  We examine the systematics affecting the X-ray mass estimators applied to a
set of five simulated galaxy clusters. They have been processed through the
X-ray Map Simulator, X-MAS, to provide Chandra-like long exposures that are
analyzed to reconstruct the gas temperature, density, and mass profiles used as
input. We find that at R_2500 the mass profile obtained via a direct
application of the hydrostatic equilibrium equation is consistent within 1
sigma with the actual mass; although we notice this estimator shows high
statistical errors due to high level of Chandra background. Instead, the
poorness of the beta-model in describing the gas density profile makes the
evaluated masses to be underestimated by \sim 40 per cent with respect to the
true mass, both with an isothermal and a polytropic temperature profile. We
also test ways to recover the mass by adopting an analytic mass model, such as
those proposed by Navarro et al. (1997) and Rasia et al. (2004), and fitting
the temperature profile expected from the hydrostatic equilibrium equation to
the observed one. These methods and the one of the hydrostatic equilibrium
equation provide a more robust mass estimation than the ones based on the
beta-model. In the present work the main limitation for a precise mass
reconstruction is to ascribe to the relatively high level of the background
chosen to reproduce the Chandra one. After artificially reducing it by a factor
of 100, we find that the estimated mass significantly underestimates the true
mass profiles. This is manly due (i) to the neglected contribution of the gas
bulk motions to the total energy budget and (ii) to the bias towards lower
values of the X-ray temperature measurements because of the complex thermal
structure of the emitting plasma.
