The old pulsar PSR B0943+10 (P = 1.1 s, characteristic age τ = 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. During the radio fainter state (Q mode), the X-ray flux is more than a factor of 2 larger than during the B mode and X-ray pulsations with ˜50 per cent 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 hotspot with luminosity of 7 × 1028 erg s-1, 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 per cent 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.
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