  Transitional pulsars provide us with a unique laboratory to study the physics
of accretion onto a magnetic neutron star. PSR J1023+0038 (J1023) is the best
studied of this class. We investigate the X-ray spectral properties of J1023 in
the framework of a working radio pulsar during the active state. We modelled
the X-ray spectra in three modes (low, high, and flare) as well as in
quiescence, to constrain the emission mechanism and source parameters. The
emission model, formed by an assumed pulsar emission (thermal and
magnetospheric) plus a shock component, can account for the data only adding a
hot dense absorber covering ~30% of the emitting source in high mode. The
covering fraction is similar in flaring mode, thus excluding total enshrouding,
and decreases in the low mode despite large uncertainties. This provides
support to the recently advanced idea of a mini-pulsar wind nebula (PWN), where
X-ray and optical pulsations arise via synchrotron shock emission in a very
close (~100 km, comparable to a light cylinder), PWN-like region that is
associated with this hot absorber. In low mode, this region may expand,
pulsations become undetectable, and the covering fraction decreases.
