The merger history of clusters and its effect on the X-ray properties of the intracluster medium

We investigate the growth over time of 20 massive (>3 keV) clusters in a hydrodynamical simulation of the ? cold dark matter cosmology with radiative cooling. The clusters show a variety of formation histories: some accrete most of their mass in major mergers; others, more gradually. During major mergers, the long-term (temporally smoothed) luminosity increases such that the cluster moves approximately along the LX–TX relation; between times, it slowly decreases, tracking the drift of the LX–TX relation. We identify several different kinds of short-term luminosity and temperature fluctuations associated with major mergers including double-peaked mergers, in which the global intracluster medium merges first (LX and TX increase together) and then the cluster cores merge (LX increases and TX decreases). At both luminosity peaks, clusters tend to appear to be spherical and relaxed, which may lead to biases in high-redshift, flux-limited samples. There is no simple relationship between scatter in the LX–TX relation nor in the recent or overall merger activity or cluster formation time. The scatter in the LX–M and TX–M relations is reduced if properties are measured within R500 rather than Rvir.