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Search for a non-equilibrium plasma in the merging galaxy cluster Abell 754

Author:
Inoue, Shota  Hayashida, Kiyoshi  Ueda, Shutaro  Nagino, Ryo  Tsunemi, Hiroshi  Koyama, Katsuji  


Journal:
PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF JAPAN


Issue Date:
2016


Abstract(summary):

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(e)t > 10(13) s cm(-3)) except for the specific region with the highest temperature (kT =3D 13.3(-1.1)(+1.4) keV) where n(e)t =3D 10(-1.7)(11.6)(+0.6) 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 timescale 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. However, we 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 =3D 4.2(-1.5)(+4.2) 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 the non-equilibrium ionization plasma.


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