Time delays in the images of gravitationally lensed quasars play crucial role in understanding the geometry and physical context of the gravitational lens systems (GLS). In case of the short time delays on hourly/daily timescales, correlating the X-ray/gamma-ray data is the best way to determine them as the variability of quasars at these energies is usually faster than at lower ones. Here, we demonstrate the usage of our web tool for correlation function analysis, applying the cross-correlation (asymmetrical) function to the Chandra and auto-correlation (symmetrical) one to the XMM-Newton light curves of the images of the quasar in the famous GLS Q2237+0305 (“Huchra lens”/“Einstein Cross”). We also describe the way to distinguish between the GLS time delay and periodicity in light curves based on translational symmetry of cross-correlation function in case of periodicity of the signal. We have estimated the delays between the gravitationally lensed images and the timescales of (quasi)periodical flux variations of the quasar in the Einstein Cross.
Fedorova et al. (Thu,) studied this question.