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Geometry of X-ray sources in accreting black-hole binaries
  Nordita Astrophysics seminars

Friday 12 August 2016
from 10:30 to 11:30
at FC61
Speaker : Andrzej Zdziarski (N. Copernicus Astronomical Center, Poland)
Abstract : Accreting binary stellar systems containing black holes and low-mass donors are transient, i.e., they outburst after a period of quiescence, and those with high-mass donors are persistent. Both of them exhibit two main luminous states, spectrally soft and hard. Their X-ray spectra in the soft and hard states are dominated by blackbody emission of accretion discs peaking around 1 keV, and by a component peaking around 100 keV from Compton scattering by mildly relativistic electrons, respectively. There is a general consensus about the nature of the soft state, in which an optically thick accretion disc emitting blackbody radiation extends down to the innermost stable circular orbit (ISCO) around the black hole. However, there is currrently a heated controversy regarding the nature of the hard state. According to a long-dominant paradigm, the accretion disc in this state is truncated at a radius >> ISCO and replaced by a hot flow emitting hard X-rays. This explains many observed phenomena, e.g., spectral and variability differences between the states and transition to the hard state from quiescence (when the disc is certainly truncated) in transients. On the other hand, there have been numerous claims that the disc extends to ISCO also in the hard state, and the hard X-ray source is located on the black-hole rotation axis and very close to the horizon (a `lamppost'). I will discuss both the theoretical and observational arguments for the disc truncation and against the `lamppost' geometry.

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