The impact of cosmic rays on the sensitivity of JWST/NIRSpec

Abstract

The focal plane of the NIRSpec instrument on board the James Webb Space Telescope (JWST) is equipped with two Teledyne H2RG near-IR detectors, state-of-the-art HgCdTe sensors with excellent noise performance. Once JWST is in space, however, the noise level in NIRSpec exposures will be affected by the cosmic ray (CR) fluence at the JWST orbit and our ability to detect CR hits and to mitigate their effect. We have simulated the effect of CRs on NIRSpec detectors by injecting realistic CR events onto dark exposures that were recently acquired during the JWST cryo-vacuum test campaign undertaken at Johnson Space Flight Center. Here we present the method we have implemented to detect the hits in the exposure integration cubes, to reject the affected data points within our ramp-to-slope processing pipeline (the prototype of the NIRSpec official pipeline), and assess the performance of this method for different choices of the algorithm parameters. Using the optimal parameter set to reject CR hits from the data, we estimate that, for an exposure length of 1,000 s, the presence of CRs in space will lead to an increase of typically ~7% in the detector noise level with respect to the on-ground performance, and the corresponding decrease in the limiting sensitivity of the instrument, for the medium and high-spectral resolution modes.

arXiv commentAccepted by Publications of the Astronomical Society of the Pacific - 25 pages, 12 figures

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• JADES NIRSpec Initial Data Release for the Hubble Ultra Deep Field: Redshifts and Line Fluxes of Distant Galaxies from the Deepest JWST Cycle 1 NIRSpec Multi-Object Spectroscopy
• APERO: A PipelinE to Reduce Observations -- Demonstration with SPIRou
• The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope IV. Capabilities and predicted performance for exoplanet characterization
• Beyond Gaussian Noise: A Generalized Approach to Likelihood Analysis with non-Gaussian Noise
• Bayesian approach to radio frequency interference mitigation
• Photobombing Earth 2.0: Diffraction Limit Related Contamination and Uncertainty in Habitable Planet Spectra
• JWST NIRSpec's Cosmic Ray Experience at L2