  The supernova remnant (SNR) RX J0852-4622 (Vela Jr., G266.6-1.2) is one of
the most important SNRs for investigating the acceleration of multi-TeV
particles and the origin of Galactic cosmic rays because of its strong
synchrotron X-ray and TeV gamma-ray emission, which show a shell-like
morphology similar to each other. Using the XMM-Newton archival data consisting
of multiple pointing observations of the northwestern rim of the remnant, we
investigate the spatial properties of the nonthermal X-ray emission as a
function of distance from an outer shock wave. All X-ray spectra are well
reproduced by an absorbed power-law model above 2 keV. It is found that the
spectra show gradual softening from a photon index 2.56 in the rim region to
2.96 in the interior region. We show that this radial profile can be
interpreted as a gradual decrease of the cutoff energy of the electron spectrum
due to synchrotron cooling. By using a simple spectral evolution model that
includes continuous synchrotron losses, the spectral softening can be
reproduced with the magnetic field strength in the post-shock flow to less than
several tens of uG. If this is a typical magnetic field in the SNR shell,
gamma-ray emission would be accounted for by inverse Compton scattering of
high-energy electrons that also produce the synchrotron X-ray emission. Future
hard X-ray imaging observations with Nustar and ASTRO-H and TeV gamma-ray
observations with the Cherenkov Telescope Array (CTA) will allow to us to
explore other possible explanations of the systematic softening of the X-ray
spectra.
