The galaxy cluster outskirts probed by Chandra
  We studied the physical properties of the intracluster medium in the
virialization region of a sample of 320 clusters ($0.056 <z< 1.24$, $kT>3$ keV)
in the Chandra archive. With the emission measure profiles from this large
sample, the typical gas density, gas slope and gas fraction can be constrained
out to and beyond $R_{200}$. We observe a steepening of the density profiles
beyond $R_{500}$ with $\beta \sim 0.68$ at $R_{500}$ and $\beta \sim 1$ at
$R_{200}$ and beyond. By tracking the direction of the cosmic filaments
approximately with the ICM eccentricity, we report that galaxy clusters deviate
from spherical symmetry, with only small differences between relaxed and
disturbed systems. We also did not find evolution of the gas density with
redshift, confirming its self-similar evolution. The value of the baryon
fraction reaches the cosmic value at $R_{200}$: however, systematics due to
non-thermal pressure support and clumpiness might enhance the measured gas
fraction, leading to an actual deficit of the baryon budget with respect to the
primordial value. This study has important implications for understanding the
ICM physics in the outskirts.
