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E. Chapman, R.M. Sambruna (GMU)
We present X-ray observations of the nearby TeV blazar Mrk 501 obtained in 1998 and 1999 with the Rossi X-ray Timing Explorer. We are using proprietary as well as archival RXTE data. In 1998, the source was observed starting February 25 every three days for two months, then daily in May and June, and finally with a more irregular sampling through September. In 1999, Mrk 501 was observed daily starting May 7 for a month, and with a more sparse sampling throughout June and July. Significant spectral variations were observed, with the well-known "clockwise" and "anti-clockwise" loops when the spectral index is plotted versus the intensity. These results support a synchrotron origin for the X-ray emission, and give insights into the cooling and acceleration of the jet electrons. This work is supported by NASA grant NAG5-10244.