Charge Migration and Residual Non-Linearity in NIRSpec BOTS Observations

TL;DR

This study examines how charge migration and residual non-linearity affect JWST/NIRSpec detectors during long-duration observations of exoplanet WASP-121b, revealing significant differences between two detectors and their impact on spectral trace stability.

Contribution

It provides the first detailed analysis of charge migration and non-linearity effects on NIRSpec detectors over extended observations, highlighting differences between NRS1 and NRS2.

Findings

NRS1 shows ~3x larger non-linearity deviations than NRS2.

Charge distribution varies with planetary orbital phase for NRS1.

Trace shape stability differs between NRS1 and NRS2 during observations.

Abstract

We investigate the effect of charge migration and residual non-linearity on the JWST/NIRSpec G395H NRS1 and NRS2 detectors using Bright Object Time Series (BOTS) observations of the ultra-hot Jupiter WASP-121b. These full-orbit phase curve observations were taken over 37.8 hours (1.57 days), and provide an excellent testbed of the non-linearity behavior of NRS1 and NRS2 over long timescales. For both detectors, our analysis demonstrates charge losses at the center of the spectral trace and charge excesses at the trace edges. We find that the NRS1 detector displays ~3x larger deviations from linearity compared to NRS2. Given the large transit (~1.5%) and eclipse (~0.5%) signals for WASP-121b, we also investigate variations in the distribution of charge throughout the time-series observation. Our results show that charge distribution varies at different planetary orbital phases for NRS1, which manifests as a change in the morphology and shape of the spectral trace over the course of the time-series. The effect of charge distribution on the trace shape is not evident for NRS2.