Pulsars

In 1967, PhD student Jocelyn Bell discovered an unusual radio signal, one containing a pulse that repeated with very high precision every one-and-a-third seconds. The subsequent discovery of similar signals, two of which came from supernova remnants, convinced astronomers that the signals were coming from rapidly rotating neutron stars.

Neutron stars are incredibly dense stars, created in the collapse of the cores of massive stars that have ended their stellar lives as supernovas. They have diameters on the order of ten kilometres and their rotational periods range from tens of seconds to thousandths of a second.

A neutron star is highly magnetized and its intense magnetic field focuses beams of radiation in the direction of the magnetic poles. When the magnetic poles and the axis of rotation are not aligned, the beams sweep through space like the beams of a lighthouse. When a beam points at us, we see the star as a steadily repeating, pulsating signal: a pulsar.

Pulsars act as incredibly accurate timing devices and, as such, have proven to be valuable tools of astronomical discovery.

In 1974, observations of a pulsar and neutron-star in orbit around each other revealed that the stars’ orbits were decaying as they lost energy. Einstein’s General Theory of Relativity predicted that such decay was due to the loss of energy through the propagation of gravitational waves and the discovery is considered the first evidence of the existence of those ripples in spacetime.

In 1992, astronomers made the first detection of exoplanets when they analyzed variations in the period of a pulsar and saw that the variations could be explained by the presence of three planets in orbit around the pulsar.

Today, U of T astronomers are studying pulsars using Very Long Baseline Interferometry (VLBI) which combines observations from radio telescopes located around the world to increase resolving power. This in turn is combined with a technique called scintillometry which mixes the signals from distant pulsars that have been scattered by the Interstellar Medium (ISM) and taken different paths to reach us. Together, these techniques result in observations with the spatial resolution of a radio telescope with a one-astronomical-unit diameter dish.

As well, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) will be one of many instruments comprising a Pulsar Timing Array, a program that will monitor dozens of pulsars closely over many years. In addition to providing insight into the nature of these cosmic lighthouses, the work will help in the detection of the extremely low-frequency background of gravitational waves from the mergers of supermassive galactic black holes.

At the Dunlap:

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In an artist’s impression, the pulsar PSR B1957+20 is seen in the background through a cloud of gas enveloping its brown dwarf star companion. Image: Dr. Mark A. Garlick; Dunlap Institute for Astronomy & Astrophysics