We study linear polarization from scattering of light on a cloud of relativistic electrons. We assume that the cloud is hovering above an accretion disc, or it is in an accelerated motion underthe combined influence of the radiation and gravitational fields near an accreting black hole. Atfirst we derive simple and general analytical formulae for the Stokes parameters. These formulae are then used in calculations of the temporal evolution of the observed signal.

We find that higher-order images can significantly enhance the observed flux. Possible targets where the effect should be searched are accreting super-massive black holes and Galactic microquasars exhibiting episodic accretion/ejection events.