EDP Sciences
Free access
Volume 419, Number 2, May IV 2004
Page(s) 459 - 467
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20034320

A&A 419, 459-467 (2004)
DOI: 10.1051/0004-6361:20034320

Radio spectra of a sample of X-ray selected BL Lacs

F. Cavallotti1, A. Wolter1, J. T. Stocke2 and T. Rector3

1  Osservatorio Astronomico di Brera, via Brera 28, 20121, Milano, Italy
    e-mail: cavallot@brera.mi.astro.it
2  Center for Astrophysics and Space Astronomy, University of Colorado, Boulder, CO 80309-0389, USA
3  National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA

(Received 15 September 2003 / Accepted 18 February 2004)

We present simultaneous multifrequency radio observations for a complete subsample of 26 XBLs from the Einstein Extended Medium-Sensitivity Survey, obtained with the Very Large Array (VLA). Spectra are computed using fluxes at 20, 6 and 3.6 cm. Unlike many radio selected samples, the EMSS did not impose any criterion on the radio spectrum to identify BL Lac objects. It is therefore possible to investigate the intrinsic radio spectral slope distribution and to determine the effect produced by this selection criterion. We find that 15% of the observed objects do not meet the flat-spectrum criterion imposed on some other BL Lac samples. A dataset that includes non-simultaneous data (that are also taken with different VLA configurations) shows an even higher percentage of steep spectrum sources. This effect can be ascribed to a larger fraction of extended flux detected with the more compact VLA configuration. Possible biases introduced by the flat-radio-spectrum criterion in the radio-selected BL Lac samples cannot explain the discrepancies observed in the evolutionary properties of Radio and X-ray selected samples of BL Lacs.

Key words: galaxies: BL Lacertae objects: general -- radio continuum: galaxies

Offprint request: A. Wolter, anna@brera.mi.astro.it

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© ESO 2004