FREQUENCY DEPENDENCE OF PULSE WIDTH FOR 150 RADIO NORMAL PULSARS

The frequency dependence of the pulse width is studied for 150 normal pulsars, mostly selected from the European Pulsar Network, for which the 10% multifrequency pulse widths can be well fit with the Thorsett relationship W-10 = A nu(mu) + W-10,W-min. The relative fraction of pulse width change between 0.4 GHz and 4.85 GHz, eta = (W-4.85 - W-0.4)/W-0.4, is calculated in terms of the best-fit relationship for each pulsar. It is found that 81 pulsars (54%) have eta < -10% (group A), showing considerable profile narrowing at high frequencies, 40 pulsars (27%) have -10% <= eta <= 10% (group B), meaning a marginal change in pulse width, and 29 pulsars (19%) have eta > 10% (group C), showing a remarkable profile broadening at high frequencies. The fractions of the group-A and group-C pulsars suggest that the profile narrowing phenomenon at high frequencies is more common than the profile broadening phenomenon, but a large fraction of the group-B and group-C pulsars (a total of 46%) is also revealed. The group-C pulsars, together with a portion of group-B pulsars with slight pulse broadening, can hardly be explained using the conventional radius-to-frequency mapping, which only applies to the profile narrowing phenomenon. Based on a recent version of the fan beam model, a type of broadband emission model, we propose that the diverse frequency dependence of pulse width is a consequence of different types of distribution of emission spectra across the emission region. The geometrical effect predicting a link between the emission beam shrinkage and spectrum steepening is tested but disfavored.