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Register a new userGeometrically thin accretion disk around Maclaurin spheroid
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We investigated a semi-analytic and numerical model to study the geometrically thin and optically thick accretion disk around Maclaurin spheroid (MS). The main interest is in the inner region of the so called {alpha}-disk, {alpha} being the viscosity parameter. Analytical calculations are done assuming radiation pressure and gas pressure dominated for close to Eddington mass accretion rate and $dot{M}lesssim 0.1dot{M_{Edd}}$ respectively. We found that the change in eccentricity of MS gives a change at high frequency region in the emitted spectra. We found that disk parameters are dependent on eccentricity of MS. Our semi-analytic results show that qualitatively an increase in eccentricity of MS has same behavior as decrease in mass accretion rate. Numerical work has been carried out to see the viscous time evolution of the accretion disk around MS. In numerical model we showed that if the eccentricity of the object is high the matter will diffuse slowly during its viscous evolution. This gives a clue that how spin-up or spin-down can change the time evolution of the accretion disk using a simple Newtonian approach. The change in spectra can be used to determine the eccentricity of MS and thus period of the MS.
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Nagendra Kumar
, Banibrata Mukhopadhyay (Department of Physics
, n Indian Institute of Science
2021
Many astrophysical sources, e.g., cataclysmic variables, X-ray binaries, active galactic nuclei, exhibit a wind outflow, when they reveal a multicolor blackbody spectrum, hence harboring a geometrically thin Keplerian accretion disk. Unlike an advective disk, in the thin disk, the physical environment, like, emission line, external heating, is expected to play a key role to drive the wind outflow. We show the wind outflow in a thin disk attributing a disk irradiation effect, probably from the inner to outer disks. We solve the set of steady, axisymmetric disk model equations in cylindrical coordinates along the vertical direction for a given launching radius $(r)$ from the midplane, introducing irradiation as a parameter. We obtain an acceleration solution, for a finite irradiation in the presence of a fixed but tiny initial vertical velocity (hence thin disk properties practically do not alter) at the midplane, upto a maximum height ($z^{max}$). We find that wind outflow mainly occurs from the outer region of the disk and its density decreases with increasing launching radius, and for a given launching radius with increasing ejection height. Wind power decreases with increasing ejection height. For $z^{max} < 2r$, wind outflow is ejected tangentially (or parallel to the disk midplane) in all directions with the fluid speed same as the azimuthal speed. This confirms mainly, for low mass X-ray binaries, (a) wind outflow should be preferentially observed in high-inclination sources, (b) the expectation of red and blue shifted absorption lines.
We perform detailed variability analysis of two-dimensional viscous, radiation hydrodynamic numerical simulations of Shakura-Sunyaev thin disks around a stellar mass black hole. Disk models are initialized on both the gas-, as well as radiation-, pressure-dominated branches of the thermal equilibrium curve, with mass accretion rates spanning the range from $dot{M} = 0.01 L_mathrm{Edd}/c^2$ to $10 L_mathrm{Edd}/c^2$. An analysis of temporal variations of the numerically simulated disk reveals multiple robust, coherent oscillations. Considering the local mass flux variability, we find an oscillation occurring at the maximum radial epicyclic frequency, $3.5times 10^{-3},t_mathrm{g}^{-1}$, a possible signature of a trapped fundamental ${it g}$-mode. Although present in each of our simulated models, the trapped ${it g}$-mode feature is most prominent in the gas-pressure-dominated case. The total pressure fluctuations in the disk suggest strong evidence for standing-wave ${it p}$-modes, some trapped in the inner disk close to the ISCO, others present in the middle/outer parts of the disk. Knowing that the trapped ${it g}$-mode frequency and maximum radial epicyclic frequency differ by only $0.01%$ in the case of a non-rotating black hole, we simulated an additional initially gas-pressure-dominated disk with a dimensionless black hole spin parameter $a_* = 0.5$. The oscillation frequency in the spinning black hole case confirms that this oscillation is indeed a trapped ${it g}$-mode. All the numerical models we report here also show a set of high frequency oscillations at the vertical epicyclic and breathing mode frequencies. The vertical oscillations show a 3:2 frequency ratio with oscillations occurring approximately at the radial epicyclic frequency, which could be of astrophysical importance in observed twin peak, high-frequency quasi-periodic oscillations.
We study the oscillations of an axisymmetric, viscous, radiative, general relativistic hydrodynamical simulation of a geometrically thin disk around a non-rotating, $6.62,M_odot$ black hole. The numerical setup is initialized with a Novikov-Thorne, gas-pressure-dominated accretion disk, with an initial mass accretion rate of $dot{m} = 0.01,L_mathrm{Edd}/c^2$ (where $L_mathrm{Edd}$ is the Eddington luminosity and $c$ is the speed of light). Viscosity is treated with the $alpha$-prescription. The simulation was evolved for about $1000$ Keplerian orbital periods at three Schwarzschild radii (ISCO radius). Power density spectra of the radial and vertical fluid velocity components, the total (gas $+$ radiation) midplane pressure, and the vertical component of radiative flux from the photosphere, all reveal strong power at the local breathing oscillation frequency. The first, second and third harmonics of the breathing oscillation are also clearly seen in the data. We quantify the properties of these oscillations by extracting eigenfunctions of the radial and vertical velocity components and total pressure. This confirms that these oscillations are associated with breathing motion.
The analysis of the thermal spectrum of geometrically thin and optically thick accretion disks of black holes, the so-called continuum-fitting method, is one of the leading techniques for measuring black hole spins. Current models normally approximate the disk as infinitesimally thin, while in reality the disk thickness is finite and increases as the black hole mass accretion rate increases. Here we present an XSPEC model to calculate the multi-temperature blackbody spectrum of a thin accretion disk of finite thickness around a Kerr black hole. We test our new model with an RXTE observation of the black hole binary GRS 1915+105. We find that the spin value inferred with the new model is slightly higher than the spin value obtained with a model with an infinitesimally thin disk, but the difference is small and the effect is currently subdominant with respect to other sources of uncertainties in the final spin measurement.
PSR J1023+0038 is an exceptional system for understanding how slowly rotating neutron stars are spun up to millisecond rotational periods through accretion from a companion star. Observed as a radio pulsar from 2007-2013, optical data showed that the system had an accretion disk in 2000/2001. Starting at the end of 2013 June, the radio pulsar has become undetectable, suggesting a return to the previous accretion-disk state, where the system more closely resembles an X-ray binary. In this Letter we report the first targeted X-ray observations ever performed of the active phase and complement them with UV/Optical and radio observations collected in 2013 October. We find strong evidence that indeed an accretion disk has recently formed in the system and we report the detection of fast X-ray changes spanning about two orders of magnitude in luminosity. No radio pulsations are seen during low flux states in the X-ray light-curve or at any other times.
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