Using a hydrodynamics plus N-body simulation of galaxy cluster formation within a large volume and mock Chandra X-ray observations, we study the form and evolution of the intrinsic scatter about the best-fit X-ray temperature-mass relation for cluste
rs. We investigate the physical origin of the scatter by correlating it with quantities that are closely related to clusters formation and merging histories. We also examine the distribution of the scatter for merging and nonmerging populations, identified using halo merger trees derived from the simulation as well as X-ray substructure measures. We find a strong correlation between the scatter in the M-T_X relation and the halo concentration, in the sense that more concentrated clusters tend to be cooler than clusters with similar masses. No bias is found between the merging and relaxed clusters, but merging clusters generally have greater scatter, which is related to the properties of the distribution of halo concentrations. We also detect a signature of non-lognormality in the distribution of scatter for our simulated clusters both at z=0 and at z=1. A detailed comparison of merging clusters identified by substructure measures and by halo merger trees is given in the discussion. We conclude that, when cooling-related effects are neglected, the variation in halo concentrations is a more important factor for driving the intrinsic scatter in the M-T_X relation, while departures from hydrostatic equilibrium due to cluster mergers have a minor effect.
