Search for a Non-equilibrium Plasma in the Merging Galaxy Cluster Abell 754

Abstract

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.