A Characterization of JWST MIRI Detector Persistence and Implications for High-Contrast Imaging

TL;DR

The paper addresses flux-deficit persistence in JWST MIRI detectors, a nonstandard persistence mode at long imaging wavelengths that can bias faint-source photometry and hamper high-contrast imaging. It analyzes full-field F2100W observations after a saturation event from a bright nearby M-dwarf, applying a Bayesian exponential decay model to quantify the persistence timescale using two post-saturation epochs. The results show an initial persistence of $1.69 \pm 0.10$% that decays to one-tenth its value in $5.16^{+1.49}_{-0.94}$ hours, constraining the temporal behavior of the effect. These findings have practical implications for planning MIRI high-contrast imaging campaigns, highlighting the risks of false positives and degraded PSF subtraction, and guiding mitigation strategies for future observations.

Abstract

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 $μ$m (F2100W) to quantify detector persistence following a saturation event by a bright (K = 5.65 mag) nearby (8.12 $\pm$ 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 post-saturation epochs: shortly after saturation (11.6 minutes) and an hour later (1.39 hours). Immediately after the saturation event, we detect a persistence level of $1.69 \pm 0.10$%. By fitting a Bayesian exponential decay model to the two epochs, we estimate that persistence decreases to one-tenth of its initial value after $5.16^{+1.49}_{-0.94}$ hours. 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 PSF subtraction, particularly at small inner working angles. We also outline mitigation strategies to avoid these impacts in future observations.

Figures (1)

• Figure 1: Saturation event occurs when imaging WD 2151+591 in the F770W band (left). Sequential dither position images with MIRI at F2100W of WD 2151+591 ( middle) and WD 1748+708 (right) illustrate the flux-deficit persistence caused by saturation from the bright M-dwarf IRAS 21500+5903 in the WD 2151+591 field. This source saturated the detector most severely in 7.7 $\mu$m (F770W; left) imaging taken 12 minutes prior to the imaging sequence on the middle and 1.39 hours prior to the sequence on the right. The flux-deficit residual trace from the M-dwarf is visible between dither positions as it moves across the MIRI detector. Dither pattern used for this observation was cycling with dither points $1-4$ (top to bottom).