Astron. Astrophys. 318, L67-L70 (1997)

4. Discussions and conclusion

The low-resolution spectra for two components of SBS 1520+530, taken with the 6m telescope, show that both images are quasars with the same redshift: 1.855 0.002 and 1.854 0.002, separated by 1:0056. Both spectra demonstrate presence of identical emission and absorption line profiles: such as the broad feature in the blue wing of CIV 1549Å and two narrow metal-line absorption systems at redshifts 0.715 0.001 and 0.815 0.001 in each of them. The fact that the redshifts of both components are identical within the measurement errors and the similarity of BAL features in their spectra practically exclude the possibility for this system to be a physical pair. The probability that two independent quasars, even connected physically, may have the same structure of BAL regions is negligibly small. Some difference between the profiles of the blend AlIII/CIII] 1858/1909ÅA in the spectra of the components suggests possible microlensing effects (Refsdal & Surdej 1994). Significant differences in (B-V) and (V-R) colours and the continuum shapes between of two components can be interpreted on the basis of gravitational lens hypothesis as due to differences of absorbing matter along two paths of deflected light beams. These differences could also be explained by the emission of the deflecting galaxy. The possibility of strong absorption origin in lens matter should also be noted.

Two neighbouring objects, NW and SE, are probably connected with the SBS 1520+530 system. The spectrum of the NW component was detected, but with low signal-to-noise ratio. We could not identify any details in its spectrum, but the shape of the continuum and its colours are similar to those of the brighter components. The SE object exhibited colours that are redder then that of the other components, and may be connected with a lensing galaxy, if it is located at z=0.7 0.8. Its colours are in good agreement with those expected from spiral galaxies at this distance (Schild 1984).

Further photometric and spectral study of this system may allow us to detect the lens itself and to define the nature of the faint components, NW and SE.

Hence, we can conclude that SBS 1520+530 is a gravitationally lensed BAL QSO, the second known system after CloverLeaf H1413+117 (Magain et al. 1988), at z=1.855 with a separation of 1:0056 between the companions, having V=18:m2 and 18:m6, respectively. The identity of the emission and absorption line profiles strongly supports our contention. The apparent brightness and angular separation between lensed images, SBS 1520+530 provides an opportunity for both ground and space based investigations of microlensing effects, and measurements of the global parameters of the gravitational lens.

The basic data, concerning new gravitationally lensed system SBS 1520+530 are summarized in Table 4.

Table 4. Summary of SBS 1520+530 data

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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