X-ray emission from non-Keplerian magnetically dominated accretion disks |
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Authors: | G Belvedere and D Molteni |
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Institution: | (1) Istituto di Astronomia dell'Università, Città Universitaria, I-95125 Catania, Italia;(2) Istituto di Fisica dell'Università, Via Archirafi 36, I-90143 Palermo, Italia |
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Abstract: | Summary Most accretion disk models do assume Keplerian rotation as the ?natural? one. This is not so obvious and is somewhat aprioristic,
as shown by some attempts to follow different approaches. In the present work we suggest a new approach to the problem, pointing
out that the disk structure is deeply related to the balance between the magnetic energy produced by dynamo action and the
rotational energy associated with the accreting flow. This balance, together with the conservation laws, allows us to derive
analytically the angular-velocity field, the radial velocity field and the magnetic-energy distribution inside the disk. As
far as the disk X-ray emission is concerned, we adopt the point of view that the buoyancy and emergence at the disk surface
of the azimuthal magnetic field generated by differential rotation gives rise to looplike structures in a hot magnetically
confined corona, which is not merely overimposed, as in other models, but ?naturally? fits to the disk structure. The magnetic-energy
dissipation into the coronal medium, which sustains the X-ray luminosity, is due to twisting of the magnetic loops. we give
an exact analytical expression for the integrated luminosityL
x
in terms of the mass of the compact object, the accretion rate, the disk size and rotational velocity, and show that in the
limitr
i≪r
e (wherer
i andr
e are the inner and the outer radius of the accretion disk, respectively) it agrees fairly well with the characteristic observed
values of both galactic and extragalactic strong compact sources, within the reasonable range commonly accepted for the relevant
parameters.
Paper presented at the 2° Convegno Nazionale di Fisica Cosmica, held at L'Aquila, 29 May–2 June 1984. |
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Keywords: | Cosmic rays |
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