# The Brighter-Fatter Effect in the JWST MIRI Si:As IBC detectors I. Observations, impact on science, and modelling

**Authors:** Ioannis Argyriou, Craig Lage, George H. Rieke, Danny Gasman, Jeroen Bouwman, Jane Morrison, Mattia Libralato, Daniel Dicken, Bernhard R. Brandl, Javier Álvarez-Márquez, Alvaro Labiano, Michael Regan, Michael E. Ressler  
**arXiv:** [2303.13517](https://arxiv.org/abs/2303.13517)  
**Date:** 2023-03-23  
**Summary page:** https://www.summarizepaper.com/en/arxiv-id/2303.13517v1/

## Key points

- The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) uses three Si:As impurity band conduction (IBC) detector arrays to record the output voltage level of each MIRI detector pixel by sampling-up-the-ramp.
- Pixel ramps become non-linear in a predictable way for uniform or low-contrast illumination, but become much more complex in areas of high contrast, known as the Brighter-Fatter Effect (BFE).
- Observational evidence of BFE is found in MIRI conventional and high-contrast coronographic imaging, low-resolution spectroscopy, and medium-resolution spectroscopy data.
- The physical mechanism behind the MIRI BFE is fundamentally different from that of CCDs and Hawaii-2RG detectors due to most photo-excited electric charges not being stored at pixels but at the input to the MIRI detector unit cell buffer amplifier capacitance.
- The resulting debiasing-induced BFE alters the electrostatic potential inside the infrared active layer, causing new photo excited electrons generated at a bright pixel to be guided towards neighboring fainter pixels.
- Debiasing induced BFE makes JWST MIRI data yield 10 - 25% larger and 0.5 - 8% brighter point sources and spectral line profiles as a function of output level covered by detector pixels.
- Profile shrinking detector depletion regions have implications for developing a pixel ramp non linearity model for point sources observed with MIRI.
- A new numerical model is presented that can be used to account for this effect when analyzing data from JWST/MIRI commissioning and Cycle 1 phase.

## Summary

The Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) uses three Si:As impurity band conduction (IBC) detector arrays to record the output voltage level of each MIRI detector pixel by sampling-up-the-ramp. While pixel ramps become non-linear in a predictable way for uniform or low-contrast illumination, they become much more complex in areas of high contrast. This phenomenon is known as the Brighter-Fatter Effect (BFE). We provide observational evidence of it in MIRI conventional and high-contrast coronographic imaging, low-resolution spectroscopy, and medium-resolution spectroscopy data. We investigate the physical mechanism that gives rise to this effect on the detector pixel raw voltage integration ramps using public data from the JWST/MIRI commissioning and Cycle 1 phase. We also develop a numerical electrostatic model of the MIRI detectors using a modified version of the public Poisson_CCD code. Our findings show that the physical mechanism behind the MIRI BFE is fundamentally different from that of CCDs and Hawaii-2RG (H2RG) detectors due to most photo-excited electric charges not being stored at pixels but at the input to the MIRI detector unit cell buffer amplifier capacitance. The resulting debiasing-induced BFE alters the electrostatic potential inside the infrared active layer, causing new photo excited electrons generated at a bright pixel to be guided towards neighboring fainter pixels. Observationally, we find that this debiasing induced BFE makes JWST MIRI data yield 10 - 25% larger and 0.5 - 8% brighter point sources and spectral line profiles as a function of output level covered by detector pixels. Additionally, we note that profile shrinking detector depletion regions have implications for developing a pixel ramp non linearity model for point sources observed with MIRI. Our study provides a more detailed understanding of the BFE in MIRI detectors and its impact on science. We also present a new numerical model that can be used to account for this effect when analyzing data from JWST/MIRI commissioning and Cycle 1 phase.