Detection of chromatic microlensing in Q 2237 + 0305 A

We present narrow band images of the gravitational lens system Q 2237+0305 made with the Nordic Optical Telescope in eight different filters covering the wavelength interval 3510-8130 Å. Using PSF photometry fitting we have derived the difference in magnitude vs. wavelength between the 4 images of Q 2237+0305. At λ = 4110 Å, the wavelength range covered by the Strömgren-v filter coincides with the position and width of the CIV emission line. This allows us to determine the existence of microlensing in the continuum and not in the emission lines for two images of the quasar. Moreover the brightness of image A shows a significant variation with wavelength which can only be explained as consequence of chromatic microlensing. To perform a complete analysis of this chromatic event our observations were used together with OGLE light curves. Both data sets can not be reproduced by the simple phenomenology described under the caustic crossing approximation; using more realistic representations of microlensing at high optical depth we found solutions consistent with simple thin disk models (rs ∝ λ ), however, other accretion disk size-wavelength relationships also lead to good solutions. New chromatic events from the ongoing narrow band photometric monitoring of Q 2237+0305 are needed to constrain accurately the physical properties of the accretion disk for this system. Subject headings: gravitational lensing — accretion, accretion disks — quasars: individual (Q 2237+0305)