Robin McGary (Furman University)

PROPER MOTIONS OF HIGH VELOCITY PULSARS

The focus of this project has been to determine the proper motions of a number of high velocity pulsars. A precise measure of the proper motions is necessary to associate the pulsars with supernova remnants. It is difficult to associate pulsars with supernova remnants for many reasons. The high velocity of pulsars causes them to quickly leave the area in which they were born. In addition pulsars have lifetimes on the order of 10^6 years while the supernova remnants are visible for only 10^4 years. This accounts for the fact that there are ~600 known pulsars today to only ~200 known supernova remnants. An accurate determination of the proper motions of these pulsars would improve our estimations of the locations of their birthplaces.

It has also been conjectured by Narayan and Ostriker (1990) that there are two different populations of pulsars in the Galaxy, one born near the plane and a second born high above the Galactic plane with high velocities. This would account for the observations by Harrison, Lyne and Anderson (1993) of four pulsars with velocities towards the Galactic plane. Some of the pulsars are on the order of 1Myr old and thus are too young to have completed 1/4 of an oscillation about the galactic plane. In addition, their velocities of up to 300 km/s approach the escape velocity of the Galaxy. Many opinions exist on the origin of these pulsars including theories that the Galaxy is more massive than models predict or that the distances to these pulsars have been miscalculated. However, the most popular theory is that these pulsars are actually born high above the Galactic plane from "runaway" OB stars (Lyne and Lorimer 1993).

Recently, new calculations by Cordes and Chernoff (in press) contend that the apparent velocities of the pulsars towards the plane are the result of projection effects. They conclude that all pulsars may have been born at a height of less than 0.3 kpc above the Galactic plane. By calculating a precise proper motion for a number of pulsars that are high above the galactic plane we hope to produce some insight on the origin of these pulsars and their velocity perpendicular to the Galactic plane.

The data used in this experiment was taken at three epochs in 1992.98, 1994.24, and 1995.52 by Andy Fruchter, Steve Thorsett and Miller Goss. A total of 31 pulsars were observed with 13 of the pulsars being observed at two epochs. All 13 pulsars were observed in 1992.98 with 12 having a second epoch in 1995.52. Thus, the resulting baseline for proper motion determination is 2.54yr for these pulsars. Only B1540-06 had its second epoch in 1994.24 resulting in a baseline of 1.26yr.

Observations were made in the L-Band in the A array at the VLA in Socorro, NM. The A array was chosen because Galactic sources in the field would be resolved and thus could be eliminated from the reference source list. Reference sources would be QSO's and distant radio galaxies that were unresolved by the VLA in this configuration. The pulsar would also remain unresolved because its angular size is determined by scatter broadening on the mas level. In the L-Band, 16 channels of 1.56 MHz were used. This relatively narrow bandwidth was chosen to produce a large delay beam of ~30 arc min. The large delay beam resulted in an average of 3 reference sources in the field for each pulsar. Reference sources were chosen from within the delay beam in order to cancel out many of the errors due to the troposphere. Since a relative ofsett between two epochs as large as .1 ec can result from errors due to the troposphere, it is advantageous to use sources in the delay beam as reference soureces instead of the calibrator which can be tens of degrees away (Fomalont etal 1992). The proper motion of the pulsar relative to these in-field reference sources will eliminate this effect and result in accuracies on a 10 mas level.

The data was taken at the VLA using a "matched filter" in the RR polarization. The pulsar was detected by the Princeton Mark III timing machine. Then the correlator was gated with the correct phase so that it only gathered data when the pulsar was "on." This method increased the signal to noise ratio on average by a factor of 6. The increased signal to noise that we have in our data will enable us to accurately determine pulsars that are as weak as 2 mJy. Thus this method will be useful for looking at the pulsars high above the galactic plane. The LL polarization was ungated so that good signals could be obtained for the reference sources. Positions between the RR and LL polarizations do agree the 5mas level.

Calibrations were performed on the CH_0 data and then applied to the LINE data. Pulsar positions are obtained from the RR LINE data. Wide field images are then made of the LL LINE data in order to detect reference source candidates. Once all sources in the field have been detected, they are re-imaged using subfields in IMAGR. Sources that remain unresolved in these final images are used as reference sources. For those sources that were not gated in our data, we use stokes "I" for the positions of the pulsar and the reference sources.

Since the different epochs were at different times of the year, some additional corrections were made to the data. A correction for the annual aberration of the Earth was made using UVFIX. This error is a result of the approximations made when AIPS calculated the u, v, and w positions of sources. UVFIX recalculates the positions taking into account annual aberration. In addition effects due to Lorentz contraction were removed using a Fortran program by Ed Fomalont.

The difference in RA and DEC between a reference source and the pulsar was then calculated for each epoch. By looking at how these differences changed between the two epochs we were able to determine a relative proper motion. By repeating this procedure with a number of reference sources we were able to test our methods and produce a more reliable estimate of the proper motion of the pulsar. We are initially looking at pulsars with proper motions previously published by Fomalont etal (1996) and/or Taylor, Manchester & Lyne (1993). At this point it appears that we are getting reasonable agreement with their results.

I will be installing AIPS at Furman in the fall in order to complete the work on these 13 pulsars. I will also be looking at images in which the calibrator was offset from the field center in one direction. These images will be used to test for any second order effects that may have effected the positions of our reference sources that were far from the field center. This effect will ultimately be accounted for in our proper motion calculations.

We are also applying for additional time on the VLA to gather data on a third epoch for these 13 pulsars as well as get a second epoch for the remaining 18 pulsars in our set. A third epoch will greatly reduce the errors in our proper motion calculations. I will be returning to Socorro for the month of February to complete any remaining work on this data as well as prepare for our next run. I will be presenting a poster at the AAS meeting in January discussing the accomplishments of my project so far.