ROSAT PSPC observations of the outer regions of the Perseus cluster of galaxies

We present an analysis of four off-axis ROSAT Position Sensitive Proportional Counter (PSPC) observations of the Perseus cluster of galaxies (Abell 426). We detect the surface brightness profile to a radius of 80 arcmin (similar to 2.4 h(50)(-1) Mpc) from the X-ray peak. The profile is measured in various sectors and in three different energy bands. First, a colour analysis highlights a slight variation of N-H over the region, and cool components in the core and in the eastern sector. We apply the beta-model to the profiles from different sectors and present a solution to the, so-called, beta-problem. The residuals from an azimuthally-averaged profile highlight extended emission both in the east and in the west, With estimated luminosities of about 8 and 1 x 10(43) erg s(-1), respectively. We fit several models to the surface brightness profile, including the one obtained from the Navarro, Frenk & White potential. We obtain the best fit with the gas distribution described by a power law in the inner, cooling region and a beta-model for the extended emission. Through the best-fitting results and the constraints from the deprojection of the surface brightness profiles, we define the radius where the overdensity inside the cluster is 200 times the critical value, r(200), at 2.7 h(50)(-1) Mpc. Within 2.3 h(50)(-1) Mpc (085r(200)), the total mass in the Perseus cluster is 1.2 x 10(15) M-. and its gas fraction is about 30 per cent.