  We present a systematic study of the density profiles of LCDM halos, focusing
on the outer regions, 0.1 < r/Rvir < 9. We show that the median and mean
profiles of halo samples of a given peak height exhibit significant deviations
from the universal analytic profiles discussed previously in the literature,
such as the Navarro-Frenk-White and Einasto profiles, at radii r > 0.5 R200m.
In particular, at these radii the logarithmic slope of the median density
profiles of massive or rapidly accreting halos steepens more sharply than
predicted. The steepest slope of the profiles occurs at r ~ R200m, and its
absolute value increases with increasing peak height or mass accretion rate,
reaching slopes of -4 and steeper. Importantly, we find that the outermost
density profiles at r > R200m are remarkably self-similar when radii are
rescaled by R200m. This self-similarity indicates that radii defined with
respect to the mean density are preferred for describing the structure and
evolution of the outer profiles. However, the inner density profiles are most
self-similar when radii are rescaled by R200c. We propose a new fitting formula
that describes the median and mean profiles of halo samples selected by their
peak height or mass accretion rate with accuracy < 10% at all radii, redshifts
and masses we studied, r < 9 Rvir, 0 < z < 6 and Mvir > 1.7E10 Msun/h. We
discuss observational signatures of the profile features described above, and
show that the steepening of the outer profile should be detectable in future
weak-lensing analyses of massive clusters. Such observations could be used to
estimate the mass accretion rate of cluster halos.
