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LibraryD. Gasman (KU Lueven), I. Argyriou (KU Leuven), G.C. Sloan (STScI, UNC), B. Aringer (Univ. Vienna), and 17 others
2023, A&A, 673, 102
Full manuscript available locally (PDF) or from the arXiv (2212.03596).
Context. The Medium Resolution Spectrometer (MRS) is one of the four
observing modes of JWST/MIRI. Using JWST in-flight data of unresolved
(point) sources, we can derive the MRS absolute spectral response function
(ASRF) starting from raw data. Spectral fringing, caused by coherent
reflections inside the detector arrays, plays a critical role in the
derivation and interpretation of the MRS ASRF. The fringe corrections
implemented in the current pipeline are not optimal for non-extended
sources, and a high density of molecular features particularly inhibits an
accurate correction.
Aims: In this paper, we present an alternative way to calibrate the
MIRI/MRS data. Firstly, we derive a fringe correction that accounts for the
dependence of the fringe properties on the MIRI/MRS pupil illumination and
detector pixel sampling of the point spread function. Secondly, we derive
the MRS ASRF using an absolute flux calibrator observed across the full 5-28
µm wavelength range of the MRS. Thirdly, we apply the new ASRF to the
spectrum of a G dwarf and compare it with the output of the JWST/MIRI
default data reduction pipeline. Finally, we examine the impact of the
different fringe corrections on the detectability of molecular features in
the G dwarf and K giant.
Methods: The absolute flux calibrator HD 163466 (A-star) was used to derive
tailored point source fringe flats at each of the default dither locations
of the MRS. The fringe-corrected point source integrated spectrum of
HD 163466 was used to derive the MRS ASRF using a theoretical model for the
stellar continuum. A cross-correlation was run to quantify the uncertainty
on the detection of CO, SiO, and OH in the K giant and CO in the G dwarf for
different fringe corrections.
Results: The point-source-tailored fringe correction and ASRF are found to
perform at the same level as the current corrections, beating down the
fringe contrast to the sub-percent level in the G dwarf in the longer
wavelengths, whilst mitigating the alteration of real molecular features.
The same tailored solutions can be applied to other MRS unresolved targets.
Target acquisition is required to ensure the pointing is accurate enough to
apply this method. A pointing repeatability issue in the MRS limits the
effectiveness of the tailored fringe flats is at short wavelengths. Finally,
resulting spectra require no scaling to make the sub-bands match, and a
dichroic spectral leak at 12.2 µm is removed.
Last modified 26 May, 2023. © Gregory C. Sloan and others.