Line and continuum variability of two intermediate-redshift, high-luminosity quasars

Context. It has been shown that the luminosity of active galactic nuclei and the size of their broad line region obey a simple relation of the type R-BLR = aL(gamma), from faint Seyfert nuclei to bright quasars, allowing single-epoch determination of the central black hole mass M-BH = bL(gamma) Delta(2) H-beta from their luminosity L and width of H-beta emission line. Adopting this mass determination for cosmological studies requires the extrapolation to high redshift and luminosity of a relation whose calibration relies so far on reverberation mapping measurements performed for L less than or similar to 1046 erg s(-1) and redshift z less than or similar to 0.4. Aims. We initiated a campaign for the spectrophotometric monitoring of a few luminous, intermediate redshift quasars whose apparent magnitude, V < 15.7, allows observations with a 1.8 m telescope, aimed at proving that emission lines vary and respond to continuum variations even for luminosities less than or similar to 1047 erg s(-1), and determining eventually their MBH from reverberation mapping. Methods. We have repeatedly performed simultaneous spectrophotometric observations of quasars and reference stars to determine relative variability of continuum and emission lines. We describe the observations and methods of analysis. Results. For the quasars PG 1634+ 706 and PG 1247+ 268 we obtain light-curves respectively for CIII](lambda lambda 1909 angstrom), MgII(lambda lambda 2798 angstrom) and for CIV(lambda lambda 1549 angstrom), CIII](lambda lambda 1909 angstrom) emission lines with the relevant continua. During 3.2 years of observation, in the former case no continuum variability was detected and the evidence for line variability is marginal, while in the latter case both continuum and line variability are detected with high significance and the line variations appear correlated with continuum variations. Conclusions. The detection of the emission line variability in a quasar with L similar to 10(47) erg s(-1) encourages the continuation of the monitoring campaign which should provide a black hole mass estimate in another 5-6 years, constraining the mass-luminosity relation in a poorly explored range of luminosity.