Fast variability as a probe of the smallest regions around accreting black holes. (arXiv:1301.0627v1 [astro-ph.HE])

Fast variability as a probe of the smallest regions around accreting black holes. (arXiv:1301.0627v1 [astro-ph.HE]):
We extract the spectra of the fastest variability (above 10 Hz) from the
black hole XTE J1550-564 during a transition from hard to soft state on the
rise to outburst. We confirm previous results that the rapid variability
contains no significant disc component despite this being strongly present in
the total spectrum of the softer observations. We model ionised reflection
significantly better than previous work, and show that this is also suppressed
in the rapid variability spectrum compared to the total emission. This is
consistent with the fast variability having its origin in a hot inner flow
close to the black hole rather than in the accretion disc or in a corona above
it. However, the rapid variability spectrum is not simply the same as the total
Comptonised emission. It is always significantly harder, by an amount which
increases as the spectrum softens during the outburst. This adds to evidence
from time lags that the Comptonisation region is inhomogeneous, with harder
spectra produced closest to the black hole, the same region which produces the
fastest variability.