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Register a new userNew solution to viscous evolution of accretion disks in binary systems
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Analytical investigation of time-dependent accretion in disks is carried out. We consider a time-dependent disk in a binary system at outburst which has a fixed tidally-truncated outer radius. The standard Shakura-Sunyaev model of the disk is considered. The vertical structure of the disk is accurately described in two regimes of opacity: Thomson and free-free. Fully analytical solutions are obtained, characterized by power-law variations of accretion rate with time. The solutions supply asymptotic description of disk evolution in flaring sources in the periods after outbursts while the disk is fully ionized. The X-ray flux of multicolor (black-body) alpha-disk is obtained as varying quasi-exponentially. Application to X-ray novae is briefly discussed concerning the observed faster-than-power decays of X-ray light curves. The case of time-dependent advective disk when the exponential variations of accretion rate can occur is discussed.
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Variation of mass supply rate from the companion can be smeared out by viscous processes inside an accretion disk. By the time the flow reaches the inner edge, the variation in X-rays needs not reflect the true variation of the rate at the outer edge. However, if the viscosity fluctuates around a mean value, one would expect the viscous time scale also to spread around a mean value. In HMXBs, the size of the viscous Keplerian disk is smaller & thus such a spread could be lower as compared to the LMXBs. If there is an increasing or decreasing trend in viscosity, the interval between enhanced emission would be modified systematically. In the absence of a full knowledge about the variation of mass supply rates at the outer edge, we study ideal circumstances where modulation must take place exactly in orbital time scales when there is an ellipticity in the orbit. We study a few compact binaries using long term RXTE/ASM(1.5-12 keV) & Swift/BAT(15-50keV) data to look for such effects & to infer what these results can tell us about the viscous processes inside the respective disks. We employ three different methods to seek imprints of periodicity on the X-ray variation & found that in all the cases, the location of the peak in the power density spectra is consistent with the orbital frequencies. Interestingly, in HMXBs the peaks are sharp with high rms values, consistent with a small Keplerian disk in a wind fed system. However, in LMXBs with larger Keplerian disk, the peaks are spread out with much lower rms values. X-ray reflections, or superhump phenomena which may also cause such X-ray modulations would not be affected by the size of the Keplerian disk. Our result confirms different sizes of Keplerian disks in 2 classes of binaries. If the orbital period of a binary system is not known, it may be obtained with reasonable accuracy for HMXBs & with lesser accuracy for LMXBs by our method.
Axisymmetric magnetorotational instability (MRI) in viscous accretion disks is investigated by linear analysis and two-dimensional nonlinear simulations. The linear growth of the viscous MRI is characterized by the Reynolds number defined as $R_{rm MRI} equiv v_A^2/ uOmega $, where $v_A$ is the Alfv{e}n velocity, $ u$ is the kinematic viscosity, and $Omega$ is the angular velocity of the disk. Although the linear growth rate is suppressed considerably as the Reynolds number decreases, the nonlinear behavior is found to be almost independent of $R_{rm MRI}$. At the nonlinear evolutionary stage, a two-channel flow continues growing and the Maxwell stress increases until the end of calculations even though the Reynolds number is much smaller than unity. A large portion of the injected energy to the system is converted to the magnetic energy. The gain rate of the thermal energy, on the other hand, is found to be much larger than the viscous heating rate. Nonlinear behavior of the MRI in the viscous regime and its difference from that in the highly resistive regime can be explained schematically by using the characteristics of the linear dispersion relation. Applying our results to the case with both the viscosity and resistivity, it is anticipated that the critical value of the Lundquist number $S_{rm MRI} equiv v_A^2/etaOmega$ for active turbulence depends on the magnetic Prandtl number $S_{{rm MRI},c} propto Pm^{1/2}$ in the regime of $Pm gg 1$ and remains constant when $Pm ll 1$, where $Pm equiv S_{rm MRI}/R_{rm MRI} = u/eta$ and $eta$ is the magnetic diffusivity.
We calculate the chemical evolution of protoplanetary disks considering radial viscous accretion, vertical turbulent mixing and vertical disk winds. We study the effects on the disk chemical structure when different models for the formation of molecular hydrogen on dust grains are adopted. Our gas-phase chemistry is extracted from the UMIST Database for Astrochemistry (Rate06) to which we have added detailed gas-grain interactions. We use our chemical model results to generate synthetic near- and mid-infrared LTE line emission spectra and compare these with recent Spitzer observations. Our results show that if H2 formation on warm grains is taken into consideration, the H2O and OH abundances in the disk surface increase significantly. We find the radial accretion flow strongly influences the molecular abundances, with those in the cold midplane layers particularly affected. On the other hand, we show that diffusive turbulent mixing affects the disk chemistry in the warm molecular layers, influencing the line emission from the disk and subsequently improving agreement with observations. We find that NH3, CH3OH, C2H2 and sulphur-containing species are greatly enhanced by the inclusion of turbulent mixing. We demonstrate that disk winds potentially affect the disk chemistry and the resulting molecular line emission in a similar manner to that found when mixing is included.
We aim to examine the detailed disc structure that arises in a misaligned binary system as a function of the disc aspect ratio h, viscosity parameter alpha, disc outer radius R, and binary inclination angle gamma_F. We also aim to examine the conditions that lead to an inclined disc being disrupted by strong differential precession. We use a grid-based hydrodynamic code to perform 3D simulations. This code has a relatively low numerical viscosity compared with the SPH schemes that have been used previously to study inclined discs. This allows the influence of viscosity on the disc evolution to be tightly controlled. We find that for thick discs (h=0.05) with low alpha, efficient warp communication in the discs allows them to precess as rigid bodies with very little warping or twisting. Such discs are observed to align with the binary orbit plane on the viscous evolution time. Thinner discs with higher viscosity, in which warp communication is less efficient, develop significant twists before achieving a state of rigid-body precession. Under the most extreme conditions we consider (h=0.01, alpha=0.005 and alpha=0.1), we find that discs can become broken or disrupted by strong differential precession. Discs that become highly twisted are observed to align with the binary orbit plane on timescales much shorter than the viscous timescale, possibly on the precession time. We find agreement with previous studies that show that thick discs with low viscosity experience mild warping and precess rigidly. We also find that as h is decreased substantially, discs may be disrupted by strong differential precession, but for disc thicknesses that are significantly less (h=0.01) than those found in previous studies (h=0.03).
Discoveries of two new white dwarf plus M star binaries with striking optical cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to six the total number of X-ray faint, magnetic accretion binaries that accrete at rates < 10^{-13} Msun/yr, or <1% of the values normally encountered in cataclysmic variables. This fact, coupled with donor stars that underfill their Roche lobes and very cool white dwarfs, brand the binaries as post common-envelope systems whose orbits have not yet decayed to the point of Roche-lobe contact. They are pre-magnetic CVs, or pre-Polars. The systems exhibit spin/orbit synchronism and apparently accrete by efficient capture of the stellar wind from the secondary star, a process that has been dubbed a ``magnetic siphon. Because of this, period evolution of the binaries will occur solely by gravitational radiation, which is very slow for periods >3 hr. Optical surveys for the cyclotron harmonics appear to be the only means of discovery, so the space density of pre-Polars could rival that of Polars, and the binaries provide an important channel of progenitors (in addition to the asynchronous Intermediate Polars). Both physical and SDSS observational selection effects are identified that may help to explain the clumping of all six systems in a narrow range of magnetic field strength around 60 MG.
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