Open AccessThe pattern of accretion flow on to Sgr A*
Author(s) -
Mościbrodzka Monika,
Das Tapas K.,
Czerny Bozena
Publication year - 2006
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1111/j.1365-2966.2006.10470.x
Subject(s) - physics , accretion (finance) , astrophysics , angular momentum , torus , specific relative angular momentum , active galactic nucleus , supermassive black hole , precession , astronomy , flow (mathematics) , galaxy , classical mechanics , mechanics , total angular momentum quantum number , geometry , mathematics , angular momentum coupling
The material accreting on to Sgr A* most probably comes from the nearby stars. We analyse the pattern of this flow at distances of a fraction of a parsec, and we argue that the net angular momentum of this material is low but non‐negligible, and the initially supersonic disc accretion changes into subsonic flow with constant angular momentum. Next, we estimate the flow parameters at a distance R BHL from the black hole, and we argue that for the plausible parameter range the accretion flow is non‐stationary. The inflow becomes supersonic at a distance of ∼10 4 R g , but the solution does not continue below the horizon and the material piles up forming a torus, or a ring, at a distance of a few, up to tens of Schwarzchild, radii. Such a torus is known to be unstable and may explain strong variability of the flow in Sgr A*. Our considerations show that the temporary formation of such a torus seems to be unavoidable. Our best‐fitting model predicts a rather large accretion rate of around 4 × 10 −6 M ⊙ yr −1 directly on Sgr A*. We argue that magnetic fields in the flow are tangled, and this allows our model to overcome the disagreement with the Faraday rotation limits.