  A 57Fe nucleus in the solar core could emit a 14.4-keV monochromatic axion
through the M1 transition if a hypothetical elementary particle, axion, exists
to solve the strong CP problem. Transition edge sensor (TES) X-ray
microcalorimeters can detect such axions very efficiently if they are again
converted into photons by a 57Fe absorber. We have designed and produced a
dedicated TES array with 57Fe absorbers for the solar axion search. The iron
absorber is set next to the TES, keeping a certain distance to reduce the
iron-magnetization effect on the spectroscopic performance. A gold thermal
transfer strap connects them. A sample pixel irradiated from a 55Fe source
detected 698 pulses. In contrast to thermal simulations, we consider that the
pulses include either events produced in an iron absorber or gold strap at a
fraction dependent on the absorption rate of each material. Furthermore,
photons deposited on the iron absorber are detected through the strap as
intended. The identification of all events still needs to be completed.
However, we successfully operated the TES with the unique design under iron
magnetization for the first time.
