Vol. 637
4. Extragalactic astronomy

Liverpool-Maidanak monitoring of the Einstein Cross in 2006–2019. I. Light curves in the gVrRI optical bands and microlensing signatures

The Einstein Cross is a gravitationally lensed quasar at z= 1.695. The quasar QSO 2237+0305 is lensed in four images (A, B, C, and D) that are arranged like a cross around the nucleus of a nearly face-on spiral galaxy (the lens) at z =0.039. In addition to the global strong lensing, compact objects, such as stars, in the lensing galaxy produce microlensing and differential magnification of the four images. In order to gain more insight into these compact objects, the authors monitored the Einstein Cross using 4 374 optical frames (gVrRI) taken with the 2 m Liverpool and the 1.5 m Maidanak telescopes, over the 2006-2019 period, yielding light curves of the four images A-D. The differences between A-D light curves reveal strong microlensing signatures, affecting the C image, which experienced a double caustic crossing. The UV continuum of the quasar is modeled by a standard accretion disk of radius R, which increases with wavelength as a power law of exponent alpha=1.33. More monitoring data are required to draw a more robust conclusion.