Detection of chromatic microlensing in Q 2237+0305 A

TL;DR

This paper reports the detection of chromatic microlensing in the gravitational lens system Q 2237+0305, using narrowband imaging and light curves to analyze wavelength-dependent brightness variations, providing insights into the quasar's accretion disk.

Contribution

First detection of chromatic microlensing in Q 2237+0305 using narrowband imaging and combined light curve analysis, constraining accretion disk models.

Findings

Chromatic microlensing affects the continuum but not emission lines.

Brightness variation in image A is wavelength-dependent, indicating chromatic microlensing.

Data are consistent with thin disk models but other size-wavelength relations also fit.

Abstract

We present narrowband 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 \AA. Using point-spread function photometry fitting we have derived the difference in magnitude versus wavelength between the four images of Q~2237+0305. At $\lambda=4110$ \AA, the wavelength range covered by the Str\"omgren-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 Optical Gravitational Lensing Experiment light curves. Both data sets cannot 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 ($r_{s}\varpropto \lambda^{4/3}$); 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 accurately constrain the physical properties of the accretion disk for this system.