The influence of gravitational lensing on the spectra of lensed quasi-stellar objects

We consider the influence of (milli/micro) lensing on the spectra of lensed quasi-stellar objects (QSOs). We propose a method for the observational detection of microlensing in the spectra of lensed QSOs and apply it to the spectra of three lensed QSOs (PG 1115 + 080, Q1413 + 117 and Q0957 + 561) observed with the Hubble Space Telescope. We find that the flux ratio between images A1 and A2 of PG 1115 + 080 is wavelength-dependent and shows differential magnification between the emission lines and the continuum. We interpret this magnification as arising from millilensing. We also find that the temporal variations in the continuum of image C of Q1413 + 117 may be caused by microlensing, while the temporal variation observed in Q0957 + 561 was probably an intrinsic one.