Characterizing JWST NIRSpec’s Noise Performance with a Lab-Measured Time Series Observation

While it is expected that JWST’s NIRSpec instrument will revolutionize the study of temperate terrestrial exoplanets, this revolution can only take place if subtle instrumental systematics allow it. Moreover, virtually all exposure time calculators ignore noise systematics and assume a noise floor. We have used laboratory JWST data to create two realistic transiting exoplanet time series observations and assessed the effects of real instrument systematics. We find that jitter detrending can effectively negate systematic noise resulting from intrapixel sensitivity variations and PSF motions. We are able to recover injected spectral features similar to those expected for GJ 436 b and TRAPPIST-1 d, and place a 3-sigma upper limit on the detector noise floor of 14 ppm. Our findings confirm that instrumental systematics are unlikely to be the limiting factor in future observations of targets with small-amplitude atmospheric features.