  We present a catalog of galaxy clusters detected in a new ROSAT PSPC survey.
The survey is optimized to sample, at high redshifts, the mass range
corresponding to T> keV clusters at z=0. Technically, our survey is the
extension of the 160 square degrees survey (Vikhlinin etal 98a, Mullis etal
2003). We use the same detection algorithm, thus preserving high quality of the
resulting sample; the main difference is a significant increase in sky
coverage. The new survey covers 397 square degrees and is based on 1610 high
Galactic latitude ROSAT PSPC pointings, virtually all pointed ROSAT data
suitable for the detection of distant clusters. The search volume for X-ray
luminous clusters within z<1 exceeds that of the entire local Universe (z<0.1).
We detected 287 extended X-ray sources with fluxes f>1.4e-13 erg/s/cm^2 in the
0.5-2 keV energy band, of which 266 (93%) are optically confirmed as galaxy
clusters, groups or individual elliptical galaxies. This paper provides a
description of the input data, the statistical calibration of the survey via
Monte-Carlo simulations, and the catalog of detected clusters. We also compare
the basic results to those from previous, smaller area surveys and find good
agreement for the log N - log S distribution and the local X-ray luminosity
function. Our sample clearly shows a decrease in the number density for the
most luminous clusters at z>0.3. The comparison of our ROSAT-derived fluxes
with the accurate Chandra measurements for a subset of high-redshift clusters
demonstrates the validity of the 400 square degree survey's statistical
calibration.

Astrophysics