Astron. Astrophys. 331, L13-L16 (1998)

Letter to the Editor

Markarian 501 in X-ray bright state - RXTE observations

G. Lamer ¹ and S.J. Wagner <sup>2, 3

1</sup> Institut für Astronomie und Astrophysik, Abt. Astronomie, Universität Tübingen, Waldhäuserstr. 64, D-72076 Tübingen, Germany (e-mail: lamer@astro.uni-tuebingen.de)
² Landessternwarte Heidelberg-Königstuhl, D-69117 Heidelberg, Germany
³ Mount Stromlo & Siding Spring Observatories, Private Bag, Weston Creek PO, ACT 2611, Australia

Received 28 November 1997 / Accepted 23 December 1997

Abstract

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
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