| Sumario: | Aims. We aim to use microlensing taking place in the lensed quasar QSO 2237 + 0305 to study the structure of the broad line region (BLR) and measure the size of the region emitting the C IV and C III] lines. Methods. Based on 39 spectrophotometric monitoring data points obtained between Oct. 2004 and Dec. 2007, we derived lightcurves for the C IV and C III] emission lines. We used three different techniques to analyse the microlensing signal. Different components of the lines (narrow, broad, and very broad) were identified and studied. We built a library of the simulated microlensing lightcurves that reproduce the signal observed in the continuum and in the lines provided only the source size is changed. A Bayesian analysis scheme is then developed to derive the size of the various components of the BLR. Results. 1. The half-light radius of the region emitting the C IV line is found to be light-days = 0.06 pc = 1.7 cm (at 68.3% CI). Similar values are obtained for C III]. Relative sizes of the carbon-line and V-band continuum emitting-regions are also derived with median values of Rline/Rcont in the range 4 to 29, depending on the FWHM of the line component. 2. The size of the C IV emitting region agrees with the radius-luminosity relationship derived from reverberation mapping. Using the virial theorem, we derive the mass of the black hole in QSO 2237 + 0305 to be MBH ~ 108.3 ± 0.3 M⊙. 3. We find that the C IV and C III] lines are produced in at least 2 spatially distinct regions, the most compact one giving rise to the broadest component of the line. The broad and narrow line profiles are slightly different for C IV and C III]. 4. Our analysis suggests a different structure for the C IV and Fe II+III emitting regions, with the latter produced in the inner part of the BLR or in a less extended emitting region than C IV.
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