We examine the systematics affecting the X-ray mass estimators applied to a set of five galaxy clusters resolved at high resolution in hydrodynamic simulations , including cooling , star formation and feedback processes .
These simulated objects are 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 .
These clusters have different dynamic state : we consider an hot cluster with temperature T = 11.4 keV , a perturbed cluster with T = 3.9 keV , a merging object with T = 3.6 keV , and two relaxed systems with T = 3.3 keV and T = 2.7 keV , respectively .
These systems are located at z = 0.175 so that their emission fits within the Chandra ACIS-S3 chip between 0.6 and 1.2 R _ { 500 } .

We find that the mass profile obtained via a direct application of the hydrostatic equilibrium equation is dependent upon the measured temperature profile .
An irregular radial distribution of the temperature values , with associated large errors , induces a significant scatter on the reconstructed mass measurements .
At R _ { 2500 } , the actual mass is recovered within 1 \sigma , 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 ( 26 ) and ( 30 ) , and fitting the temperature profile expected from the hydrostatic equilibrium equation to the observed one .
We conclude that the methods of the hydrostatic equilibrium equation and those of the analytic fits 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 the total background 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 .