[Paper Review] A First Look at the JWST MIRI/LRS Phase Curve of WASP-43b

Taylor J. Bell, Laura Kreidberg+8 more|arXiv (Cornell University)|Jan 16, 2023

Geophysics and Gravity Measurements8 citations

TL;DR

This paper presents preliminary JWST MIRI/LRS phase-curve observations of WASP-43b, evaluates instrument performance, and offers planning recommendations for Cycle 2 based on observed systematics and stability.

ABSTRACT

We observed a full-orbit phase curve of the hot Jupiter WASP-43b with MIRI/LRS as part of the Transiting Exoplanet Community Early Release Science Program. Here we report preliminary findings for the instrument performance from the team's MIRI Working Group. Overall we find that MIRI's performance for phase curve observations is excellent, with a few minor caveats. The key takeaways for Cycle 2 planning with MIRI/LRS are: (1) long-duration observations (> 24 hours) have now been successfully executed; (2) for phase curves, we recommend including a one-hour burn-in period prior to taking science data to mitigate the effects of the ramp systematic; and (3) we do not yet recommend partial phase curve observations. In addition, we also find that: the position of the spectrum on the detector is stable to within 0.03 pixels over the full 26.5-hour observation; the light curves typically show a systematic downward ramp that is strongest for the first 30 minutes, but continues to decay for hours; from 10.6-11.8 microns, the ramp effect has remarkably different behavior, possibly due to a different illumination history for the affected region of the detector; after trimming the integrations most affected by the initial ramps and correcting the remaining systematics with analytic models, we obtain residuals to the light-curve fits that are typically within 25% of the photon noise limit for 0.5-micron spectroscopic bins; non-linearity correction is not a significant source of additional noise for WASP-43, though it may be an issue for brighter targets; the gain value of 5.5 electrons/DN currently on CRDS and JDox is known to be incorrect, and the current best estimate for the gain is approximately 3.1 electrons/DN; new reference files for the JWST calibration pipeline reflecting these findings are under development at STScI.

Motivation & Objective

• Assess JWST MIRI/LRS performance for full-orbit phase-curve observations of WASP-43b.
• Characterize instrumental systematics (ramps, stability, non-linearity) during a long-duration observation.
• Provide practical guidance for Cycle 2 planning and calibration needs.
• Identify detector behavior and required calibration updates to support accurate light-curve extraction.

Proposed method

• Conduct a full-orbit phase curve observation of WASP-43b with JWST MIRI/LRS over ~26.5 hours.
• Assess and correct ramp systematics using analytic models after trimming the most affected integrations.
• Evaluate the stability of the spectrum position on the detector (to ~0.03 pixels).
• Analyze wavelength-dependent ramp behavior (notably 10.6–11.8 μm) and its illumination history implications.
• Estimate residuals of light-curve fits relative to photon noise in 0.5 μm spectroscopic bins after corrections.
• Report on non-linearity relevance and update calibration reference files (gain value) accordingly.

Experimental results

Research questions

• RQ1 How well can JWST MIRI/LRS deliver long-duration phase-curve observations for a hot Jupiter?
• RQ2 What are the dominant instrumental systematics (ramps) affecting phase-curve photometry, and how can they be mitigated?
• RQ3 How stable is the spectral trace on the detector over a long observation, and what calibration updates are required?
• RQ4 What is the impact of detector non-linearity on WASP-43b phase-curve measurements and gain calibration?
• RQ5 What guidance should be issued for Cycle 2 planning based on early MIRI/LRS performance?

Key findings

• Long-duration (>24 hours) MIRI/LRS phase-curve observations have been successfully executed.
• Recommend a one-hour burn-in period before science data to mitigate ramp systematics.
• Position of the spectrum on the detector is stable to within 0.03 pixels over 26.5 hours.
• Ramps are present with a strongest initial downward trend, continuing to decay for hours; 10.6–11.8 μm shows different ramp behavior likely due to illumination history.
• After trimming the most affected integrations and modeling systematics, residuals are typically within 25% of the photon-noise limit for 0.5 μm bins.
• Non-linearity is not a significant noise source for WASP-43b, though it could affect brighter targets.
• Initial gain value (CRDS/JDox) is incorrect; current best estimate is ~3.1 e−/DN (not 5.5 e−/DN).
• New reference files for JWST calibration pipeline are being developed at STScI.