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At high luminosities black hole binaries show spectra with a strong disccomponent accompanied by an equally strong tail where at least some of theelectrons are non-thermal. We reanalyze the simultaneous ASCA-RXTE-OSSE datafrom the 1998 outburst of XTE J1550-564, which span 0.7-1000 keV and remain thebest data available of a black hole binary in this state. We reassess theimportance of electron-positron pair production using a realistically highvalue of the source compactness for the first time. The lack of an observableannihilation line together with the observed gamma-ray flux beyond 511 keVconstrains the maximum electron Lorentz factor to be leq 10 and the slope ofthe injected electrons to leq 2.5. We also use the fast (10-50 Hz) variabilityspectrum to constrain the spatial dependence of the electron heating andacceleration. We find that the spectrum of the fast variability is consistentwith being fully thermal, so that the observed non-thermal emission is producedfrom further out in the flow.

Limits on pair production and multi-zone comptonization: The broadband X/gamma-ray spectrum of XTE J1550-564 revisited