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Abstract: We present narrowband images of the gravitational lens system Q~2237+0305made with the Nordic Optical Telescope in eight different filters covering thewavelength interval 3510-8130 \AA. Using point-spread function photometryfitting we have derived the difference in magnitude versus wavelength betweenthe four images of Q~2237+0305. At $\lambda=4110$ \AA, the wavelength rangecovered by the Str\-omgren-v filter coincides with the position and width ofthe CIV emission line. This allows us to determine the existence ofmicrolensing in the continuum and not in the emission lines for two images ofthe quasar. Moreover, the brightness of image A shows a significant variationwith wavelength which can only be explained as consequence of chromaticmicrolensing. To perform a complete analysis of this chromatic event ourobservations were used together with Optical Gravitational Lensing Experimentlight curves. Both data sets cannot be reproduced by the simple phenomenologydescribed under the caustic crossing approximation; using more realisticrepresentations of microlensing at high optical depth, we found solutionsconsistent with simple thin disk models $r {s}\varpropto \lambda^{4-3}$;however, other accretion disk size-wavelength relationships also lead to goodsolutions. New chromatic events from the ongoing narrow band photometricmonitoring of Q~2237+0305 are needed to accurately constrain the physicalproperties of the accretion disk for this system.

Autor: A. M. Mosquera, J.A. Muñoz, E. Mediavilla


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