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Sergei Nayakshin, Demosthenes Kazanas (NASA/GSFC)
We show that the large amplitude X-ray variability common for GBHCs (and AGN) cannot be explained by magnetic flares if they are incoherent. Therefore, we present a cellular automata model (CAM), in which magnetic flux tubes appear randomly on the surface of the disk (with the rate proportional to the local energy generation in the disk), but do not flare until a certain criterion is reached. As is already known from Solar CAMs, under these conditions the magnetic flux tubes are in a self-organized critical state, such that an ignition of one flux tube might prompt its quasi-stable neighbors to flare as well, and a large scale outburst may develop.
We present our model light curves and power spectra. These show that the flares may only produce variability shorter than about 0.1 sec, and that longer time variations (often seen in GBHCs) must be due to temporal variations of either the accretion disk itself or the efficiency with which the flares are produced. Further, we find that there is a strong connection between RMS amplitude of variability and the state of the magnetic field in the disk. This fact allows us to test the earlier suggested explanation of the state transitions in GBHCs as due to the change in the state of the field. We show that if the field is indeed largely uniform in radiation-dominated disks (soft states of GBHCs) and is highly localized in the gas-dominated disks (hard states), then the X-ray RMS variability is self-consistently lower in the soft state than it is in the hard state, in agreement with observations.
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