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Cause of Gemini/FLAMINGOS-2 image artefact is unknown

Identify the origin of the image artefact affecting the Gemini/FLAMINGOS-2 J and Ks-band observations of GRB220831A and determine the reduction steps or instrumental effects responsible for it.

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Background

Several near-infrared detections in Gemini/FLAMINGOS-2 data were later deemed spurious. The authors attribute these to an unknown artefact likely related to data reduction (e.g., sky subtraction residuals and dither pattern), but could not identify the exact cause.

Clarifying the source of the artefact is important to ensure data reliability in similar follow-up campaigns and to avoid misinterpretation of transient signals.

References

In Section 2, we report that some of the data collected with Gemini/FLAMINGOS-2 at 9, 10 and 52d post-burst (PI: O’Connor, O’Connor et al. 2022b) with the 𝐽 and 𝐾 filter𝑠 suffers from an unknown image artefact likely introduced in the data reduction process (Kathleen Labrie, private communication), though we did not identify its exact cause in the data.

— GRB$\,$220831A: a hostless, intermediate Gamma-ray burst with an unusual optical afterglow (2411.14749 - Freeburn et al., 22 Nov 2024) in Appendix A (A persistent artefact in FLAMINGOS-2 imaging)