119,120 research outputs found

    The radiation properties of an accretion disk with a non-zero torque on its inner edge

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    The structure of the inner edge of the accretion disk around a black hole can be altered, if the matter inside the marginally stable orbit is magnetically connected to the disk. In this case, a non-zero torque is exerted on its inner edge, and the accretion efficiency ϵ\epsilon can be much higher than that in the standard accretion disk model. We explore the radiation properties of an accretion disk at its sonic point around a black hole with a time-steady torque exerted on the inner edge of the disk. The local structure of the accretion flow at the sonic point is investigated in the frame of general relativity. It is found that the accretion flow will be optically thin at its sonic point for most cases, if the additional accretion efficiency δϵ\delta\epsilon caused by the torque is as high as \sim10 %. The results imply that the variable torque may trigger transitions of the flow between different accretion types.Comment: 6 pages, to appear in PASJ, Vol. 55, No. 1 (February 25, 2003

    Self-supporting liberals and their cliques : an axiomatic characterization

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    social groupssocial status;social choice

    Profiles of thermal line emission from advection dominated accretion flows

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    Recently, Narayan & Raymond (1999) proposed that the thermal emission lines from the hot plasma in advection dominated accretion flows (ADAFs) are potentially observable with the next generation of X-ray observatories, with which the physical properties of some X-ray sources can be probed. In ADAFs, the temperature of the ion is so high that the thermal broadening of the line is important. We calculate the profiles of thermal line emission from ADAFs, in which both the thermal and Doppler broadening have been considered. It is found that the double-peaked profiles are present for high inclination angles between the axis of disk and the line of sight. The double-peaked profiles are smeared in low inclination cases, and completely disappear while the inclination angle is less than 1515^{\circ}, where the thermal and turbulent broadening dominated on the line profiles. We also note that the thermal line profile is affected by the location of the transition radius of ADAF. The self-similar height-integrated disk structure and the emissivity with power-law dependence of radius are adopted in our calculations. The results obtained in this work can be used as a diagnosis on the future X-ray observations of the thermal lines. Some important physical quantities of ADAFs could be inferred from future thermal line observations.Comment: 7 page

    An efficient hybrid model and dynamic performance analysis for multihop wireless networks

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    Multihop wireless networks can be subjected to nonstationary phenomena due to a dynamic network topology and time varying traffic. However, the simulation techniques used to study multihop wireless networks focus on the steady-state performance even though transient or nonstationary periods will often occur. Moreover, the majority of the simulators suffer from poor scalability. In this paper, we develop an efficient performance modeling technique for analyzing the time varying queueing behavior of multihop wireless networks. The one-hop packet transmission (service) time is assumed to be deterministic, which could be achieved by contention-free transmission, or approximated in sparse or lightly loaded multihop wireless networks. Our model is a hybrid of time varying adjacency matrix and fluid flow based differential equations, which represent dynamic topology changes and nonstationary network queues, respectively. Numerical experiments show that the hybrid fluid based model can provide reasonably accurate results much more efficiently than standard simulators. Also an example application of the modeling technique is given showing the nonstationary network performance as a function of node mobility, traffic load and wireless link quality. © 2013 IEEE