Zooming into the broad line region of the gravitationally lensed quasar QSO 2237 + 0305  ≡  the Einstein Cross

III. Determination of the size and structure of the C iv and C iii] emitting regions using microlensing^⋆

¹ Astronomisches Rechen-Institut am Zentrum für Astronomie der Universität Heidelberg Mönchhofstrasse 12-14, 69120 Heidelberg, Germany
e-mail: sluse@ari.uni-heidelberg.de
² Laboratoire d’Astrophysique, Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
³ F.R.S.-FNRS, Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 17, B5c, 4000 Liège, Belgium
⁴ Centro de Astro-Ingeniería, Departamento de Astronomía y Astrofísica, P. Universidad Católica de Chile, Casilla 306, Santiago, Chile
⁵ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
⁶ Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195, USA

Received: 9 November 2010
Accepted: 13 December 2010

Abstract

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 ${\mathit{R}}_{\mathrm{C}\hspace{0.17em}{iv}}\mathrm{~}\mathrm{6}{\mathrm{6}}_{-46}^{\mathrm{+}\mathrm{110}}$ light-days = 0.06$\begin{array}{}\mathrm{+}\mathrm{0.09}\\ -0.04\end{array}$ pc = 1.7$\begin{array}{}\mathrm{+}\mathrm{2.8}\\ -1.1\end{array}\mathrm{\times }{\mathrm{10}}^{\mathrm{17}}$ 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 R^line/R^cont 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 M_BH ~ 10^8.3 ± 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.