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FSRQs

An examination of Figure 5 shows that the FSRQs in the DXRBS sample cover a much wider range of parameter space than those in the two previously existing complete samples of FSRQs, the 1 Jy and S4 samples (radio data for these samples were taken from Stickel et al. 1994 and Stickel & Kühr 1994, while X-ray data for these objects were taken from the multifrequency AGN database of Padovani et al. (1997b; and references therein); it should be noted that the deeper S5 sample is $\sim 25\%$ unidentified and half of the objects lack redshifts, as noted in Stickel & Kühr 1996). We have quantified these differences by 1-dimensional and 2-dimensional Kolmogorov-Smirnov (K-S) tests. The 2-dimensional K-S test reveals that the differences in the (L_X,L_R) plane coverage between the DXRBS and 1 Jy and S4 samples are significant: the probability that the 1 Jy and DXRBS samples could emerge from the same parent population is 0.2%, where as the probability that the S4 and DXRBS samples could emerge from the same parent population is 2.2%. Given the fainter flux limits of DXRBS, this is expected.

One-dimensional K-S tests reveal that largest difference is in the radio luminosity. The probability that the 1 Jy and DXRBS radio luminosity distributions could emerge from the same parent population is $<0.1\%$, and the probability that the S4 and DXRBS radio luminosity distributions could emerge from the same parent population is 0.7%. The mean of the DXRBS L_R distribution is different from that of both the S4 and 1 Jy at greater than 99.9% significance. The situation is somewhat different for the X-ray luminosity distribution. The probability that the 1 keV luminosity distribution of the 1 Jy and DXRBS sample could emerge from the same parent population is 15%, i.e. our results are inconsistent with them emerging from a different parent population. The result is similar for the S4 (23% probability). Also, the mean of the DXRBS sample's X-ray luminosity differs from that of both the 1 Jy and S4 samples' only at the 93 and $92\%$ level respectively. Note, however, that X-ray data are available only for $\sim 53\%$ and $66\%$ of the S4 and 1 Jy FSRQs respectively, so their X-ray luminosity distributions are likely to be skewed towards the most luminous X-ray sources.

Inspection of Figure 5 reveals that the differences lie in two areas: at low luminosities (particularly low radio luminosities) and high ratios of $L_{\rm X}/L_{\rm R}$. The former regime could not be surveyed well by previous surveys due to their considerably higher flux limits. It is therefore not surprising that, as shown in Figure 5, the 1 Jy and S4 samples together have only twelve objects at radio luminosities $L_R < 10^{33.5} {\rm ~erg ~s^{-1}}$ ($\sim 3\%$), and none at $L_R < 10^{32.5} {\rm ~erg ~s^{-1}}$. The fraction of low-luminosity objects is much higher in the DXRBS sample (Fig. 5), which, while still incomplete, already contains over twice as many objects (28; or $\sim 20\%$) with $L_R < 10^{33.5} {\rm ~erg ~s^{-1}}$, six of which are at $L_R < 10^{32.5} {\rm ~erg ~s^{-1}}$. The DXRBS sample is therefore the very first sample of blazars to contain statistically significant numbers of blazars at low luminosities, approaching what should be the lower end of the FSRQ luminosity function according to unified schemes, i.e. $\sim 10^{31.5} {\rm ~erg ~s^{-1} ~Hz^{-1}}$(Urry & Padovani 1995).

The discovery of a large population of FSRQs with ratios of X-ray to radio luminosity L_X/L_R > 10^-6 ($\alpha_{\rm rx} < 0.78$), values more similar to HBLs, is more startling, as few such objects were known in previous complete samples (there are nine such objects in the 1 Jy and S4 combined; see Fig. 5). The finding of a large population of ``HBL-type'' FSRQs contradicts the prediction of Sambruna et al. (1996) that, based upon the similarities in the optical-X-ray broadband spectral characteristics of LBLs and FSRQs, there should be no HBL-type FSRQs. Padovani, Giommi & Fiore (1997b) were the first to notice that about 17% of all radio quasars with radio/optical/X-ray data (previous to DXRBS) fell in the region of the $(\alpha_{\rm ox},\alpha_{\rm ro})$ plane typical of HBLs (or X-ray selected BL Lacs) and called them ``HBL-like'' quasars.

We term these objects ``HFSRQs'', or high-energy peaked FSRQs; this terminology stresses their apparent similarity to the HBLs. These objects comprise $\sim 25\%~(32/135)$ of the DXRBS sample of FSRQs so far. However they probably comprise a somewhat larger proportion of the DXRBS FSRQ population as a whole, as $\sim 40\%$ (25/59) of the newly-identified FSRQs are HFSRQs. Padovani et al. (1997b) have proposed that the X-ray band in these objects, unlike in lower L_X/L_R FSRQs, in which inverse Compton emission prevails (Padovani, Giommi & Fiore 1997a), is dominated by synchrotron emission (see also Sambruna 1997), as the X-ray spectra of the previously-observed HFSRQs in their database were as steep as those of HBLs (Perlman et al. 1996b; Sambruna et al. 1996; Padovani & Giommi 1996). As the DXRBS sample contains a larger, more representative sample of HFSRQs than could be gleaned from previously identified samples, we will revisit this assertion and address the properties of the HFSRQ subclass in depth in a future paper (Perlman & Padovani, in prep). However, the data herein allow us the first measure of the prevalence of such objects and their proportion among FSRQs in a well defined sample, as well as the first opportunity to speculate upon their relationship to the FSRQ subclass as a whole.

In order to examine the differences between the HFSRQs and lower L_X/L_R objects, we have performed 1-dimensional K-S tests on the radio and X-ray luminosity distributions on the subsamples of DXRBS FSRQs with L_X/L_R greater and less than 10^-6. These tests reveal that the probability that the X-ray luminosity distribution of the two subsamples could emerge from the same parent population is 41% (i.e. consistent with having been drawn from the same parent population), while the probability that the radio luminosity distribution of the two subsamples could emerge from the same parent population is $\sim 0.01\%$. The same story is told by the mean X-ray and radio luminosities: The mean radio luminosities differ by 0.75 in the log and the significance of the difference is $\gt 99.99\%$,where as the difference in the X-ray luminosities is only 0.18, and is not statistically significant ($P=77\%$). This trend can also be seen on Figure 5. There is only one HFSRQ at luminosities $L_R \gt 10^{35} {\rm ~erg ~s^{-1} ~Hz^{-1}}$, compared to over two dozen lower-L_X/L_R objects. And a careful examination of the figure reveals that the lower-L_X/L_R objects are much more strongly clustered at high radio luminosities than are the HFSRQs. The fraction of HFSRQs also increases as radio luminosity decreases. These trends are similar to (but not as marked) as what is seen for BL Lacs (Urry & Padovani 1995; see also below).