The present paper investigates the temperature structure of the X-ray<br />
emitting plasma in the core of the Perseus cluster using the 1.8--20.0 keV data<br />
obtained with the Soft X-ray Spectrometer (SXS) onboard the Hitomi Observatory.<br />
A series of four observations were carried out, with a total effective exposure<br />
time of 338 ks and covering a central region $\sim7&#039;$ in diameter. The SXS was<br />
operated with an energy resolution of $\sim$5 eV (full width at half maximum)<br />
at 5.9 keV. Not only fine structures of K-shell lines in He-like ions but also<br />
transitions from higher principal quantum numbers are clearly resolved from Si<br />
through Fe. This enables us to perform temperature diagnostics using the line<br />
ratios of Si, S, Ar, Ca, and Fe, and to provide the first direct measurement of<br />
the excitation temperature and ionization temperature in the Perseus cluster.<br />
The observed spectrum is roughly reproduced by a single temperature thermal<br />
plasma model in collisional ionization equilibrium, but detailed line ratio<br />
diagnostics reveal slight deviations from this approximation. In particular,<br />
the data exhibit an apparent trend of increasing ionization temperature with<br />
increasing atomic mass, as well as small differences between the ionization and<br />
excitation temperatures for Fe, the only element for which both temperatures<br />
can be measured. The best-fit two-temperature models suggest a combination of 3<br />
and 5 keV gas, which is consistent with the idea that the observed small<br />
deviations from a single temperature approximation are due to the effects of<br />
projection of the known radial temperature gradient in the cluster core along<br />
the line of sight. Comparison with the Chandra/ACIS and the XMM-Newton/RGS<br />
results on the other hand suggests that additional lower-temperature components<br />
are present in the ICM but not detectable by Hitomi SXS given its 1.8--20 keV<br />
energy band.