Zooming into the broad line region of the gravitationally lensed quasar Q2237+0305 = the Einstein Cross: III. Determination of the size and structure of the CIV and CIII] emitting regions using microlensing

We aim to use microlensing taking place in the lensed quasar Q2237+0305 to study the structure of the broad line region and measure the size of the region emitting the CIV and CIII] lines. Methods: Based on 39 spectrophotometric monitoring data points obtained between Oct. 2004 and Dec. 2007, we derived lightcurves for the CIV and CIII] emission lines. We used three different techniques to analyse the microlensing signal. Different components of the lines (narrow, broad and very broad) are identified and studied. We built a library of 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 CIV line is found to be R_CIV ~ 66^{+110}{-46} lt-days = 0.06$^{+0.09}{-0.04}$ pc = 1.7$^{+2.8}_{-1.1}$,10$^{17}$ cm (at 68.3% CI). Similar values are obtained for CIII]. 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 of the FWHM of the line component. 2. The size of the CIV 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 Q2237+0305 to be M_BH ~ 10^{8.3+/-0.3} M_sun. 3. We find that the CIV and CIII] 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 CIV and CIII]. 4. Our analysis suggests a different structure for the CIV and FeII+III emitting regions, with the latter produced in the inner part of the BLR or in a less extended emitting region than CIV.

💡 Research Summary

The paper presents a detailed microlensing study of the gravitationally lensed quasar Q2237+0305 (the Einstein Cross) with the aim of measuring the size and internal structure of its broad‑line region (BLR), specifically the regions that emit the CIV λ1549 and CIII] λ1909 lines. Using 39 spectrophotometric epochs obtained between October 2004 and December 2007, the authors extracted light curves for the two carbon lines from each of the four lensed images. Three complementary analysis techniques were employed: (1) a differential‑spectra method to isolate pure microlensing variability between image pairs; (2) a multi‑component decomposition of each line into narrow (N), broad (B) and very broad (VB) Gaussian components, allowing the authors to track microlensing signatures for each kinematic sub‑region; and (3) the construction of an extensive library of simulated microlensing light curves that reproduce the observed continuum variability while varying only the source size. The simulations incorporate realistic stellar mass functions, surface mass densities, and shear values derived from the lens galaxy, and they are generated on high‑resolution caustic maps to capture the full stochastic nature of microlensing.

A Bayesian inference framework compares the observed line light curves with the simulated library, yielding posterior probability distributions for the half‑light radii (R½) of each component. The main results are: (i) the CIV emitting region has a half‑light radius of R_CIV ≈ 66 light‑days (≈0.06 pc), with a 68.3 % confidence interval of +110 /‑46 light‑days; the CIII] region shows a comparable size. These values are fully consistent with the radius‑luminosity (R‑L) relationship derived from reverberation mapping studies of lower‑redshift AGN. (ii) The line‑to‑continuum size ratios span a wide range, from about 4 for the very broad component to roughly 29 for the narrow component, indicating that the BLR is not a single homogeneous shell but consists of multiple stratified zones with different characteristic velocities and distances from the central engine. (iii) By combining the measured BLR size with the line width (FWHM) and assuming virial motion, the black‑hole mass is estimated as M_BH ≈ 10^{8.3±0.3} M_⊙, placing Q2237+0305 among typical luminous quasars. (iv) The microlensing signatures differ between CIV and CIII], implying that each line originates in at least two spatially distinct regions: a compact, high‑velocity zone that produces the very broad component, and a more extended, lower‑velocity zone responsible for the narrow component. The profiles of the two lines are not identical, reinforcing the notion of a complex, multi‑component BLR. (v) The Fe II+III emission appears to arise from an even more compact or less extended region than CIV, suggesting a different ionization structure or density regime within the innermost BLR.

Overall, the study demonstrates that microlensing provides an independent, high‑resolution probe of BLR geometry that complements reverberation mapping. The combination of multi‑component line decomposition, extensive microlensing simulations, and Bayesian analysis offers a powerful methodology for dissecting the kinematic and spatial sub‑structures of quasar emission regions. The authors argue that applying this approach to a larger sample of lensed quasars, together with longer‑term spectroscopic monitoring, will refine BLR models, improve black‑hole mass estimates, and deepen our understanding of the physical conditions governing line formation in active galactic nuclei.