 相似文献
14.
15.
The X‐ray spectra of luminous Seyfert 1 galaxies often appear to be reflection dominated. In a number of Narrow Line Seyfert 1 (NLS1) galaxies and galactic black holes in the very high state, the variability of the continuum and of the iron line are decoupled, the reflected component being often much less variable than the continuum. These properties have been interpreted as effects of gravitational light bending. In this framework, we present detailed Monte‐Carlo simulations of the reflection continuum in the Kerr metric. These calculations confirm that the spectra and variability behaviour of these sources can be reproduced by the light bending model. As an alternative to the light bending model, we show that similar observational properties are expected from radiation pressure dominated discs subject to violent clumping instabilities and, as a result, have a highly inhomogeneous two‐phase structure. In this model, most of the observed spectral and variability features originate from the complex geometrical structure of the inner regions of near‐Eddington accretion flows and are therefore a signature of accretion physics rather than general relativity. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
16.
Marek Gierli&#;ski Marek Niko&#;ajuk Bo&#;ena Czerny 《Monthly notices of the Royal Astronomical Society》2008,383(2):741-749
There is increasing evidence that supermassive black holes in active galactic nuclei (AGN) are scaled-up versions of Galactic black holes. We show that the amplitude of high-frequency X-ray variability in the hard spectral state is inversely proportional to the black hole mass over eight orders of magnitude. We have analysed all available hard-state data from RXTE of seven Galactic black holes. Their power density spectra change dramatically from observation to observation, except for the high-frequency (≳10 Hz) tail, which seems to have a universal shape, roughly represented by a power law of index −2. The amplitude of the tail, C M (extrapolated to 1 Hz), remains approximately constant for a given source, regardless of the luminosity, unlike the break or quasi-periodic oscillation frequencies, which are usually strongly correlated with luminosity. Comparison with a moderate-luminosity sample of AGN shows that the amplitude of the tail is a simple function of black hole mass, C M = C / M , where C ≈ 1.25 M⊙ Hz−1 . This makes C M a robust estimator of the black hole mass which is easy to apply to low- to moderate-luminosity supermassive black holes. The high-frequency tail with its universal shape is an invariant feature of a black hole and, possibly, an imprint of the last stable orbit. 相似文献
17.
分别在薄盘和厚盘两种情况下,讨论吸积盘的中心黑洞在顺行积和逆行吸积中的演化特征,结果表明,黑洞的温度和熵在逆行吸积与顺行吸积中的演化特征很不相同;在顺行吸积中黑洞的温度单调减小,而在逆行吸积中黑洞的温度存在一个极大值;在吸积的全过程中,黑洞的熵始终是增加的,而且逆行吸积中黑洞熵的变化率要比顺行吸积中的黑洞熵的变化率大得多,此外还讨论了在吸积过程中,黑洞质量的相对变化与黑洞自转的相关性。 相似文献
18.
Santabrata Das Sandip K. Chakrabarti 《Monthly notices of the Royal Astronomical Society》2008,389(1):371-378
We study the dynamical structure of a cooling dominated rotating accretion flow around a spinning black hole. We show that non-linear phenomena such as shock waves can be studied in terms of only three flow parameters, namely the specific energy , the specific angular momentum (λ) and the accretion rate of the flow. We present all possible accretion solutions. We find that a significant region of the parameter space in the plane allows global accretion shock solutions. The effective area of the parameter space for which the Rankine–Hugoniot shocks are possible is maximum when the flow is dissipation-free. It decreases with the increase of cooling effects and finally disappears when the cooling is high enough. We show that shock forms further away when the black hole is rotating compared to the solution around a Schwarzschild black hole with identical flow parameters at a large distance. However, in a normalized sense, the flow parameters for which the shocks form around the rotating black holes are produced shocks closer to the black hole. The location of the shock is also dictated by the cooling efficiency in that higher the accretion rate , the closer is the shock location. We believe that some of the high-frequency quasi-periodic oscillations may be due to the flows with higher accretion rate around the rotating black holes. 相似文献
19.
We compare standard models of accretion discs around black holes (BHs) that include the appropriate zero-torque inner boundary condition and relativistic effects on the emission and propagation of radiation. The comparison is performed adopting the multicolour disc blackbody model (MCD) as reference and looking for the parameter space in which it is in statistical agreement with 'more physical' accretion disc models. We find simple 'recipes' that can be used for adjusting the estimates of the physical inner radius of the disc, the BH mass and the accretion rate inferred using the parameters of the MCD fits. We applied these results to four ultraluminous X-ray sources for which MCD spectral fits of their X-ray soft spectral components have been published and find that, in three cases (NGC 1313 X-1, X-2 and M 81 X-9), the BH masses inferred for a standard disc around a Schwarzschild BH are in the interval ∼100–200 M⊙ . Only if the BH is maximally rotating are the masses comparable to the much larger values previously derived in the literature. 相似文献
20.
The fluctuating-accretion model of Lyubarskii and its extension by Kotov, Churazov & Gilfanov seek to explain the spectral-timing properties of the X-ray variability of accreting black holes in terms of inward-propagating mass accretion fluctuations produced at a broad range of radii. The fluctuations modulate the X-ray emitting region as they move inwards and can produce temporal-frequency-dependent lags between energy bands, and energy-dependent power spectral densities (PSDs) as a result of the different emissivity profiles, which may be expected at different X-ray energies. Here, we use a simple numerical implementation to investigate in detail the X-ray spectral-timing properties of the model and their relation to several physically interesting parameters, namely the emissivity profile in different energy bands, the geometrical thickness and viscosity parameter of the accretion flow, the strength of damping on the fluctuations and the temporal coherence (measured by the 'quality factor', Q ) of the fluctuations introduced at each radius. We find that a geometrically thick flow with large viscosity parameter is favoured, and we confirm that the predicted lags are quite robust to changes in the emissivity profile and physical parameters of the accretion flow, which may help to explain the similarity of the lag spectra in the low/hard and high/soft states of Cyg X-1. We also demonstrate the model regime where the light curves in different energy bands are highly spectrally coherent. We compare model predictions directly to X-ray data from the narrow line Seyfert 1 galaxy NGC 4051 and the black hole X-ray binary (BHXRB) Cyg X-1 in its high/soft state, and we show that this general scheme can reproduce simultaneously the time lags and energy-dependence of the PSD. 相似文献