The Criteria

We have identified as our highest priority sources those which meet the following four criteria:

1.

$\alpha_{\rm 6-20} \leq 0.7$ for $\delta \gt-15^\circ$; $\alpha_{\rm 3.6-6} \leq 0.7$ for $\delta < -15^\circ$

2.

$\vert b\vert \gt 10^\circ$;

3.

$F(20 {\rm ~cm}) \gt 100$ mJy for $0<\delta<75^\circ$;

4.

$F(6 {\rm ~cm}) \lower.5ex\hbox{$\; \buildrel \gt \over \sim \;$}50$ mJy for $\delta<0^\circ$.

These criteria were chosen in order to ensure that a well defined, flux-limited sample can be achieved. Over 200 candidate blazars met the above defining criteria (details are given below). In addition, 98 previously identified but serendipitously observed objects meet our criteria (see § 4.3). Lower-galactic-latitude and lower-flux sources were assigned to a lower-priority list, but are useful in order to fill in higher L_X/L_R areas of parameter space. No pre-selection was imposed on WGACAT in the region $75^\circ \geq \delta \geq -90^\circ$, although for ease of identification we avoided the regions within $5^\circ$ of the Large and Small Magellanic Clouds, as well as M31.

Note that although we have not imposed any cut in X-ray flux, our sample will have both radio and X-ray flux limits, the latter depending on the region of the sky surveyed due to the serendipitous nature of WGACAT (similarly to the EMSS). Depending on the length of each individual exposure and the distance from the center of the PSPC field, the X-ray flux limits appropriate for each source will vary between $\sim 10^{-14}$ and $\sim 10^{-12}$ erg cm^-2 s^-1. The slightly different radio flux limits north and south of the equator are simply explained: For $\delta\gt^\circ$ (and $\le 75^{\circ}$, the limit of the GB6 catalog), it is easy to see that all sources with $F(20 {\rm ~cm}) \gt 100$ mJy (the limit of the NORTH20CM catalog) and $\alpha_{\rm r} \leq 0.7$ will be above the much lower flux limit of the GB6 catalog ($\sim 25$ mJy; note that the converse does not hold). For $\delta<0^\circ$, on the other hand, our radio flux limit is the completeness limit of the PMN survey, which (while declination dependent; see Griffith & Wright 1993) averages about 50 mJy.