Flat-spectrum radio quasars from the SDSS DR3 quasar catalogue

We constructed a sample of 185 flat-spectrum radio quasars (FSRQs) by cross-correlating Shen et al.'s Sloan Digital Sky Survey Data Release 3 (SDSS DR3) X-ray quasar sample with the Faint Images of the Radio Sky at Twenty Centimeters (FIRST) and Green Bank 6-cm survey (GB6) radio catalogues. From the spectral energy distribution (SED) constructed using multi-band (radio, UV, optical, infrared and X-ray) data, we derived the synchrotron peak frequency and peak luminosity. The black hole mass M(BH) and the broad-line region (BLR) luminosity (then the bolometric luminosity L(bol)) were obtained by measuring the linewidth and strength of broad emission lines from SDSS spectra. We define a subsample of 118 FSRQs for which non-thermal jet emission is thought to dominate over thermal emission from the accretion disc and host galaxy. For this subsample, we found 25 FSRQs having synchrotron peak frequency nu(peak) > 10(15) Hz, which is higher than the typical value for FSRQs. These sources with high nu(peak) could be targets for the Fermi gamma-ray telescope. Only a weak anti-correlation is found between the synchrotron peak frequency and peak luminosity, while no strong correlation is present between either the synchrotron peak frequency and black hole mass or the synchrotron peak frequency and the Eddington ratio L(bol)/L(Edd). When combining the FSRQ subsample with Wu et al.'s sample of 170 BL Lac objects, a strong anti-correlation between the synchrotron peak frequency and luminosity is apparently present, covering about seven orders of magnitude in nu(peak). However, the anti-correlation differs from the blazar sequence in having large scatter. At a similar peak frequency, the peak luminosity of FSRQs with nu(peak) > 10(15) Hz is systematically higher than that of BL Lac objects, with some FSRQs outside the range covered by BL Lac objects. Although high nu(peak) values are found in some FSRQs, they do not reach the extreme value of BL Lacs. For the subsample of 118 FSRQs, we found significant correlations between the peak luminosity and black hole mass, the Eddington ratio and the BLR luminosity, indicating that the jet physics may be strongly related to the accretion process