EDP Sciences
Free access
Issue
A&A
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)

Abstract
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

SIMBAD Objects



© ESO 2004

What is OpenURL?

The OpenURL standard is a protocol for transmission of metadata describing the resource that you wish to access.

An OpenURL link contains article metadata and directs it to the OpenURL server of your choice. The OpenURL server can provide access to the resource and also offer complementary services (specific search engine, export of references...). The OpenURL link can be generated by different means.

  • If your librarian has set up your subscription with an OpenURL resolver, OpenURL links appear automatically on the abstract pages.
  • You can define your own OpenURL resolver with your EDPS Account.
    In this case your choice will be given priority over that of your library.
  • You can use an add-on for your browser (Firefox or I.E.) to display OpenURL links on a page (see http://www.openly.com/openurlref/). You should disable this module if you wish to use the OpenURL server that you or your library have defined.