SIMBAD references

Mrk 501 has been in a state of very high flux in X-rays and VHE γ-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 γ-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 100keV and is best described by a broken power law with spectral indices α₁=0.70 and α₂=0.94 below and above the break energy of E_break=5.8keV. The X-ray flux from Mrk 501 declined and flared by ∼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 100keV 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.