5 Spectroscopy of SBS 1520+530

5.1 Keck II spectroscopy

Converting the time delay into an estimate of the Hubble parameter requires good knowledge of the geometry of the lensing system, including the redshift of the source and of the lensing galaxy. For this purpose, we obtained a deep optical spectrum of SBS 1520+530 with the Keck II Telescope and the Echellette Spectrograph and Imager (ESI) (Sheinis et al. 2000). The observations were obtained on May 23, 2001 under good seeing conditions (0.6 $^{\prime\prime}$). The 1 $^{\prime\prime}$  slit was aligned to obtain simultaneously the spectrum of SBS 1520+530 and of star S2 (indicated in Fig. 1). Four 900s exposures were taken so that cosmic rays could be efficiently removed.

5.2 Redshift of the lensing galaxy

Using the spatial information contained in the spectrum of the PSF star S2, the 2D spectrum of SBS 1520+530 was spatially deconvolved, by applying the spectral version of the MCS deconvolution algorithm (Courbin et al. 2000b). The deconvolution process decomposes the data into the individual spectra of the two quasar images and of the faint lensing galaxy (see Figs. 5 and 6), as done for the lensed systems HE 2149-2745 (Burud et al. 2002) or HE 1104-1805 (Lidman et al. 2000). No emission lines were detected in the spectrum of the lensing galaxy (Fig. 5). However, an absorption doublet is clearly observed at $\lambda \sim 6700$ Å, matching within the error-bars the CaII H and K lines at redshift 0.71. This strongly supports a scenario where the lensing galaxy is responsible for the CaII absorption lines at redshift $z=0.71\pm0.005$ first found by Chavushyan et al. (1997) in the quasar spectra. Another absorption system was found by Chavushyan et al. (1997), at z=0.82. This system does not seem to be associated with the lens.

5.3 Spectra of the quasar components

The quasar spectra display the usual prominent emission lines such as AlIII/CIII] ($\lambda$ 1909 Å) and MgII ($\lambda$ 2798 Å), at redshift z=1.855. Both (deconvolved) spectra show the metal absorption lines first reported by Chavushyan et al. (1997). These are the FeII ($\lambda$ 2587, 2600 Å), MgII ($\lambda$ 2798 Å), MgI ($\lambda$ 2853 Å) absorption lines at redshifts $0.717\pm0.001$ and $0.817\pm0.001$. CaII ($\lambda$ 3933, 3968 Å) is seen only at z=0.717 and is indicated in Fig. 6. It is better seen in the normalized spectra of Fig. 7.

The general shape of the spectra towards the blue wavelengths is most probably corrupted by atmospheric refraction. The observed reddening of B compared to A at the blue wavelengths may therefore not be real. In the red parts of the spectra, we find a flux ratio of 1.82 (at $\lambda$ 9000 Å) which, compared with the ratio of 1.892 measured on R band light curves ($\lambda$ 6500 Å), indicates an almost constant flux ratio in this wavelength range.

Figure 5: Spectrum of the lensing galaxy in SBS 1520+530. The CaII ($\lambda$ 3933, 3968 Å) is identified. Also indicated are a night sky emission line and an atmospheric absorption line.

Figure 6: Spectra of the quasar components A (top) and B (bottom) of SBS 1520+530. Absorption lines corresponding to metal absorptions at z=0.71 and z=0.82 are indicated.

Figure 7: The A (top) and B (bottom, vertically shifted by -0.2 for clarity) spectra with normalized continuum. The equivalent widths of the emission lines including AlIII/CIII] and MgII are significantly smaller in B than in A. The CaII absorption lines at redshift 0.71 are indicated as well as two telluric bands.

The continuum normalized spectra, displayed in Fig. 7, show that the equivalent widths of the emission lines are much larger in A than in B, suggesting that B's continuum is amplified. Such a differential amplification can be explained by microlensing effects. Assuming that the lensed quasar follows the "standard'' AGN model and that we are observing component B through a network of caustics produced by stars in the main lensing galaxy, one can imagine that the inner continuum region of component B is enhanced by a larger amount than the much larger emission line region. This interpretation has already been proposed to explain similar spectral differences observed in the double HE 1104-1805 (Wisotzki et al. 1993; Courbin et al. 2000b) and HE 2149-2745 (Burud et al. 2002).