  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 clusters. 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.
