  Superconducting transition-edge sensor (TES) microcalorimeters have great
utility in x-ray applications owing to their high energy resolution, good
collecting efficiency and the feasibility of being multiplexed into large
arrays. In this work, we develop hard x-ray TESs to measure the absolute
energies of muonic-argon ($\mu$-Ar) transition lines around 44 keV and 20 keV.
TESs with sidecar absorbers of different heat capacities were fabricated and
characterized for their energy resolution and calibration uncertainty. We
achieved ~ 1 eV absolute energy measurement accuracy at 44 keV, and < 12 eV
energy resolution at 17.5 keV.
