Radial Profile and Lognormal Fluctuations of the Intracluster Medium as the Origin of Systematic Bias in Spectroscopic Temperature

The origin of the recently reported systematic bias in the spectroscopic temperature of galaxy clusters is investigated using a cosmological hydrodynamic simulation. We find that the local inhomogeneities of the gas temperature and density, after being corrected for their global radial profiles, have a nearly universal distribution that resembles a lognormal function. Based on this lognormal approximation for the fluctuations in the intracluster medium, we develop an analytical model that explains the bias in the spectroscopic temperature. We conclude that the multiphase nature of the intracluster medium, due not only to the radial profiles but also to the local inhomogeneities, plays an essential role in producing the systematic bias.