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Letter to the Editor
Markarian 501 in X-ray bright state - RXTE observations
G. Lamer 1 and
S.J. Wagner 2, 3
Received 28 November 1997 / Accepted 23 December 1997
Mrk 501 has been in a state of very high flux in X-rays and VHE -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 -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 and below and above the break energy of .
The X-ray flux from Mrk 501 declined and flared by 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.
Key words: galaxies: active BL Lacertae objects: individual: Mrk 501 X-rays: galaxies
Send offprint requests to: G. Lamer, Department of Physics and Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ, England
© European Southern Observatory (ESO) 1998
Online publication: February 16, 1998