Apparent correlation between extrinsic and intrinsic flux variations in the first gravitationally lensed quasar

Abstract

To better understand which sources contribute to optical passband fluxes of Q0957+561, we present and analyse light curves of the doubly imaged gravitationally lensed quasar from its discovery to 2023. After an early microlensing event, the difference light curves (describing delay-corrected flux ratios between the two quasar images) only show prominent microlensing gradients over the last 17 yr. In addition to these long time-scale gradients in the gr bands, we detect short time-scale, extrinsically induced differential variations that are highly correlated with the short time-scale intrinsic variability of the quasar in those bands. Both the accretion disc and the broad emission-line region (BELR) contribute to optical passband fluxes, and we also show that realistic contributions of the BELR account for the observed correlations in the gr bands. We would like to highlight that the BELR sources of optical passband fluxes of Q0957+561 should be taken into account when measuring accretion-disc source sizes from microlensing simulations.