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We extract the spectra of the fastest variability (above 10 Hz) from theblack hole XTE J1550-564 during a transition from hard to soft state on therise to outburst. We confirm previous results that the rapid variabilitycontains no significant disc component despite this being strongly present inthe total spectrum of the softer observations. We model ionised reflectionsignificantly better than previous work, and show that this is also suppressedin the rapid variability spectrum compared to the total emission. This isconsistent with the fast variability having its origin in a hot inner flowclose to the black hole rather than in the accretion disc or in a corona aboveit. However, the rapid variability spectrum is not simply the same as the totalComptonised emission. It is always significantly harder, by an amount whichincreases as the spectrum softens during the outburst. This adds to evidencefrom time lags that the Comptonisation region is inhomogeneous, with harderspectra produced closest to the black hole, the same region which produces thefastest variability.

Fast variability as a probe of the smallest regions around accreting black holes