  Radio pulsar subpulse drifting has been interpreted as rotation of sub-beams
(sparks) of pair plasma produced by intermittent breakdowns of an inner vacuum
gap above the pulsar polar cap. This model also predicts strong thermal X-ray
emission from the polar cap caused by inflowing particles created in spark
discharges. We have observed the best-studied drifting pulsar B0943+10 with
XMM-Newton and detected a point source coincident with the radio pulsar
position. Its spectrum could be fitted with a thermal blackbody model, although
a power-law model is also acceptable. The thermal fit gives a bolometric
luminosity L_bol ~ 5 x 10^{28} erg/s and a surface area A ~ 10^3 (T/3MK)^{-4}
m^2, much smaller than the conventional polar cap area, 6 x 10^4 m^2. Such
thermal radiation can be interpreted as emitted from footprints of sparks
drifting in an inner gap of a height h ~ 0.1 - 0.2 r_pc, where r_pc is the
polar cap radius. However, the original vacuum gap model by Ruderman and
Sutherland requires some modification to reconcile the X-ray and radio data.
