HEXTE Detections of Hard X-Ray Tails in Sco X-1
  We report the detection of a non-thermal hard X-ray component from Sco X-1 based upon the analysis of 20-220 keV spectra obtained with the HEXTE experiment on-board the RXTE satellite. We find that the addition of a power-law component to a thermal bremsstrahlung model is required to achieve a good fit in 5 of 16 observations analyzed. Using PCA data we were able to track the movement of the source along the Z diagram, and we found that the presence of the hard X-ray tail is not confined to a specific Z position. However, we do observe an indication that the power law index hardens with increasing mass accretion rate, as indicated from the position on the Z diagram. We find that the derived non-thermal luminosities are at order of 10% of that derived for the brightest of the atoll sources. 
F. D'Amico (1 and 2), W. A. Heindl (1), R. E. Rothschild (1) and D. E.   Gruber (1) ((1) UCSD, La Jolla, USA, (2) INPE, S. J...