High-Energy X-Ray Timing Experiment detections of hard X-ray tails in Scorpius X-1

We report the detection of a nonthermal hard X-ray component from Sco X-1 b ased on the analysis of 20-220 keV spectra obtained with the High-Energy X- Ray Timing Experiment on board the Rossi X-Ray Timing Explorer satellite. W e find that the addition of a power-law component to a thermal bremsstrahlu ng model is required to achieve a good fit in five of 16 observations analy zed. Using Proportional Counter Array data, we were able to track the movem ent of the source along the Z diagram, and we found that the presence of th e 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 M, a s indicated from the position on the Z diagram. We find that the derived no nthermal luminosities are similar to 10% of that derived for the brightest of the atoll sources.