It is widely held that pulsars are born in the Galactic plane from massive Population I stars ( ~solar masses ) and that their large scale heights can be attributed to a birth velocity of a few hundred km/s. Proper motion measurements are the observational support for the inference that pulsars are a high velocity population born in the plane. Such measurements, when used together with the distance and age of a pulsar, give the magnitude and direction of the transverse velocity of the pulsar. Not only does this demonstrate that the majority of pulsars are migrating out of the plane, but in some individual cases it can be used to trace the pulsar back to its origin, be that an OB association and/or a supernova remnant.
The distribution of pulsar velocities is another important area of research. Important questions can be answered on the origin of these velocities (binary disruption and/or random ``kicks") simply by knowing the true shape of the distribution. To date, proper motion measurements have been made for less than 15% of the known pulsars, producing a sample which is seriously limited by selection effects (i.e., bright, nearby pulsars). Indeed, attempts to correct for these selection effects have suggested that the mean pulsar velocity may be larger than was commonly thought by a factor of 2-3.
By necessity, astrometric observations have been made at centimeter wavelengths to avoid ionospheric effects. However, as pulsars are steep spectrum () objects, observations at 10-20cm require that we gate the correlator on pulse for increased signal-to-noise ratio. The current small sample of pulsars with proper motion measurements reflects the twin limitations of sensitivity and positional accuracy. The current VLA yields superior sensitivity but is limited in positional accuracy ( milli-arcsec/yr). Conversely, the VLBA has excellent positional accuracy but is limited to phase reference observations on the brighter pulsars (>5-10 mJy). The enhanced VLA, particularly the A+ configuration with the wider bandwidth 1.4 GHz system and the new 2.4 GHz system, will greatly increase the number of objects for which we can do accurate astrometry.
The VLA Development Plan benefits pulsar studies in two ways: through improved signal-to-noise ratio and positional accuracy. All IF improvements (receiver, fiber optics and correlator) provide a signal-to-noise increase. The 2.4 GHz receiver and the increased bandwidth of the 1.4 GHz receiver gain a factor of 2 in signal-to-noise. However, the greatest improvements come from the A+ configuration. In the best case we should improve positional accuracy by 20 at 1.4 GHz and at 2.4 GHz. To achieve these improvements for the rapidly rotating and high-dispersion-measure pulsars will require that the new correlator have a frequency dependent gate so that the pulse phase can be accurately tracked across the full available bandwidth.
With the current VLA we can observe the 40 closest and brightest of 700 known pulsars. With the proposed improvements to VLA sensitivity and the A+ configuration we can do sub-milli-arc-second astrometry for mJy pulsars, with which a significant fraction (>30%) of the known pulsar population can be studied. The expected improvements, normalized to our current capabilities with the current VLA at 20 cm, are illustrated graphically in Fig. 3.5. We estimate that the number of pulsars with proper motion measurements would double, and that a trigonometric parallax could be measured for pulsars out to a distance of 1 kpc. Serious investigations of pulsar velocities could begin, unhampered by selection effects.
Figure 3.5: A comparison of the relative capabilities of several VLA configurations for pulsar astrometry. Shown are the current VLA and VLA+VLBA capabilities in the 1.4 GHz band, with and without the gate, as well the predicted capabilities of the enhanced VLA, with and without the A+ configuration. The two axes are gain in sensitivity and gain in positional accuracy, and the numbers are gains relative to the current VLA without gating. Displayed above the plot are the number of pulsars available to each configuration.