Microlensing of the broad line region in 17 lensed quasars

When an image of a strongly lensed quasar is microlensed, the different components of its spectrum are expected to be differentially magnified owing to the different sizes of the corresponding emitting region. Chromatic changes are expected to be observed in the continuum while the emission lines should be deformed as a function of the size, geometry and kinematics of the regions from which they originate. Microlensing of the emission lines has been reported only in a handful of systems so far. In this paper we search for microlensing deformations of the optical spectra of pairs of images in 17 lensed quasars. This sample is composed of 13 pairs of previously unpublished spectra and four pairs of spectra from literature. Our analysis is based on a spectral decomposition technique which allows us to isolate the microlensed fraction of the flux independently of a detailed modeling of the quasar emission lines. Using this technique, we detect microlensing of the continuum in 85% of the systems. Among them, 80% show microlensing of the broad emission lines. Focusing on the most common lines in our spectra (CIII] and MgII) we detect microlensing of either the blue or the red wing, or of both wings with the same amplitude. This observation implies that the broad line region is not in general spherically symmetric. In addition, the frequent detection of microlensing of the blue and red wings independently but not simultaneously with a different amplitude, does not support existing microlensing simulations of a biconical outflow. Our analysis also provides the intrinsic flux ratio between the lensed images and the magnitude of the microlensing affecting the continuum. These two quantities are particularly relevant for the determination of the fraction of matter in clumpy form in galaxies and for the detection of dark matter substructures via the identification of flux ratio anomalies.

💡 Research Summary

This paper presents a systematic search for microlensing‑induced distortions in the optical spectra of 17 strongly lensed quasars. The authors assembled 13 new pairs of spectra and added four pairs taken from the literature, giving a relatively large and homogeneous sample for this type of study. Their analysis relies on a spectral decomposition technique that separates the microlensed flux component from the intrinsic (macro‑lensed) flux without requiring detailed line‑profile modeling. By fitting the two images of each quasar simultaneously, they obtain both the intrinsic flux ratio (the macro‑lensing magnification factor) and the additional microlensing magnification that affects only a fraction of the source emission.

The main results are striking. Continuum microlensing is detected in 85 % of the systems, confirming that stellar‑mass objects in the lensing galaxy frequently magnify the compact accretion‑disk region (tens of light‑days across). Among those with a detected continuum effect, 80 % also show clear signatures of microlensing in the broad emission lines. The authors focus on the most commonly covered lines, C III] λ1909 and Mg II λ2798, and examine the blue and red wings separately. Three distinct patterns emerge: (i) symmetric amplification/attenuation of both wings with the same amplitude, (ii) amplification/attenuation of only one wing (blue or red), and (iii) asymmetric changes where both wings are affected but with different amplitudes. The asymmetric cases dominate, indicating that the broad‑line region (BLR) is not a simple spherical shell.

These observations have direct implications for BLR geometry and kinematics. A symmetric wing response would be consistent with a rotating, roughly axisymmetric disk, whereas isolated wing microlensing points to anisotropic structures such as a tilted disk, a non‑uniform outflow, or localized high‑density clumps. The authors compare their findings with existing microlensing simulations of a biconical outflow and find a mismatch: the simulations predict simultaneous, comparable changes in both wings, which are rarely seen. This discrepancy suggests that current models are oversimplified and that a more complex combination of rotation, inflow/outflow, and possibly radiative transfer effects must be invoked to reproduce the data.

In addition to the qualitative insights, the paper provides quantitative measurements of the intrinsic flux ratios and the microlensing magnitudes for each image pair. These quantities are crucial for two broader astrophysical applications. First, they allow a more accurate determination of the fraction of matter in compact (stellar) form versus smooth dark matter in lens galaxies, because the microlensing signal depends sensitively on the stellar mass fraction. Second, deviations between the observed flux ratios and those predicted by smooth macro‑lens models (so‑called flux‑ratio anomalies) can be used to infer the presence of dark‑matter substructures, a key test of the cold‑dark‑matter paradigm.

Overall, the paper demonstrates that microlensing of broad emission lines is a common phenomenon, not a rare curiosity, and that it provides a powerful, indirect probe of the BLR’s three‑dimensional structure. The authors’ decomposition method, which avoids detailed line modeling, proves effective for large samples and could be applied to future spectroscopic surveys (e.g., with the ELT or JWST) to build statistically robust constraints on BLR geometry, quasar accretion physics, and the small‑scale distribution of matter in lensing galaxies.