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Institut für Astronomie und Astrophysik

Abteilung Astronomie

Sand 1, D-72076 Tübingen, Germany

Preprint 07/95

ROSAT Observations of BL Lacertae Objects

G. Lamer(1), H. Brunner(1,2), and R. Staubert(1)

(1)Institut für Astronomie und Astrophysik, Astronomie, Universität Tübingen, Waldhäuser Str. 64, D-72076 Tübingen, Germany
(2)Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany

accepted by Astronomy & Astrophysics

Abstract. We present soft X-ray spectra of 74 BL Lacertae objects observed with the PSPC detector on board of the ROSAT satellite. The sample contains all BL Lac objects detected during the pointed observation phase as a target or serendipitously. We have investigated the soft X-ray and broad band spectral properties and discuss the consequences for the X-ray emission processes.
For the first time a clear dependence of the X-ray spectral steepness on the radio to X-ray spectral energy distribution is found: alpha_RX and alpha_X are correlated in the X-ray selected (XBL) subsample and anticorrelated in the radio selected (RBL) subsample. The objects with intermediate alpha_RX values thus do have the steepest soft X-ray spectra. Simulated PSPC spectra based on a set of simple two component multifrequency spectra are in good agreement with the measurements and suggest a broad range of synchrotron cutoff energies.
We have calculated synchrotron self-Compton beaming factors for a subsample of radio bright objects and find a correlation of the beaming factors delta_IC with alpha_RX and alpha_X. The most extreme RBL objects are very similar to flat spectrum radio quasars in all their broad band and X-ray properties.

Key words: Galaxies: active - BL Lacertae objects: general - Radio continuum: general - X-rays: general

Paper (101k gzip'ed Postscript including figures)
Table 2 (2k) Table 3 (3k) (all gzip'ed ASCII text)

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