One of the key parameters necessary to help explain the geometry and evolution of radio pulsars is the angle between the magnetic axis and the rotation axis, alpha . Unfortunately, the value of this parameter cannot be read directly from observations, but must be derived. There are two different techniques for finding alpha : (1) through empirical - geometrical relationships derived statistically from the pulsar population and then applied to the particular pulsar, and (2) via fits to the swing of the position angle of polarization as the pulsar rotates. Two different techniques have been proposed to calculate alpha using empirical - geometrical relationships. Lyne & Manchester (1988) and Rankin (1990) found (different) relationships between the intrinsic beamwidth and period, and then derived approximate relations between the observed pulse width, period, and alpha . These methods are each based on different assumptions about the underlying pulse mechanisms, and find similar values of alpha for some pulsars, but disagree greatly for others. Other researchers have attempted to use another method altogether, which is based on measuring the rotation of the polarized position angle as the emission beam swings across the observer. It is then possible to fit a geometrical rotating-vector position angle model (Radhakrishnan and Cooke 1969) to the data in order to find alpha and also the magnetic pole - observer impact parameter beta . Again, there is not always agreement among the determinations of different investigators. We have done our own fits to Arecibo 1418 MHz polarized profiles in order to derive these geometrical parameters and to study their sensitivity to the data, to the assumptions employed, and to the subtleties of the fitting process. We compare the various methods, their assumptions, and their results, and we assess their strengths and weaknesses.
American Astronomical Society Meeting Abstracts
- Pub Date:
- December 1995