Geometry of X-ray sources in accreting black-hole binaries
by
Andrzej Zdziarski(N. Copernicus Astronomical Center, Poland)
→
Europe/Stockholm
FC61
FC61
Description
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.