Frequency-dependent polarization of repeating fast radio bursts-implications for their origin

The polarization of fast radio bursts (FRBs), bright astronomical transients, contains crucial information about their environments. We report polarization measurements of five repeating FRBs, the abundant signals of which enable wide-band observations with two telescopes. A clear trend of lower polarization at lower frequencies was found, which can be well characterized by a single parameter rotation-measure-scatter ({\sigma}RM) and modeled by multi-path scatter. Sources with higher {\sigma}RM have higher RM magnitude and scattering timescales. The two sources with the most substantial {\sigma}RM, FRB 20121102A and FRB 20190520B, are associated with a compact persistent radio source. These properties indicate a complex environment near the repeating FRBs, such as a supernova remnant or a pulsar wind nebula, consistent with their arising from young populations.