Heating or Cooling: Study of Advective Heat Transport in the Inflow and the Outflow of Optically Thin Advection-dominated Accretion Flows

Advection is believed to be the dominant cooling mechanism in optically thin advection-dominated accretion flows (ADAF's). When outflow is considered, however, the first impression is that advection should be of opposite sign in the inflow and the outflow, due to the opposite direction of radial motion. Then how is the energy balance achieved simultaneously? We investigate the problem in this paper, analysing the profiles of different components of advection with self-similar solutions of ADAF's in spherical coordinates ($r\theta\phi$). We find that for $n < 3\gamma/2-1$, where $n$ is the density index in $\rho \propto r^{-n}$ and $\gamma$ is the heat capacity ratio, the radial advection is a heating mechanism in the inflow and a cooling mechanism in the outflow. It becomes 0 for $n = 3\gamma/2-1$, and turns to a cooling mechanism in the inflow and a heating mechanism in the outflow for $n > 3\gamma/2-1$. The energy conservation is only achieved when the latitudinal ($\theta$-direction) advection is considered, which takes an appropriate value to maintain energy balance, so that the overall effect of advection, no matter the parameter choices, is always a cooling mechanism that cancels out the viscous heating everywhere. For the extreme case of $n=3/2$, latitudinal motion stops, viscous heating is balanced solely by radial advection, and no outflow is developed.Comment: 9 pages, 4 figures, accepted by Ap

Study of advective energy transport in the inflow and the outflow of super-Eddington accretion flows

Photon trapping is believed to be an important mechanism in super-Eddington accretion, which greatly reduces the radiative efficiency as photons are swallowed by the central black hole before they can escape from the accretion flow. This effect is interpreted as the radial advection of energy in one-dimensional height-integrated models, such as the slim disc model. However, when multi-dimensional effects are considered, the conventional understanding may no longer hold. In this paper, we study the advective energy transport in super-Eddington accretion, based on a new two-dimensional inflow-outflow solution with radial self-similarity, in which the advective factor is calculated self-consistently by incorporating the calculation of radiative flux, instead of being set as an input parameter. We found that radial advection is actually a heating mechanism in the inflow due to compression, and the energy balance in the inflow is maintained by cooling via radiation and vertical ($\theta$-direction) advection, which transports entropy upwards to be radiated closer to the surface or carried away by the outflow. As a result, less photons are advected inwards and more photons are released from the surface, so that the mean advective factor is smaller and the emergent flux is larger than those predicted by the slim disc model. The radiative efficiency of super-Eddington accretion thus should be larger than that of the slim disc model, which agrees with the results of some recent numerical simulations.Comment: 7 pages, 3 figures, submitted to MNRA

FOG COMPUTING BASED BEARING REMAINING USEFUL LIFE PROGNOSIS USING TIME SERIES NORMALIZED SIMILARITY AND RECURRENT NEURAL NETWORKS

Techniques are described for determining a remaining useful life (RUL) prognosis of bearings using a feature extraction module for extracting time series normalized similarity (TSNS) features for vibration data normalization and a prediction module utilizing a deep learning model, known as an independently recurrent neural network (IndRNN), for predicting bearing RUL. The feature extraction module and prediction module are deployed on a fog computing platform as services for determining the RUL prognosis of bearings

On the global well-posedness and scattering of the 3D Klein-Gordon-Zakharov system

In this paper we are interested in the global well-posedness of the 3D Klein-Gordon-Zakharov equations with small initial data. We show the uniform boundedness of the energy for the global solution without any compactness assumptions on the initial data. The main novelty of our proof is to apply a modified Alinhac's ghost weight method together with a newly developed normal-form type estimate to remedy the lack of the space-time scaling vector field; moreover, we give a clear description of the smallness conditions on the initial data.Comment: 17 page

On the structure of Accretion Disks with Outflows

In order to study the outflows from accretion disks, we solve the set of hydrodynamic equations for accretion disks in the spherical coordinates ($r\theta\phi$) to obtain the explicit structure along the $\theta$ direction. Using self-similar assumptions in the radial direction, we change the equations to a set of ordinary differential equations (ODEs) about the $\theta$-coordinate, which are then solved with symmetrical boundary conditions in the equatorial plane, and the velocity field is obtained. The $\alpha$ viscosity prescription is applied and an advective factor $f$ is used to simplify the energy equation.The results display thinner, quasi-Keplerian disks for Shakura-Sunyaev Disks (SSDs) and thicker, sub-Keplerian disks for Advection Dominated Accretion Flows (ADAFs) and slim disks, which are consistent with previous popular analytical models. However, an inflow region and an outflow region always exist, except when the viscosity parameter $\alpha$ is too large, which supports the results of some recent numerical simulation works. Our results indicate that the outflows should be common in various accretion disks and may be stronger in slim disks, where both advection and radiation pressure are dominant. We also present the structure dependence on the input parameters and discuss their physical meanings. The caveats of this work and possible improvements in the future are discussed.Comment: 24 pages, 20 figures. Accepted for publication in Ap

Unifying ultrafast demagnetization and intrinsic Gilbert damping in Co/Ni bilayers with electronic relaxation near the Fermi surface

The ability to controllably manipulate the laser-induced ultrafast magnetic dynamics is a prerequisite for future high speed spintronic devices. The optimization of devices requires the controllability of the ultrafast demagnetization time, , and intrinsic Gilbert damping, . In previous attempts to establish the relationship between and , the rare-earth doping of a permalloy film with two different demagnetization mechanism is not a suitable candidate. Here, we choose Co/Ni bilayers to investigate the relations between and by means of time-resolved magneto-optical Kerr effect (TRMOKE) via adjusting the thickness of the Ni layers, and obtain an approximately proportional relation between these two parameters. The remarkable agreement between TRMOKE experiment and the prediction of breathing Fermi-surface model confirms that a large Elliott-Yafet spin-mixing parameter is relevant to the strong spin-orbital coupling at the Co/Ni interface. More importantly, a proportional relation between and in such metallic films or heterostructures with electronic relaxation near Fermi surface suggests the local spin-flip scattering domains the mechanism of ultrafast demagnetization, otherwise the spin-current mechanism domains. It is an effective method to distinguish the dominant contributions to ultrafast magnetic quenching in metallic heterostructures by investigating both the ultrafast demagnetization time and Gilbert damping simultaneously. Our work can open a novel avenue to manipulate the magnitude and efficiency of Terahertz emission in metallic heterostructures such as the perpendicular magnetic anisotropic Ta/Pt/Co/Ni/Pt/Ta multilayers, and then it has an immediate implication of the design of high frequency spintronic devices