Three-dimensional simulations of supercritical black hole accretion discs – luminosities, photon trapping and variability. Issue 4 (13th January 2016)

• Record Type: Journal Article
• Authors: Sądowski, Aleksander; Narayan, Ramesh
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 456:Issue 4(2016)
• Page Start: 3929
• Page End: 3947
• Publication Date: 2016-01-13
• Subjects: accretion, accretion discs -- black hole physics -- relativistic processes -- methods: numerical; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stv2941
• Languages: English
• ISSNs: 0035-8711

Abstract

Abstract: We present a set of four three-dimensional, general relativistic, radiation magnetohydrodynamical simulations of black hole accretion at supercritical mass accretion rates, $\dot{M} > \dot{M}_{\rm Edd}$ . We use these simulations to study how disc properties are modified when we vary the black hole mass, the black hole spin, or the mass accretion rate. In the case of a non-rotating black hole, we find that the total efficiency is of the order of $3\, \mathrm{per\ cent}\ \dot{M} c^2$, approximately a factor of 2 less than the efficiency of a standard thin accretion disc. The radiation flux in the funnel along the axis is highly super-Eddington, but only a small fraction of the energy released by accretion escapes in this region. The bulk of the $3\, \mathrm{per\ cent}\ \dot{M} c^2$ of energy emerges farther out in the disc, either in the form of photospheric emission or as a wind. In the case of a black hole with a spin parameter of 0.7, we find a larger efficiency of about $8\, \mathrm{per\ cent}\ \dot{M} c^2$ . By comparing the relative importance of advective and diffusive radiation transport, we show that photon trapping is effective near the equatorial plane. However, near the disc surface, vertical transport of radiation by diffusion dominates. We compare the properties of our fiducial three-dimensional run with those of an equivalent two-dimensional axisymmetric model with a mean-field dynamo. The latter simulation runs nearly 100 times faster than the three-dimensional simulation, and gives very similar results for time-averaged properties of the accretion flow, but does not reproduce the time-variability.