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Sergei Nayakshin (NASA/GSFC)
A magnetically fed corona above a cold accretion disk is one of the most likely ways in which accretion disks produce X-rays. While the shape of the continuum and the iron line features are often fit well with this model in both AGN and GBHCs, the question of the overall normalization of X-rays relative to the bolometric luminosity has not been previously considered. Here we show that since magnetic fields contribute to the disk viscosity, and thus the local disk heating, transport of a given amount of magnetic energy into the corona via diffusive magnetic fields should lead to an even greater amount of energy being liberated inside the disk. This latter energy is then thermally radiated away, so that such disks should always have less power in hard X-rays than they do in the thermal disk component.
On the other hand, if one assumes that most of the disk magnetic field is localized in the form of strong magnetic flux tubes (as on the surface of the Sun), then the field contribution to viscosity goes down substantially. This then leads to a decrease of the local disk heating, so that it becomes possible for accretion disks to release most of their energy in the corona rather than through the common thermal radiation. We show that this situation is possible in the gas-dominated disks, whereas in radiation-dominated disks radiation penetrates into the tubes ''too easily'', which makes the field weak/diffuse.
We then point out that this consideration can explain the observed hard/soft transitions in GBHCs, since the observed luminosity corresponding to these transitions coincides with that for which the disk switches from being gas-dominated to being radiation-dominated. We also discuss implications of our results for the often observed ``steeper when brighter'' behavior in AGN.
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