Observations of the Blandford-Znajek and the MHD Penrose processes in computer simulations of black hole magnetospheres

Abstract

In this paper we report the results of axisymmetric relativistic MHD simulations for the problem of Kerr black hole immersed into a rarefied plasma with ''uniform'' magnetic field. The long term solution shows properties which are significantly different from those of the initial transient phase studied recently by Koide(2003). The topology of magnetic field lines within the ergosphere is similar to that of the split-monopole model with a strong current sheet in the equatorial plane. Closer inspection reveals a system of isolated magnetic islands inside the sheet and ongoing magnetic reconnection. No regions of negative hydrodynamic ''energy at infinity'' are seen inside the ergosphere and the so-called MHD Penrose process does not operate. Yet, the rotational energy of the black hole continues to be extracted via purely electromagnetic mechanism of Blandford and Znajek(1977). However, this is not followed by development of strong relativistic outflows from the black hole. Combined with results of other recent simulations this signals a potential problem for the standard MHD model of relativistic astrophysical jets should they still be observed at distances as small as few tens of gravitational radii from the central black hole.