Slide 51 of 59
Notes:
In addition, the relativistic motion of the source near the light cylinder may cause relativistic beaming of the radiation. In simple terms, the radiation is concentrate in a beam along the direction of motion, the beam width being approximately 1/gamma where gamma is the Lorentz factor.
For a pulsar there is a further effect due to a time compression by a factor of 2xgamma^2 when the source is travelling towards the observer so that the beam sweeps across the observer in the time given by the relationship shown (P/(2pi) is the time taken to cover 1 radian).
The observed widths of typical individual pulses can be explained with gammas of 2 or 3.