tokens: in the strength of the BLR–H β component, in addition to that observed in the Fe II strength. However, a remarkable simultaneous variation in both H β and Fe II strength is what we would expect in the case when the two emissions are strictly co-spatial, whereas we know that the BLR is stratified and that microlensing magnifies the emission from the most compact regions more strongly. Therefore, we do not exclude the microlensing hypothesis. A detailed analysis of the different BLR spectra from the two images is beyond the scope of this work and will be presented in a forthcoming paper. Therefore, in this work, we focus on how we accounted for this effect during the spectral analysis. Consequently, in SDSS J1353+1138, we extracted two distinct nuclear spectra, namely, spectrum A and B that are shown in Fig. 3 , using a 03 radius aperture centred on the emission peak of each lensed image. We proceeded to fit them separately. In both spectra we used a first-degree polynomial to model the AGN continuum that is still dominant over the stellar continuum. Unlike the BLR modelling of HS 0810+2554, the multiple Gaussian fit was not sufficient to reproduce the broader and complex profile of the broad Balmer emission lines, especially the H β line in spectrum B. In fact, even though both H β prominent wings are likely due to the Fe II emission, as discussed above, the Fe II templates employed by the code were not able to reproduce such observed emission. Therefore, we modelled both wings as part of the broad H β line profile without any focus on their physical interpretation, as we were simply interested in identifying the overall BLR spectrum in order to remove it. For the modelling of the broad Balmer emission lines observed in SDSS J1353+1138 (i.e., H β and H γ ), we used a broken power law distribution convolved with a Gaussian profile ( Nagao et al. 2006 ): where the free parameters of the fit, for each line, are the central wavelength λ 0 , the two power-law indices α and β , the normalisation F 0 , and the width σ of the Gaussian kernel. In spectrum A (upper panel), the H β and H γ were modelled separately through the line profile described in Eq. ( 1 ). Spectrum B (lower panel) required instead an additional broad Gaussian component to suitably reproduce both the more extended red wing and the broad peak ( σ ∼ 800 km s −1 ) in the H β profile. Moreover, we had to constrain the H γ line profile through that of H β . Different Fe II templates have been selected in the BLR best-models for the two nuclear spectra. To model the narrow emission line profiles, we used three Gaussian components in spectrum A, while a single Gaussian component in spectrum B, since we did not actually observe any narrow component. Then we proceeded to fit the whole data cube following the procedure described in Sect. 3.1.2 , with the main difference that in each spatial pixel, pPXF considered a linear combination of the two BLR models, weighing their relative contribution
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