  The old pulsar PSR B0943+10 (P=1.1 s, characteristic age tau=5 Myr) is the
best example of mode-switching radio pulsar. Its radio emission alternates
between a highly organized state with regular drifting subpulses (B mode) and a
chaotic emission pattern (Q mode). We present the results of XMM-Newton
observations showing that the X-ray properties of PSR B0943+10 depend on its
radio state (Hermsen et al. 2013). During the radio fainter state (Q mode) the
X-ray flux is more than a factor two larger than during the B-mode and X-ray
pulsations with about 50% pulsed fraction are detected. The X-ray emission of
PSR B0943+10 in the B-mode is well described by thermal emission with blackbody
temperature kT=0.26 keV coming from a small hot spot with luminosity of 7x10^28
erg/s, in good agreement with the prediction of the partially screened gap
model, which also explains the properties of the radio emission in this mode.
We derived an upper limit of 46% on the X-ray pulsed fraction in the B-mode,
consistent with the geometry and viewing angle of PSR B0943+10 inferred from
the radio data. The higher flux observed during the Q-mode is consistent with
the appearance of an additional component with a power-law spectrum with photon
index 2.2. We interpret it as pulsed non-thermal X-rays produced in the star
magnetosphere. A small change in the beaming pattern or in the efficiency of
acceleration of the particles responsible for the non-thermal emission can
explain the reduced flux of this component during the radio B-mode.
