  We report on Chandra observations of the black widow pulsar, PSR B1957+20.
Evidence for a binary-phase dependence of the X-ray emission from the pulsar is
found with a deep observation. The binary-phase resolved spectral analysis
reveals non-thermal X-ray emission of PSR B1957+20, confirming the results of
previous studies. This suggests that the X-rays are mostly due to intra-binary
shock emission which is strongest when the pulsar wind interacts with the
ablated material from the companion star. The geometry of the peak emission is
determined in our study. The marginal softening of the spectrum of the
non-thermal X-ray tail may indicate that particles injected at the termination
shock is dominated by synchrotron cooling.
