The separation \Delta ^ { peak } between two peaks in the gamma-ray pulse profile is calculated as a function of energy for several polar cap models with curvature-radiation-induced cascades .
The Monte Carlo results are interpreted with the help of analytical approximations and discussed in view of the recent data analysis for the Vela pulsar [ 1999 ] .
We find that the behaviour of \Delta ^ { peak } as a function of photon energy \varepsilon depends primarily on local values of the magnetic field , B _ { local } , in the region where electromagnetic cascades develop .
For low values of B _ { local } ( < 10 ^ { 12 } G ) , \Delta ^ { peak } ( \varepsilon ) is kept constant .
However , for stronger magnetic fields ( \ga 10 ^ { 12 } G ) in the hollow-column model \Delta ^ { peak } decreases with increasing photon energy at a rate dependent on maximum energy of beam particles as well as on viewing geometry .
There exists a critical photon energy \varepsilon _ { turn } above which the relation \Delta ^ { peak } ( \varepsilon ) changes drastically : for \varepsilon > \varepsilon _ { turn } , in hollow-column models the separation \Delta ^ { peak } increases ( whereas in filled-column model it decreases ) rapidly with increasing \varepsilon , at a rate of \sim 0.28 of the total phase per decade of photon energy .
The existence of critical energy \varepsilon _ { turn } is a direct consequence of one-photon magnetic absorption effects .
In general , \varepsilon _ { turn } is located close to the high-energy cutoff of the spectrum , thus photon statistics at \varepsilon _ { turn } should be very low .
That will make difficult to verify an existence of \varepsilon _ { turn } in real gamma-ray pulsars .
Spectral properties of the Vela pulsar would favour those models which use low values of magnetic field in the emission region ( B _ { local } \mathrel { \hbox { \raise 2.15 pt \hbox { $ < $ } \hbox to 0.0 pt { \lower 2.15 % pt \hbox { $ \sim$ } } } } 10 ^ { 11 } G ) which in turn implies a constant value of the predicted \Delta ^ { peak } within EGRET range .