arXiv:0709.2828v1 [astro-ph] 18 Sep 2007 Astronomy & Astrophysics manuscript no. Q2237˙v09 c  ESO 2019 February 12, 2019 Microlensing variability in the Einstein Cross, QSO 2237+0305: a spectrophotometric monitoring with the VLT ⋆ A. Eigenbrod 1 , F. Courbin 1 , D. Sluse 1 , G. Meylan 1 , and E. Agol 2 1 Laboratoire d’Astrophysique, Ecole Polytechnique F´ ed´ erale de Lausanne (EPFL), Observatoire, 1290 Sauverny, Switzerland 2 Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195, USA Received ... ; accepted ... ABSTRACT Aims. We present the results of the first long-term spectroscopic monitoring of a gravitationally lensed quasar, for QSO 2237+0305: the Einstein Cross. We show that chromatic microlensing-induced variations constantly affect the spectra of all four images of the lensed quasar. The goal of the present paper is to present the observational facts to later be compared with theoretical models in order to constrain the inner structure of the source quasar. Methods. We spatially deconvolve deep VLT/FORS1 spectra in order to accurately separate the spectrum of the lensing galaxy from the spectra of the quasar images. Accurate cross-calibration of the 31-epoch observations is carried out using non-variable foreground stars observed simultaneously to the quasar. The quasar spectra are further decomposed into several components in order to infer the variations in the continuum, and in the broad emission lines. Results. We find the most prominent microlensing events in quasar images A and B, while C and D are almost quiescent on a time scale of a few months. The strongest variations are observed in the continuum. Their amplitude is larger in the blue than in the red, consistent with microlensing of an accretion disk. Variations in the intensity and profile of the broad emission lines are also reported, most prominently in the wings of the C III] and center of the C IV emission lines. During a strong microlensing episode in quasar image A, the broad component of the C III] is more highly magnified than the narrow component. In addition, the emission lines with higher ionization potentials are more magnified than the lines with lower ionization potentials, consistent with the results obtained with reverberation-mapping. Finally, we find that the V-band differential extinction by the lens, between the quasar images is in the range 0.1-0.3. Key words. Gravitational lensing: quasar, microlensing — Quasars: general. Quasars: individual QSO 2237+0305, Einstein Cross 1. Introduction The gravitational lens QSO 2237+0305, also known as “Huchra’s lens” or the “Einstein Cross”, was discovered by Huchra et al. (1985) during the Center for Astrophysics Redshift Survey. It consists of a z s = 1.695 quasar gravita- tionally lensed into four images arranged in a crosslike pattern around the nucleus of a z l = 0.0394 barred Sab galaxy. A few years after this discovery, Schneider et al. (1988) and Kent & Falco (1988) computed the first simple models of the system, leading to the conclusion that this system was very promising to study microlensing. Indeed, the predicted time de- lays between the four quasar images are of the order of a day, meaning that intrinsic variability of the quasar can easily be distinguished from microlensing events. In addition, the par- ticularly small redshift of the lensing galaxy implies large tan- ⋆ Based on observations made with the ESO-VLT Unit Telescope 2 Kueyen (Cerro Paranal, Chile; Proposals 073.B-0243(A&B), 074.B- 0270(A), 075.B-0350(A), 076.B-0197(A), 177.B-0615(A&B), PI: F. Courbin) gential velocities for the microlenses and a high stellar density where the quasar images form, i.e., right in the bulge of the lens. The combination of these properties makes microlensing events very likely in the Einstein Cross and very rapid, with time-scales of a few weeks to a few months. Indeed, Irwin et al. (1989) report significant brightness variations of the bright- est quasar image A, which they interpreted as the first detection ever of microlensing in the images of a multiply-imaged quasar. Microlensing has been observed in several other gravi- tationally lensed quasars, and is expected to occur in virtu- ally any quadruply lensed quasar (Witt et al. 1995). Probably the most compelling examples of microlensing light curves are given by the Optical Gravitational Lensing Experiment (OGLE) (Wo´ zniak et al. 2000a). Since 1997, this project mon- itors regularly the four quasar images of QSO 2237+0305, showing beautiful and perpetual microlensing-induced varia- tions in the light curves. Most of the quasar microlensing studies so far are based exclusively on broad-band photometric monitoring (e.g., Wo´ zniak et al. 2000b; Schechter et al. 2003; Colley & Schild