Markarian 501 in X-ray bright state - RXTE observations
  Mrk 501 has been in a state of very high flux in X-rays and VHE gamma-rays during 1997. In July 1997 near its hitherto maximum X-ray brightness intense multifrequency observations of Mrk 501 have been performed at radio, near infrared, optical, X-ray, and VHE gamma-ray frequencies.   Here we report on Rossi X-ray Timing Explorer (RXTE) observations carried out in 1997 between July 11 and July 16. The X-ray spectrum has been well detected up to 100 keV and is best described by a broken power law with spectral indices alpha1=0.70 and alpha2=0.94 below and above the break energy 5.8 keV.   The X-ray flux from Mrk 501 declined and flared by about 30% within about 3 days each, showing an unusual anti-correlation between flux and spectral hardening of both power law components. The break energy remained constant.   The observed broad band and X-ray spectra confirm recent observations, that the synchrotron component in Mrk 501 can extend up to energies of 100 keV with the maximum power being emitted at hard X-rays. On longer time-scales the cut-off frequency of the synchrotron spectrum changes by more than two orders of magnitude. 
G. Lamer and S.J. Wagner