  Electronic $K$ x rays emitted by muonic Ar atoms in the gas phase were
observed using a superconducting transition-edge-sensor microcalorimeter. The
high-precision energy spectra provided a clear signature of the presence of
muonic atoms accompanied by a few electrons, which have never been observed
before. One-, two-, and three-electron bound, i.e., H-like, He-like, and
Li-like, muonic Ar atoms were identified from electronic $K$ x rays and
hyper-satellite $K$ x rays. These $K$ x rays are emitted after the charge
transfer process by the collisions with surrounding Ar atoms. With the aid of
theoretical calculations, we confirmed that the peak positions are consistent
with the x-ray energies from highly charged Cl ions, and the intensities
reflecting deexcitation dynamics were successfully understood by taking into
account the interaction between the muon and bound electrons.
