Journal Article

General relativistic magnetohydrodynamic simulations of accretion on to Sgr A*: how important are radiative losses?

S. Dibi, S. Drappeau, P. C. Fragile, S. Markoff and J. Dexter

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 426, issue 3, pages 1928-1939
Published in print November 2012 | ISSN: 0035-8711
Published online November 2012 | e-ISSN: 1365-2966 | DOI: http://dx.doi.org/10.1111/j.1365-2966.2012.21857.x
General relativistic magnetohydrodynamic simulations of accretion on to Sgr A*: how important are radiative losses?

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Abstract

We present general relativistic magnetohydrodynamic numerical simulations of the accretion flow around the supermassive black hole in the Galactic Centre, Sagittarius A* (Sgr A*). The simulations include for the first time radiative cooling processes (synchrotron, bremsstrahlung and inverse Compton) self-consistently in the dynamics, allowing us to test the common simplification of ignoring all cooling losses in the modelling of Sgr A*. We confirm that for Sgr A*, neglecting the cooling losses is a reasonable approximation if the Galactic Centre is accreting below ∼10−8 M yr−1, i.e. M ̇ < 10−7Edd. However, above this limit, we show that radiative losses should be taken into account as significant differences appear in the dynamics and the resulting spectra when comparing simulations with and without cooling. This limit implies that most nearby low-luminosity active galactic nuclei are in the regime where cooling should be taken into account.

We further make a parameter study of axisymmetric gas accretion around the supermassive black hole at the Galactic Centre. This approach allows us to investigate the physics of gas accretion in general, while confronting our results with the well-studied and observed source, Sgr A*, as a test case. We confirm that the nature of the accretion flow and outflow is strongly dependent on the initial geometry of the magnetic field. For example, we find it difficult, even with very high spins, to generate powerful outflows from discs threaded with multiple, separate poloidal field loops.

Keywords: accretion, accretion discs; black hole physics; MHD; radiation mechanisms: thermal; methods: numerical; Galaxy: centre

Journal Article.  8697 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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