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Issue 4.05

The Astrophysics Spectator

March 21, 2007

This week a page is added that discusses how the young radio pulsars are distributed in the Milky Way Galaxy. While this page is starting its life as part of the “Stars” topical path, it will eventually be part of a planned topical path on the structure of the Milky Way Galaxy.

The Galactic disk we see in the sky as the Milky Way is very narrow, of order 100 parsecs in width, compared to its radius of about 15 kiloparsecs. The stars in this disk orbit the Galactic center at an average velocity of about 230 km s-1. Individual stars have velocities that deviate from this average by only of order 10 km s-1, which is why the Galactic disk is so narrow; the velocity of a star perpendicular to the Galactic plane is too small to escape the gravitational pull of the galactic plane on the star. The stars in the Galactic disk oscillate above and below the Galactic plane in the Galactic disk.

The young radio pulsars have an entirely different distribution within the Milky Way Galaxy. They are not confined to the thin Galactic disk. Instead they constitute a very fat disk that extends 1 kiloparsec above and below the galactic plane. Unlike the small velocities perpendicular to the Galactic plane possessed by the stars in the Galactic disk, the radio pulsars have tremendous velocities perpendicular to the Galactic disk. These velocities are generally a couple of hundred kilometers per second, so the radio pulsars are not bound to the Galactic disk. The pulsars are also moving away from the Galactic plane, so we are seeing them shortly after their birth within the Galactic disk. The distance above the Galactic plane along with the velocity perpendicular to the Galactic plane give the best measure of a radio pulsar's age, which is generally no more than 10 million years. By 10 million years, the pulsar is shutting down its radio emission, becoming invisible to us as it travels 1 kiloparsec above or below the Galactic plane. This implies that the disk of radio pulsars reflects only a small fraction of the volume occupied by neutron star; we simply cannot seem the vast majority of the neutron stars in our Galaxy. The distribution of radio pulsars within the Galaxy is a direct reflects the dynamics of supernovae and the physics of radio emission from a pulsar.

Next Issue: The next issue of The Astrophysics Spectator is scheduled for April 4.

Jim Brainerd


Young Radio Pulsars within the Milky Way. The distance to a radio pulsar within our Galaxy can be derived by measuring how the pulsar's radio pulses are delayed as they travel through the hot interstellar gas. The pulses seen at low radio frequencies travel more slowly to Earth than those seen at high radio frequencies. With a model for the distribution of hot gas in the Galaxy, the distance to a pulsar can be derived by comparing difference in arrival of pulses at two different frequencies. The picture that emerges is of radio pulsars traveling at very high velocity away from the Galactic plane. By measuring the distance traveled by a pulsar since its birth in the Galactic disk, the age of the pulsar can be derived. Such measurements show that the radio pulsars are no more than 10 million years old. This kinetic age for a pulsar is often much less than the age implied by the slowing of a pulsar's spin. These results imply that the magnetic field of a pulsar decreases over time, and the mechanism responsible for the pulsar's radio emission shuts off after 10 million years. (continue)

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