  The source 4U 1901+03 is a high-mass X-ray pulsar than went into outburst in
2003. Observation performed with the Rossi X-ray Timing Explorer showed
spectral and timing variability, including the detection of flares,
quasi-periodic oscillations, complex changes in the pulse profiles, and pulse
phase dependent spectral variability. We re-analysed the data covering the 2003
X-ray outburst and focused on several aspects of the variability that have not
been discussed so far. These are the 10 keV feature and the X-ray spectral
states and their association with accretion regimes, including the transit to
the propeller state at the end of the outburst. We find that 4U 1901+03 went
through three accretion regimes over the course of the X-ray outburst. At the
peak of the outburst and for a very short time, the X-ray flux may have
overcome the critical limit that marks the formation of a radiative shock at a
certain distance above the neutron star surface. Most of the time, however, the
source is in the subcritical regime. Only at the end of the outburst, when the
luminosity decreased below ~10^{36} (d/10 kpc)^2 erg/s, did the source enter
the propeller regime. Evidence for the existence of these regimes comes from
the pulse profiles, the shape of the hardness-intensity diagram, and the
correlation of various spectral parameters with the flux. The 10 keV feature
appears to strongly depend on the X-ray flux and on the pulse phase, which
opens the possibility to interpret this feature as a cyclotron line.
