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We present the results of the first long-term (2.2 years) spectroscopicmonitoring of a gravitationally lensed quasar, namely the Einstein CrossQ2237+0305. The goal of this paper is to present the observational facts to becompared in follow-up papers with theoretical models to constrain the innerstructure of the source quasar. We spatially deconvolve deep VLT/FORS1 spectrato accurately separate the spectrum of the lensing galaxy from the spectra ofthe quasar images. Accurate cross-calibration of the 58 observations at31-epoch from October 2004 to December 2006 is carried out with non-variableforeground stars observed simultaneously with the quasar. The quasar spectraare further decomposed into a continuum component and several broad emissionlines to infer the variations of these spectral components. We find prominentmicrolensing events in the quasar images A and B, while images C and D arealmost quiescent on a timescale of a few months. The strongest variations areobserved in the continuum of image A. Their amplitude is larger in the blue(0.7 mag) than in the red (0.5 mag), consistent with microlensing of anaccretion disk. Variations in the intensity and profile of the broad emissionlines are also reported, most prominently in the wings of the CIII] and centerof the CIV emission lines. During a strong microlensing episode observed inJune 2006 in quasar image A, the broad component of the CIII] is more highlymagnified than the narrow component. In addition, the emission lines withhigher ionization potentials are more magnified than the lines with lowerionization potentials, consistent with the results obtained with reverberationmapping. Finally, we find that the V-band differential extinction by the lens,between the quasar images, is in the range 0.1-0.3 mag.

Microlensing variability in the gravitationally lensed quasar Q2237+0305 = the Einstein Cross, I. Spectrophotometric monitoring with the VLT