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Pulsar Observing with the HartRAO 26m Telescope

What is a Pulsar?

Pulsars originate when an old, massive star explodes as a supernova. In this process, the core of the star implodes. The core collapses through the white dwarf stage until its constituent protons and electrons combine to form neutrons. If it is not too massive (less than 1.4 times the mass of the Sun) it can stabilise at this point as a very hot, incredibly dense ball of neutrons with a thin crust of ordinary matter on its surface. Neutron stars are spinning rapidly when they form - up to about 800 rotations per second - and slow down as they age.

Because they are so small, most neutron stars cannot be seen with optical telescopes, even very large ones. However, they have extremely strong magnetic fields. The magnetic field creates a hot-spot above each magnetic pole, which produces an intense beam of radiation that is strongest at radio wavelengths. The magnetic field axis is offset from the rotation (spin) axis. Hence if the pulsar is oriented suitably so that the beam can shine in our direction, as the pulsar spins we detect a pulse of radio waves each time the beam flashes past the radio telescope. The rotating lights on emergency vehicles produces flashes in the same way.


So a pulsar is a star - a very small, unusual star, though, with a diameter of about 20km. Compare this with our Sun, which is an "ordinary" star:


As pulsars age and slow down, their magnetic field weakens and they stop producing detectable beams. Hence all pulsars are neutron stars, but not all neutron stars are pulsars. The slowest detectable pulsars spin once in about ten seconds.

Pulsar observations at HartRAO

Pulsar observations are carried out with a single channel pulsar timing unit. A number of pulsars are monitored at 18 cm and 13 cm wavelength, particularly for detecting glitches - sudden changes in the spin rate, precession of their spin, and timing noise.

New pulsar timers are under construction to replace the old single-channel unit.

For more information pulsar observations, contact Sarah Buchner or Dr. Claire Flanagan

A workshop on "Pulsar Studies in Africa" was held at HartRAO in 2001 December.

Observing the Vela pulsar

The Vela pulsar (PSR B0833-45) has been monitored daily at HartRAO since the 1980's. Real-time glitch detection software has enabled us to capture the behaviour of the pulsar from almost immediately after several of its glitches.

This is what we observe from the Vela pulsar with the radio telescope at Hartebeesthoek. These pulses occur roughly every tenth of a second.

Vela pulsar glitch recovery
Image courtesy of Claire Flanagan

The changing rotation rate of the Vela pulsar over ten years is shown above. The glitches are the sudden decreases in the rotation rate. These occur every few years in this pulsar. These glitches are followed by a recovery which usually exhibits two or three exponential recovery rates, the fastest measured being of the order of one day, and the slowest, months.

The most recent glitch occured on 2006 August 12:
"C. S. Flanagan, Johannesburg Planetarium; and S. J. Buchner, Hartebeesthoek Radio Astronomy Observatory (HRAO), report that an increase in the rotation frequency of PSR 0833-45 (cf. IAUC 8730) of magnitude [Delta(nu)]/nu = +2.62 x 10^{-6} has been observed from the HRAO, with the "glitch" occurring on Aug. 12.93 UT. Ten hours of observations, commencing six hours after the event, indicate an accompanying fractional increase in the spin-down rate of +0.23 +/- 0.04."
This glitch came just over two years after the previous glitch, on 2004 July 7.


Publications on pulsars that involve HartRAO are listed here.


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