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1.
We calculate the broad-band radio–X-ray spectra predicted by microblazar and microquasar models for ultraluminous X-ray sources (ULXs), exploring the possibility that their dominant power-law component is produced by a relativistic jet, even at near-Eddington mass accretion rates. We do this by first constructing a generalized disc–jet theoretical framework in which some fraction of the total accretion power, P a, is efficiently removed from the accretion disc by a magnetic torque responsible for jet formation. Thus, for different black hole masses, mass accretion rates and magnetic coupling strength, we self-consistently calculate the relative importance of the modified disc spectrum, as well as the overall jet emission due to synchrotron and Compton processes. In general, transferring accretion power to a jet makes the disc fainter and cooler than a standard disc at the same mass accretion rate; this may explain why the soft spectral component appears less prominent than the dominant power-law component in most bright ULXs. We show that the apparent X-ray luminosity and spectrum predicted by the microquasar model are consistent with the observed properties of most ULXs. We predict that the radio synchrotron jet emission is too faint to be detected at the typical threshold of radio surveys to date. This is consistent with the high rate of non-detections over detections in radio counterpart searches. Conversely, we conclude that the observed radio emission found associated with a few ULXs cannot be due to beamed synchrotron emission from a relativistic jet.  相似文献   

2.
There is accumulating evidence for mass ejection in low-mass X-ray binaries(LMXBs)driven by radio pulsar activity during X-ray quiescence.We consider the condition for mass ejection by comparing the radiation pressure from a millisecond pulsar,and the gas pressure at the inner Lagrange point or at the surrounding accretion disk.We calculate the critical spin period of the pulsar below which mass ejection is allowed.Combining with the evolution of the mass transfer rate,we present constraints on the orbital ...  相似文献   

3.
We present computed radiation spectra for the boundary layer (BL) of the accretion disk that is formed near the surface of a neutron star. Both free-free processes and Comptonization were taken into account. Our computations are based on the hydrodynamic solution obtained by Popham and Sunyaev (2001) for the BL structure. The computed spectra are highly diluted compared to the Planck spectra of the same surface temperature. They are complex in shape; in particular, an intense Wien emission component is formed in their high-energy region at high accretion rates. In general, the computed spectra are harder than those observed in actual X-ray sources. This is the result of a very high temperature found by Popham and Sunyaev (2001) for the BL. We show that such temperatures could result from an oversimplified treatment of radiative transfer in their paper, which completely ignored the frequency dependence of the matter opacity and radiation intensity. Our computations indicate that at moderate accretion rates, a proper treatment of radiative transfer with allowance for Comptonization leads to appreciably lower plasma temperatures and to softer radiation spectra.  相似文献   

4.
Variability of the central region in active galactic nuclei   总被引：3，自引：0，他引：3  
Summary We review implications of the observed optical and X-ray variability (periodicities and light-curves), relevant for the understanding of physical conditions in the deep interiors of active galactic nuclei. We discuss in detail kinematical, hydrodynamical, thermal and radiative transfer effects which may be responsible for observed variability patterns. We put emphasis on theoretical options which can predict basic accretion parameters, such as the mass of the central black hole, the accretion rate and the inclination angle, in terms of observable quantities. Closed analytical results are given whenever available.  相似文献   

5.
We present a numerical analysis of the spin evolution of neutron stars in low-mass X-ray binaries, trying to explain the discrepancy in the spin period distribution between observations of millisecond pulsars and theoretical results. In our calculations, we take account of possible effects of radiation pressure and irradiation-induced instability on the structure of the disk, and the evolution of the mass transfer rate, respectively. We report the following results: (1) The radiation pressure in the accretion disk leads to a slight increase of spin periods, and the variation of mass transfer rate caused by the neutron star irradiation can shorten the spin-down phase of evolution. (2) The calculated results of the model combining radiation pressure and irradiation show that the accretion is strongly limited by the radiation pressure in the high mass transfer phase. (3) The accreted mass and fastness parameter can affect the number of systems in the equilibrium state.  相似文献   

6.
The observed spin periods of the primaries in intermediate polars require the presence of torques additional to the standard expectation of material and magnetic torques from an accretion disc. In an extension of earlier work by Wickramasinghe, Wu and Ferrario, from rates of mass transfer deduced from optical and X-ray fluxes, and assuming surface magnetic fields similar to those observed in the most rapidly rotating low mass main sequence stars, it is shown that the magnetohydrodynamic torque between the partially shielded primary and the secondary is sufficient to account for the observed spin periods. It is further found that there is a range of magnetic moments and mass transfer rates in which synchronized rotation of the primary can occur even though it possesses an accretion disc. This may account for the structures deduced for V795 Her and V2051 Oph.An analysis of the observed outbursts (or lack of) in the intermediate polars shown that wheres many systems are explainable in terms of the standard theory of -discs, in some systems the magnetic structure is supressing expected outbursts and instabilities that are as yet not understood appear in their place.  相似文献   

7.
Eccentric-orbit binary models for transient X-ray sources are investigated. In these models, a compact star is in an eccentric orbit around a more massive star. As the compact star accretes mass from the stellar wind of the massive star, the accretion rate becomes time-dependent. The accretion rate is determined by Bondi's accretion radius, which depends on both the relative velocity of the stellar wind to the compact star and the sound velocity through the stellar wind. With reasonable sets of the eccentricity, the semi-major axis, the stellar wind velocity and the sound velocity, we obtain the variations of the light curves compatible with observations for the transient X-ray sources. It is likely that many transient X-ray sources are explainable by eccentric-orbit binary models.  相似文献   

8.
The peaks in the spectra of the accretion disks surrounding massive black holes in quasars are in the far-UV or soft X-ray band, which are usually not observed. However, in the disk corona model, soft photons from the disk are Comptonized to high energy in the hot corona, and the hard X-ray spectra(luminosity and spectral shape) contain information on the incident spectra from the disk. The values of black hole spin parameter a*are inferred from the spectral fitting, which are spread over a large range, ~-0.94 to 0.998. We find that the inclination angles and mass accretion rates are well determined by the spectral fitting, but the results are sensitive to the accuracy of black hole mass estimates. No tight constraints on the black hole spins are achieved, if the uncertainties in black hole mass measurements are a factor of four,which are typical for the single-epoch reverberation mapping method. Recently, the accuracy of black hole mass measurement has been significantly improved to 0.2- 0.4 dex with the velocity resolved reverberation mapping method. The black hole spin can be well constrained if the mass measurement accuracy is50%. In the accretion disk corona scenario, a fraction of power dissipated in the disk is transported into the corona, and therefore the accretion disk is thinner than a bare disk for the same mass accretion rate,because the radiation pressure in the disk is reduced. We find that the thin disk approximation, H/R0.1,is still valid if 0.3 m 0.5, provided half of the dissipated power is radiated in the corona above the disk.  相似文献   

9.
The standstill luminosity in Z Cam systems   总被引：1，自引：0，他引：1  
We consider accretion discs in close binary systems. We show that the heating of a disc at the impact point of the accretion stream contributes significantly to the local energy budget at its outer edge. As a result, the thermal balance relation between local accretion rate and surface density (the 'S-curve') changes; the critical mass transfer rate above which no dwarf nova outbursts occur can be up to 40 per cent smaller than without impact heating. Standstills in Z Cam systems thus occur at smaller mass transfer rates than otherwise expected, and are fainter than the peak luminosity during the dwarf nova phase as a result.  相似文献   

10.
具有不同质量的恒星在耗尽其热核能源后,最终可能会坍缩成为性质完全不同的致密天体,如白矮星、中子星或者黑洞。从20世纪30年代起,黑洞的观测及其证认一直是天体物理学的研究热点之一。首先简要地回顾了恒星级黑洞的形成及其候选天体的研究历史;然后介绍了如何从观测上证认恒星级黑洞:接着详细讨论了恒星级黑洞的质量和自转参数的测量方法;最后介绍恒星级黑洞观测及其证认的最新研究进展,并做出结论:目前已经有充分的证据宣告在部分吸积X射线双星中存在恒星级黑洞。  相似文献   

11.
Patrick Cassen  Ann Moosman 《Icarus》1981,48(3):353-376
An analysis is presented of the hydrodynamic aspects of the growth of protostellar disks from the accretion (or collapse) of a rotating gas cloud. The size, mass, and radiative properties of protostellar disks are determined by the distribution of mass and angular momentum in the clouds from which they are formed, as well as from the dissipative processes within the disks themselves. The angular momentum of the infalling cloud is redistributed by the action of turbulent viscosity on a shear layer near the surface of the disk (downstream of the accretion shock) and on the radial shear across cylindrical surfaces parallel to the rotation axis. The fraction of gas that is fed into a central core (protostar) during accretion depends on the ratio of the rate of viscous diffusion of angular momentum to the accretion rate; rapid viscous diffusion (or a low accretion rate) promotes a large core-to-disk mass ratio. The continuum radiation spectrum of a highly viscous disk is similar to that of a steady-state accretion disk without mass addition. It is possible to construct models of the primitive solar nebula as an accretion disk, formed by the collapse of a slowly rotating protostellar cloud, and containing the minimum mass required to account for the planets. Other models with more massive disks are also possible.  相似文献   

12.
We present a case study of the relevance of the radially pulsational instability of a two-temperature accretion disk around a neutron star to anomalous X-ray pulsars (AXPs). Our estimates are based on the approximation that such a neutron star disk with mass in the range of 10^-6-10^-5M⊙ is formed by supernova fallback. We derive several peculiar properties of the accretion disk instability: a narrow interval of X-ray pulse periods; lower X-ray luminosities; a period derivative and an evolution time scale. All these results are in good agreement with the observations of the AXPs.  相似文献   

13.
We discuss radiation properties of plasmas in high-energy astrophysics with a keyword nonequilibrium: non-LTE level populations, nonequilibrium ionization, and non-Maxwellian distribution function, beginning with radiative transfer. We focus particularly on supernova remnants interacting with the circumstellar/interstellar matter, and also mention line emission processes in accretion gas onto a neutron star or black hole, and in the X-ray afterglow of γ-ray bursts.  相似文献   

14.
We study numerically the interaction of the infalling gas stream and the rim of the accretion disc in cataclysmic variables. The simulations were performed with a smoothed particle hydrodynamics scheme with high spatial resolution. Parameters of the systems AM CVn, OY Car, DQ Her, U Gem and IP Peg were used for the simulations. The simulations cover a wide range of orbital periods, mass ratios and mass transfer rates, as well as different thermal states of the accretion disc. The main result of this study is that the accretion stream is not stopped at the impact region (the bright spot at the outer rim of the disc). In fact, after undergoing the shock interaction, most of the matter is deflected vertically and flows in a more or less diffuse stream to inner parts of the disc, hitting the disc surface close to the circularization radius at orbital phase 0.5. This is a common feature in all systems for all simulated parameters. This stream overflow can cause the X-ray absorption dips observed in cataclysmic variables (CVs) and low-mass X-ray binaries (LMXBs) around orbital phase 0.7, if the inclination is at least 65°. Under certain circumstances, namely a sudden increase of the mass transfer rate from the secondary or a rather small disc, parts of the overflowing stream bounce off the disc surface after hitting it at orbital phase ≈0.5. Another absorption region can be expected around orbital phase 0.2.
In our simulations most of the infalling matter reaches the inner disc very quickly. This must alter the evolution of the quiescent disc and the outburst behaviour considerably compared with purely viscous transport of the material through the disc from the outer rim, and therefore should be taken into account in dwarf nova outburst cycle calculations. To our knowledge, the consequences of such a massive stream overflow for the dwarf nova outburst cycle have not been considered yet.  相似文献   

15.
We have developed a stellar wind model for OB supergiants to investigate the effects of accretion from a clumpy wind on the luminosity and variability properties of high-mass X-ray binaries. Assuming that the clumps are confined by ram pressure of the ambient gas and exploring different distributions for their mass and radii, we computed the expected X-ray light curves in the framework of the Bondi–Hoyle accretion theory, modified to take into account the presence of clumps. The resulting variability properties are found to depend not only on the assumed orbital parameters but also on the wind characteristics. We have then applied this model to reproduce the X-ray light curves of three representative high-mass X-ray binaries: two persistent supergiant systems (Vela X−1 and 4U 1700−377) and the supergiant fast X-ray transient IGR J11215−5952. The model can reproduce the observed light curves well, but requiring in all cases an overall mass loss from the supergiant about a factor of 3–10 smaller than the values inferred from ultraviolet lines studies that assume a homogeneous wind.  相似文献   

16.
Chandra and XMM–Newton observations of the Cartwheel galaxy show ∼17 bright X-ray sources  (≳5 × 1038 erg s−1)  , all within the gas-rich outer ring. We explore the hypothesis that these X-ray sources are powered by intermediate-mass black holes (IMBHs) accreting gas or undergoing mass transfer from a stellar companion. To this purpose, we run N -body/smoothed particle hydrodynamics simulations of the galaxy interaction which might have led to the formation of Cartwheel, tracking the dynamical evolution of two different IMBH populations: halo and disc IMBHs. Halo IMBHs cannot account for the observed X-ray sources, as only a few of them cross the outer ring. Instead, more than half of the disc IMBHs are pulled in the outer ring as a consequence of the galaxy collision. However, also in the case of disc IMBHs, accretion from surrounding gas clouds cannot account for the high luminosities of the observed sources. Finally, more than 500 disc IMBHs are required to produce ≲15 X-ray sources via mass transfer from very young stellar companions. Such number of IMBHs is very large and implies extreme assumptions. Thus, the hypothesis that all the observed X-ray sources in Cartwheel are associated with IMBHs is hardly consistent with our simulations, even if it is still possible that IMBHs account for the few (≲1–5) brightest ultraluminous X-ray sources.  相似文献   

17.
We perform 3D hydrodynamical numerical simulations of the winds interaction process in the massive binary system η Carinae, and find the secondary star to accrete mass from the dense primary wind close to periastron passage. This accretion is thought to result in the spectroscopic event and X-ray minimum observed in the system every revolution. In this study we limit ourselves to explore the role of clumps in the primary wind in triggering the accretion process. We include the gravity of the secondary star and the orbital motion starting 19 days (90°) before periastron passage. The accretion process is triggered by dense clumps that cannot be decelerated by the ram pressure of the secondary wind. The dense clumps are formed by instabilities in the thin dense shell formed by the shocked primary wind gas. We explore the role of the numerical viscosity and some physical parameters on the initiation of the accretion process, and explain the unique properties of η Car that allow for the periastron accretion process to occur. The accretion starts about a week before periastron passage, as is required to explain the several weeks long X-ray minimum.  相似文献   

18.
We examine the spectra of the persistent emission from anomalous X-ray pulsars (AXPs) and their variati on with the spin-down rate Ω. Based onan accretion-powered model, the influences of both the magnetic field and the mass accretion rate on the spectral properties of AXPs are addressed. We then investigate the relation between the spectral property of AXPs and mass accretion rate M. The result shows that there exists a linear correlation between the photon index and the mass accretion rate: the spectral hardness increases with increasing M. A possible emission mechanism for the explanation of the spectral properties of AXPs is also discussed.  相似文献   

19.
Two-dimensional calculations of the hydrodynamics produced by nuclear starbursts, taking into consideration the accretion or infall of disc matter on to the heart of the starburst, are here shown to lead to stationary solutions that naturally account for the kpc-scale biconical X-ray and optically detected filamentary structure. The calculated flows are critically compared with former models and with observations of nuclear starbursts. For the infall models, we find that the mechanical energy power of the nuclear cluster must exceed a threshold value, imposed by the rate of disc mass accretion, to undergo blowout. This, combined with an initial mass function (IMF), is shown to regulate the minimum amount of mass that a starburst needs to generate kpc-scale outflows.  相似文献   

20.
UV and X-ray space-based interferometry will open unprecedented possibilities for spectral and spatial studies of a wide range of currently unresolvable interacting systems. Ultra-high angular resolution direct imaging of individual} components and transport processes in interacting binary systems is essential for detailed studies and modeling of accretion and activity. Understanding the mass loss characteristics of both components, and the dynamics of the system as a function of time, will provide key inputs to evolutionary models and will revolutionize our view and understanding of the Universe.  相似文献   

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