2026ApJS..282...57V

The JWST MIRI detector exhibits a flux-deficit persistence, but its timescales and impacts remain largely uncharacterized, particularly at the longest imaging wavelengths. In this study, we analyze full-field MIRI imager observations at 21 μm (F2100W) to quantify detector persistence following a saturation event by a bright (K = 5.65 mag) nearby (8.12 ± 0.04 pc) mid-M dwarf star, IRAS 21500+5903. Unlike typical persistence that appears as excess flux, this effect presents as a flux deficit in pixels previously illuminated by the saturating or near-saturating source. We measure persistence at two postsaturation epochs: shortly after saturation (11.6 minutes) and an hour later (1.39 hr). Immediately after the saturation event, we detect a persistence level of 1.69% ± 0.10%. By fitting a Bayesian exponential decay model to the two epochs, we estimate that persistence decreases to one-tenth of its initial value after $5.1{6}_{-0.94}^{+1.49}$hr. We examine the implications of persistence for MIRI high-contrast imaging using the imager (not coronagraphy). Specifically, we discuss how MIRI detector persistence can produce false-positive exoplanet signals in direct imaging surveys, as well as degrade point-spread function subtraction, particularly at small inner-working angles. We also outline mitigation strategies to avoid these impacts in future observations.