  Abell 754 is a galaxy cluster in which an ongoing merger is evident on the
plane of the sky, from the southeast to the northwest. We study the spatial
variation of the X-ray spectra observed with Suzaku along the merging
direction, centering on the Fe Ly-alpha / Fe He-alpha line ratio to search for
possible deviation from ionization equilibrium. Fitting with a single
temperature collisional non-equilibrium plasma model shows that the electron
temperature increases from the southeast to the northwest. The ionization
parameter is consistent with that in equilibrium (n_et>10^{13} s cm^{-3})
except for a specific region with the highest temperature
(kT=13.3^{+1.4}_{-1.1} keV) where n_et=10^{11.6^{+0.6}_{-1.7}} s cm^{-3}. The
elapsed time from the plasma heating estimated from the ionization parameter is
0.36-76 Myr at the 90% confidence level. This time scale is quite short but
consistent with the traveling time of a shock to pass through that region. We
thus interpret that the non-equilibrium ionization plasma in Abell 754 observed
is a remnant of the shock heating in the merger process. We, however, note that
the X-ray spectrum of the specific region where the non-equilibrium is found
can also be fitted with a collisional ionization plasma model with two
temperatures, low kT=4.2^{+4.2}_{-1.5} keV and very high kT > 19.3 keV. The
very high temperature component is alternatively fitted with a power law model.
Either of these spectral models is interpreted as a consequence of the ongoing
merger process as in the case of that with the non-equilibrium ionization
plasma.
