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1.
In this short paper we show that making turbulence two‐rather than three‐dimensional may increase the effective turbulent viscosity by about 40 %. Dimensionless hydrodynamical viscosity parameters up to αmax = 0.25 Mt2 may be obtained in this approach, which are in better agreement with the observational data on non‐stationary accretion than the values obtained in numerical simulations. However, the α ‐parameter values known from observations are still several times higher (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
2.
Self-consistent computation of γ-ray spectra due to proton–proton interactions in black hole systems
S. Bhattacharyya N. Bhatt R. Misra 《Monthly notices of the Royal Astronomical Society》2006,371(1):245-251
In the inner regions of an accretion disc around a black hole, relativistic protons can interact with ambient matter to produce electrons, positrons and γ-rays. The resultant steady-state electron and positron particle distributions are self-consistently computed taking into account Coulomb and Compton cooling, e− e+ pair production (due to γ–γ annihilation) and pair annihilation. While earlier works used the diffusion approximation to obtain the particle distributions, here we solve a more general integro-differential equation that correctly takes into account the large change in particle energy that occurs when the leptons Compton scatter off hard X-rays. Thus this formalism can also be applied to the hard state of black hole systems, where the dominant ambient photons are hard X-rays. The corresponding photon energy spectrum is calculated and compared with broad-band data of black hole binaries in different spectral states. The results indicate that the γ-ray spectra ( E > 0.8 MeV) of both the soft and hard spectral states and the entire hard X-ray/γ-ray spectrum of the ultrasoft state could be due to p–p interactions. These results are consistent with the hypothesis that there always exists in these systems a γ-ray spectral component due to p–p interactions that can contribute between 0.5 and 10 per cent of the total bolometric luminosity. The model predicts that GLAST would be able to detect black hole binaries and provide evidence for the presence of non-thermal protons, which in turn would give insight into the energy dissipation process and jet formation in these systems. 相似文献
3.
Ding Shi-Xue Yang Lan-Tian Wu Xue-Bing Lu Ye 《Monthly notices of the Royal Astronomical Society》2000,317(4):737-742
The radial–azimuthal instability of a hot two-temperature accretion disc with advection is examined in this paper. We find that the inclusion of very little advection has significant effects on two acoustic modes for a geometrically thin, cooling-dominated two-temperature disc, but has no effect on acoustic modes for a geometrically slim, cooling-dominated two-temperature disc. We also find that, when azimuthal perturbations are considered, the stability properties of the disc are different from those in the pure radial perturbation case. An increase of the azimuthal wavenumber will stabilize the acoustic modes but make the viscous and thermal modes more unstable for a geometrically thin, cooling-dominated two-temperature disc. It makes the thermal mode more unstable and the acoustic mode more stable, but only affects the instability of the viscous mode for short-wavelength perturbations for a geometrically slim, cooling-dominated two-temperature disc. For a geometrically slim, advection-dominated two-temperature disc, the increase of the azimuthal perturbation makes the I- and O-modes more stable and the thermal mode more unstable, but has no effect on the viscous mode. 相似文献
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M. M. Kotze P. A. Charles L. A. Crause 《Monthly notices of the Royal Astronomical Society》2009,395(3):1579-1584
Long-term monitoring of the recently discovered X-ray transient, IGR J17098−3628, by the All-Sky Monitor on-board the Rossi X-Ray Timing Explorer , has shown that it displays a long-term (≈163 d) quasi-periodic modulation in the data spanning its 'active' state (i.e. approximately MJD 53450–54200). Furthermore, this light curve is not typical of 'classical' soft X-ray transients, in that J17098−3628 has remained active since its initial discovery, and may be more akin to the pseudo-transient EXO 0748−676, which is now classified as a persistent low-mass X-ray binary (LMXB). However, EXO 0748−676 recently entered a more active phase (since approximately MJD 53050), and since then we find that it too displays a quasi-periodic modulation (≈181 d) in its light curve. This must be a 'superorbital' modulation, as the orbital period of EXO 0748−676 is well established (3.8 h), and hence we interpret both objects' long periods as representing some intrinsic properties of the accretion disc (such as coupled precessional and warping effects). By analogy, we therefore suggest that IGR J17098−3628 is another member of this class of pseudo-transient LMXBs and is likely to have a <1 d orbital period. 相似文献
6.
Rebecca G. Martin Christopher A. Tout J. E. Pringle 《Monthly notices of the Royal Astronomical Society》2008,387(1):188-196
The microquasar GRO J1655−40 has a black hole with spin angular momentum apparently misaligned to the orbital plane of its companion star. We analytically model the system with a steady-state disc warped by Lense–Thirring precession and find the time-scale for the alignment of the black hole with the binary orbit. We make detailed stellar evolution models so as to estimate the accretion rate and the lifetime of the system in this state. The secondary can be evolving at the end of the main sequence or across the Hertzsprung gap. The mass-transfer rate is typically 50 times higher in the latter case but we find that, in both the cases, the lifetime of the mass-transfer state is at most a few times the alignment time-scale. The fact that the black hole has not yet aligned with the orbital plane is therefore consistent with either model. We conclude that the system may or may not have been counter aligned after its supernova kick but that it is most likely to be close to alignment rather than counter alignment now. 相似文献
7.
Marek A. Abramowicz Igor V. Igumenshchev & Jean-Pierre Lasota 《Monthly notices of the Royal Astronomical Society》1998,293(4):443-446
One of the necessary matching conditions between an optically thin, hot, advectively dominated part of an accretion disc (ADAF) and the standard Shakura–Sunyaev disc (SSD) part is that the matching transition region must rotate with an orbital speed that is faster than the Keplerian one. We discuss properties of the SSD–ADAF transition in terms of a simple two-dimensional hydrodynamical model. 相似文献
8.
Gordon E. Sarty László L. Kiss Richard Huziak Lionel J. J. Catalan Diane Luciuk Timothy R. Crawford David J. Lane Roger D. Pickard Thomas A. Grzybowski Pere Closas Helen Johnston David Balam Kinwah Wu 《Monthly notices of the Royal Astronomical Society》2009,392(3):1242-1252
The high-mass X-ray binary RX J0146.9+6121, with optical counterpart LS I+61°235 (V831 Cas), is an intriguing system on the outskirts of the open cluster NGC 663. It contains the slowest Be type X-ray pulsar known with a pulse period of around 1400 s and, primarily from the study of variation in the emission line profile of Hα, it is known to have a Be decretion disc with a one-armed density wave period of approximately 1240 d. Here we present the results of an extensive photometric campaign, supplemented with optical spectroscopy, aimed at measuring short time-scale periodicities. We find three significant periodicities in the photometric data at, in order of statistical significance, 0.34, 0.67 and 0.10 d. We give arguments to support the interpretation that the 0.34 and 0.10 d periods could be due to stellar oscillations of the B-type primary star and that the 0.67 d period is the spin period of the Be star with a spin axis inclination of 23+10 −8 degrees. We measured a systemic velocity of −37.0 ± 4.3 km s−1 confirming that LS I+61°235 has a high probability of membership in the young cluster NGC 663 from which the system's age can be estimated as 20–25 Myr. From archival RXTE All Sky Monitor (ASM) data we further find 'super' X-ray outbursts roughly every 450 d. If these super outbursts are caused by the alignment of the compact star with the one-armed decretion disc enhancement, then the orbital period is approximately 330 d. 相似文献
9.
Disc-accreting neutron stars come in two distinct varieties, atolls and Z sources, named after their differently shaped tracks on a colour–colour diagram as the source luminosity changes. Here we present analysis of three transient atoll sources showing that there is an additional branch in the colour–colour diagram of atoll sources which appears at very low luminosities. This new branch connects to the top of previously known C-shaped (atoll) path, forming a horizontal track where the average source flux decrease from right to left. This turns the C-shape into a Z. Thus both atolls and Z sources share the same topology on the colour–colour diagram and evolve in similar way, as a function of increasing averaged mass accretion rate. This strongly favours models in which the underlying geometry of these sources changes in similar ways. A possible scenario is one where the truncated disc approaches the neutron star when the accretion rate increases, but in the atolls the disc is truncated by evaporation (similarly to black holes), and in the Z sources it is truncated by the magnetic field. 相似文献
10.
T. Belloni J. Homan S. Motta E. Ratti M. Méndez 《Monthly notices of the Royal Astronomical Society》2007,379(1):247-252
We have monitored the atoll-type neutron star low-mass X-ray binary 4U 1636−53 with the Rossi X-ray Timing Explorer ( RXTE ) for more than 1.5 yr. Our campaign consisted of short (∼2 ks) pointings separated by 2 d, regularly monitoring the spectral and timing properties of the source. During the campaign we observed a clear long-term oscillation with a period of ∼30–40 d, already seen in the light curves from the RXTE All-Sky Monitor, which corresponded to regular transitions between the hard (island) and soft (banana) states. We detected kilohertz (kHz) quasi-periodic oscillations (QPOs) in about a third of the observations, most of which were in the soft (banana) state. The distribution of the frequencies of the peak identified as the lower kHz QPO is found to be different from that previously observed in an independent data set. This suggests that the kHz QPOs in the system shows no intrinsically preferred frequency. 相似文献
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S. Mendoza E. Tejeda E. Nagel † 《Monthly notices of the Royal Astronomical Society》2009,393(2):579-586
We construct a steady analytic accretion flow model for a finite rotating gas cloud that accretes material to a central gravitational object. The pressure gradients of the flow are considered to be negligible, and so the flow is ballistic. We also assume a steady flow and consider the particles at the boundary of the spherical cloud to be rotating as a rigid body, with a fixed amount of inwards radial velocity. This represents a generalization to the traditional infinite gas cloud model described by Ulrich. We show that the streamlines and density profiles obtained deviate largely from the ones calculated by Ulrich. The extra freedom in the choice of the parameters on the model can naturally account for the study of protostars formed in dense clusters by triggered mechanisms, where a wide variety of external physical mechanisms determine the boundary conditions. Also, as expected, the model predicts the formation of an equatorial accretion disc about the central object with a radius different from the one calculated by Ulrich. 相似文献
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Cool weakly ionized gaseous rotating disk form the basis for many models in astrophysics objects. Instabilities against perturbations in such disks play an important role in the theory of the formation of stars and planets. Traditionally, axisymmetric magnetohydrodynamic (MHD) and recently Hall‐MHD instabilities have been thoroughly studied as providers of an efficient mechanism for radial transfer of angular momentum, and of density radial stratification. In the current work, the Hall instability against axisymmetric perturbations in incompressible rotating fluid in external poloidal and toroidal magnetic field is considered. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
15.
I. G. Kovalenko & M. A. Eremin 《Monthly notices of the Royal Astronomical Society》1998,298(3):861-870
We examine the spatial stability of spherical adiabatic Bondi accretion on to a point gravitating mass against external perturbations. Both transonic critical and subsonic subcritical accretion are shown to be stable against purely radial acoustic, vortex or entropy perturbations. In the case of non-radial perturbations the amplitude of the perturbations grows without limit with smaller radii. Instability manifests itself only if the size of the accreting body is much less than the Bondi radius so that the inflow is highly supersonic or highly subsonic at the surface of the accretor in the case of critical or subcritical accretion respectively. These asymptotics hold and consequently the instability may develop for adiabatic index of accreting gas γ < 5/3. We suggest that this instability may lead to an essential thermalization of accreting flow thus, particularly, solving the problem of otherwise inefficient energy release in spherical accretion on to a black hole. 相似文献
16.
T. Heinemann J. C. B. Papaloizou 《Monthly notices of the Royal Astronomical Society》2009,397(1):52-63
We study and elucidate the mechanism of spiral density wave excitation in a differentially rotating flow with turbulence which could result from the magneto-rotational instability. We formulate a set of wave equations with sources that are only non-zero in the presence of turbulent fluctuations. We solve these in a shearing box domain, subject to the boundary conditions of periodicity in shearing coordinates, using a WKBJ method. It is found that, for a particular azimuthal wavelength, the wave excitation occurs through a sequence of regularly spaced swings during which the wave changes from leading to trailing form. This is a generic process that is expected to occur in shearing discs with turbulence. Trailing waves of equal amplitude propagating in opposite directions are produced, both of which produce an outward angular momentum flux that we give expressions for as functions of the disc parameters and azimuthal wavelength.
By solving the wave amplitude equations numerically, we justify the WKBJ approach for a Keplerian rotation law for all parameter regimes of interest. In order to quantify the wave excitation completely, the important wave source terms need to be specified. Assuming conditions of weak non-linearity, these can be identified and are associated with a quantity related to the potential vorticity, being the only survivors in the linear regime. Under the additional assumption that the source has a flat power spectrum at long azimuthal wavelengths, the optimal azimuthal wavelength produced is found to be determined solely by the WKBJ response and is estimated to be 2π H , with H being the nominal disc scaleheight. In a following paper by Heinemann & Papaloizou, we perform direct three-dimensional simulations and compare results manifesting the wave excitation process and its source with the assumptions made and the theory developed here in detail, finding excellent agreement. 相似文献
By solving the wave amplitude equations numerically, we justify the WKBJ approach for a Keplerian rotation law for all parameter regimes of interest. In order to quantify the wave excitation completely, the important wave source terms need to be specified. Assuming conditions of weak non-linearity, these can be identified and are associated with a quantity related to the potential vorticity, being the only survivors in the linear regime. Under the additional assumption that the source has a flat power spectrum at long azimuthal wavelengths, the optimal azimuthal wavelength produced is found to be determined solely by the WKBJ response and is estimated to be 2π H , with H being the nominal disc scaleheight. In a following paper by Heinemann & Papaloizou, we perform direct three-dimensional simulations and compare results manifesting the wave excitation process and its source with the assumptions made and the theory developed here in detail, finding excellent agreement. 相似文献
17.
Marek Gierli&#;ski Chris Done Kim Page 《Monthly notices of the Royal Astronomical Society》2009,392(3):1106-1114
We build a simple model of the optical/ultraviolet (UV) emission from irradiation of the outer disc by the inner disc and coronal emission in black hole binaries. We apply this to the broad-band Swift data from the outburst of the black hole binary XTE J1817−330 to confirm previous results that the optical/UV emission in the soft state is consistent with a reprocessing a constant fraction of the bolometric X-ray luminosity. However, this is very surprising as the disc temperature drops by more than a factor of 3 in the soft state, which should produce a marked change in the reprocessing efficiency. The easiest way to match the observed constant reprocessed fraction is for the disc skin to be highly ionized (as suggested 30 yr ago by van Paradijs), so that the bulk of the disc flux is reflected and only the hardest X-rays heat the disc. The constant reprocessed fraction also favours direct illumination of the disc over a scattering origin as the optical depth/solid angle of any scattering material (wind/corona) over the disc should decrease as the source luminosity declines. By contrast, the reprocessed fraction increases very significantly (by a factor of ∼6) as the source enters the hard state. This dramatic change is not evident from X-ray/UV flux correlations as it is masked by bandpass effects. However, it does not necessarily signal a change in emission, for example, the emergence of the jet dominating the optical/UV flux as the reflection albedo must change with the dramatic change in spectral shape. 相似文献
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Pasi Hakala Linnea Hjalmarsdotter Diana C. Hannikainen Panu Muhli † 《Monthly notices of the Royal Astronomical Society》2009,394(2):892-899
Since its discovery in 1990, UW CrB (also known as MS1603+2600) has remained a peculiar source without firm classification. Our current understanding is that it is an accretion disc corona (ADC) low-mass X-ray binary. In this paper, we present results from our photometric campaign dedicated to studying the changing morphology of the optical light curves. We find that the optical light curves show remarkable evidence for strongly evolving light curve shapes. In addition, we find that these changes show a modulation at a period of ∼5 d. We interpret these changes as either due to strong periodic accretion disc warping or due to other geometrical changes because of disc precession at a period of 5 d. Finally, we have detected 11 new optical bursts, the phase distribution of which supports the idea of a vertically extended asymmetric accretion disc. 相似文献
20.
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