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Institut für Astronomie und Astrophysik

Abteilung Astronomie

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Preprint 01/98

Markarian 501 in X-ray bright state - RXTE observations

G. Lamer(1) and S. Wagner(2,3)

(1) Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(2) Landessternwarte Heidelberg-Königstuhl, D-69117 Heidelberg, Germany
(3) Mount Stromlo & Siding Springs Observatories, Private Bag, Weston Creek PO, ACT 2611, Australia

1998, Astronomy & Astrophysics, 331, L13

Abstract. 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 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 July 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 alpha_1=0.70 and alpha_2=0.94 below and above the break energy of E_break=5.8 keV.
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 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 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

Paper (51k gzip'ed Postscript including figures)

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Jürgen Barnstedt (barnstedt AT astro.uni-tuebingen.de)
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