Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.9908m&link_type=abstract
American Astronomical Society, 181st AAS Meeting, #99.08; Bulletin of the American Astronomical Society, Vol. 24, p.1276
Astronomy and Astrophysics
Astronomy
Scientific paper
Contrary to what one might expect for the random orientation of pulsar magnetic and rotation axes, recent measurements of pulsar inclination angles show that the distribution of inclination angles is skewed towards moderate values with a peak near 30deg . Since these measurements assumed pulsar beams are circular in cross section, the contradiction may have originated in the failure to account for the quasi-elliptical geometry of the pulsar polar cap. An attempt is made to compensate existing data for the polar cap ellipticity, and the resulting distribution of inclination angles, alpha , is consistent with the expected function, sin alpha . The braking torque which causes pulsar periods to increase with time may be due to vacuum dipole radiation or plasma currents flowing along open magnetic field lines. In addition to slowing pulsar rotation, the vacuum-wave torque aligns the pulsar magnetic and rotation axes while the plasma-current torque causes the axes to counteralign. The hypotheses of alignment and counteralignment of the pulsar rotation and magnetic axes predict distinctly different analytical relationships between a pulsar's period, P, and its inclination angle. Consequently, the period dependence of the ellipse minor axis angular width varies as the pulsar inclination angle evolves. If measured pulsar beam widths are affected by the polar cap ellipse axial ratio, the beam width-period dependence of aligned and orthogonal rotators may be very different, and can be used to test the hypotheses. The width-period dependences determined from measured pulsar periods and inclination angles imply that neither pure alignment nor pure counteralignment occur. The widths vary as P(-1/2) , suggesting that the limiting corotation radius of pulsar magnetospheres varies linearly with pulsar period.
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