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The variability of accretion onto Schwarzschild black holes from turbulent magnetized discs

By Philip J. Armitage and Christopher S. Reynolds


We use global magnetohydrodynamic simulations, in a pseudo-Newtonian potential, to investigate the temporal variability of accretion discs around Schwarzschild black holes. We use the vertically-averaged magnetic stress in the simulated disc as a proxy for the rest-frame dissipation, and compute the observed emission by folding this through the transfer function describing the relativistic beaming, light bending and time delays near a non-rotating black hole. The temporal power spectrum of the predicted emission from individual annuli in the disc is described by a broken power law, with indices of ≈ −3.5 at high frequency and ≈ 0 to −1 at low frequency. Integrated over the disc, the power spectrum is approximated by a single power law with an index of −2. Increasing inclination boosts the relative power at frequencies around ≈ 0.3fms, where fms is the orbital frequency at the marginally stable orbit, but no evidence is found for sharp quasi-periodic oscillations in the lightcurve. Assuming that fluorescent iron line emission locally tracks the continuum flux, we compute simulated broad iron line profiles. We find that relativistic beaming of the non-axisymmetric emission profile, induced by turbulence, produces high-amplitude variability in the iron line profile. We show that this substructure within the broad iron line profile can survive averaging over a number of orbital periods, and discuss the origin of the anomalous X-ray spectral features, recently reported by Turner et al. (2002) for the Seyfert galaxy NGC 3516, in the context of turbulent disc models. Key words: accretion, accretion discs — black hole physics — MHD — turbulence — X-rays: binaries — galaxies: active

Year: 2003
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