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Appendix A: Microlensing scenarios for different D/B macro-magnification ratios

We investigated microlensing scenarios corresponding to the macro-magnification ratios derived by Sluse et al. (2012), M_D/B ~ 0.7, and Fadely & Keeton (2011), M_D/B ~ 0.8.

For M_D/B ~ 0.7 (and μ ~ 1.1 in the continuum), the whole line profile would be affected by microlensing so that a rigorous MmD cannot be performed. Simultaneous micro-magnification of the Hα red wing (although to a lesser extent than for M_D/B ~ 0.5) and micro-de-magnification of the blue wing would reproduce the distortions observed in the Hα line profile of image D (red curve in Fig. A.1). Caustics can cause such microlensing effect since they can delineate magnification and de-magnification areas.

For M_D/B ~ 0.8, only the blue wing of Hα is micro-de-magnified (magenta curve in Fig. A.1) and the continuum is not microlensed (μ ~ 1 in the continuum). A similar microlensing effect that magnifies only the broad emission line region, has presumably been observed for the system J1004-4142 (Richards et al. 2004). In that interpretation, a de-magnification area stands close enough to affect the BLR but far enough not to affect the continuum source. This agrees with the stability of the D/B flux ratio over time and wavelength (see Table 3). However, considering the unification model for active galactic nuclei, in which a dusty torus surrounds the BLR (regardless of its geometry), we would expect a broad de-magnification region to de-magnify the dusty torus as well. Thus, the L′-band flux should be affected, which is not observed.

Fig. A.1 Ratio between the Hα line profiles observed in the spectrum of image D and the spectrum of image B, corrected for the macro-magnification ratio. The continua that compose the spectra of images B and D were previously subtracted. This ratio is a proxy for the effect of microlensing on the Hα line profile in D. At a given wavelength, a ratio higher than unity corresponds to the micro-magnification of the BLR emission. Conversely, a lower ratio indicates a micro-de-magnification. The spectra were smoothed with a fifteen-pixel-wide median filter for clarity.

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