A Steadily Declining Dispersion Measure for the Repeating Fast Radio Burst FRB 20220529A Evidence for a Fast Radio Burst Engine Embedded in an Expanding Supernova Remnant

We present the discovery and subsequent 3.2 yr monitoring campaign of the repeating fast radio burst (FRB) FRB 20220529A with CHIME/FRB. We observe a gradual decline in dispersion measure (DM) of −0.881 ± 0.001 pc cm⁻³ yr⁻¹ (−1.235 ± 0.001 pc cm⁻³ yr⁻¹ in the rest frame), implying a ≥ 3.5% ± 0.2% decrease in the total electron column in the source environment, and we see scattering timescale variations over weeks to years. We observe a short-lived excursion in which the DM rises by ∼1 pc cm⁻³, immediately preceding an increase in transient Faraday rotation measure (RM) of ∼2000 rad m⁻² previously reported for this source, before returning to its gradual decline in DM. During this DM/RM excursion, we identify a local line-of-sight magnetic field of 3.4 ± 0.2 mG around FRB 20220529A, corresponding to one of the most strongly magnetized FRB environments. We measure a decrease in the linear polarization fraction of FRB 20220529A bursts with decreasing frequency that we attribute to depolarization from multipath propagation in the source environment. We also place a 5σ upper limit on the spectral luminosity of an associated persistent radio source of ≤5 × 10²⁸ erg s⁻¹ Hz⁻¹ at 1.5 GHz. These observations are consistent with FRB 20220529A originating from a young (approximately years to centuries old) expanding supernova remnant, with short-lived DM and RM variability arising from interactions with the supernova remnant or with a binary companion.