We report on XMM-Newton observations of the “ Black Widow pulsar ” , PSR B1957+20 .
The pulsar ’ s X-ray emission is non-thermal and best modeled with a single powerlaw spectrum of photon index 2.03 ^ { +0.51 } _ { -0.36 } .
No coherent X-ray pulsations at the pulsar ’ s spin-period could be detected , though a strong binary-phase dependence of the X-ray flux is observed for the first time .
The data suggest that the majority of the pulsar ’ s X-radiation is emitted from a small part of the binary orbit only .
We identified this part as being near to where the radio eclipse takes place .
This could mean that the X-rays from PSR B1957+20 are mostly due to intra-shock emission which is strongest when the pulsar wind interacts with the ablated material from the companion star .