Determine whether the post-2022 oxygen accumulation reflects a compositional change

Determine whether the increase in the oxygen accumulation rate measured in high-resolution Mrk 421 spectra beginning in 2022 reflects a change in the composition of the ACIS contamination layer or the presence of a new contaminant species, and confirm the statistical significance of the effect using future observations or improved analysis techniques.

Background

The contamination layer on the Chandra ACIS optical blocking filters has accumulated over the mission, with carbon, oxygen, and fluorine contributing to the measured absorption. High-resolution Mrk 421 spectra indicate that the oxygen accumulation rate may have increased beginning in 2022, potentially signaling a change in the contaminant rather than merely continued accumulation of the previously modeled material.

The authors explicitly leave the interpretation unresolved because the available measurements have substantial uncertainties. Future calibration observations and new analysis techniques are required to establish whether the apparent increase is statistically significant and whether it corresponds to a compositional change or a new contaminant species.