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The Deep X-ray Radio Blazar Survey (DXRBS) uses
a cross-correlation of all serendipitous sources in the publicly available
database of ROSAT sources, WGACAT (White, Giommi & Angelini 1995),
having quality flag (to avoid problematic detections) with a
number of publicly available radio catalogs. North of the celestial
equator, we used the 20 cm and 6 cm Green Bank survey catalogs NORTH20CM
and GB6 (White & Becker 1992; Gregory et al. 1996), while south of the
equator, we used the Parkes-MIT-NRAO catalog PMN (Griffith & Wright
1993). All sources with radio spectral index
at a few
GHz were selected as blazar candidates.
For objects north of the
celestial equator, 6-20 cm radio spectral indices
were obtained directly from the cross-correlation
of the GB6 and NORTH20CM catalogs. For sources at southern
declinations, the lack of a comparably deep radio survey at a second
frequency required a different strategy. In the band , we cross-correlated the sources with the public
NVSS database (Condon et al. 1997); our selection of
candidates is still not completed in this declination range, since
the NVSS is not yet 100% complete. Further
south, the positional accuracy of the NVSS, which covers the sky north of
, decreases somewhat (Condon
et al. 1997). In this region,
we conducted a snapshot survey with the
Australia Telescope Compact Array (ATCA) at 3.6 and 6 cm (note that the
positional accuracy of the ATCA snapshots deteriorate significantly
at
due to the East-West nature of the array), to get also radio spectral
indices unaffected by variability (see § 7). The first set of ATCA
observations (of 163 X-ray/radio sources) took place 11-13 November 1995.
A second set of 55 X-ray/radio sources (some of which have preliminarily
been classified as blazar candidates based upon the 6-20 cm spectral index
computed from their PMN and NVSS fluxes) were observed in October 1997
to complete the coverage of the southern sample. These ATCA observations
will be discussed in a future paper.
We had originally requested observations
at 6 and 20 cm at the ATCA as well, to match our northern sample, but the time
allocation committee decided otherwise, based on the instrumental
configuration. Note that, as the NVSS has a much smaller beam
size than the GB6 survey, it is preferable to use, whenever possible,
the NORTH20CM
20 cm fluxes to derive spectral indices. Extra care was taken in the
region, where we had to resort to the NVSS for this
purpose, to include the flux from all sources in a 2 arcmin radius. We stress,
however, that this problem is severe only for extended, steep-spectrum
radio sources, and not for the core-dominated, flat-spectrum sources we
are interested in.