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Pulsars, Black Holes and Gravitational Waves

A pulsar is the remnant core of a star that has undergone a supernova explosion.

A pulsar is the remnant core of a star that has undergone a supernova explosion. The pulsar’s extremely strong magnetic field accelerates particles to almost the speed of light. As these relativistic particles stream away from the pulsar, they produce focused beams of radio emission. Astronomers study these objects with the aim of discovering ripples in space and time known as gravitational waves. Credit: Swinburne Astronomy Productions

By George Hobbs

The SKA will be able to study thousands of pulsars in sufficient detail to detect gravity waves.

In 1915 Albert Einstein published his general theory of relativity, which states that massive objects such as black holes cause a distortion in space–time that is felt as gravity. If such objects accelerate, such as two black holes orbiting each other, then the space–time warping propagates outwards as a gravitational wave.

How can we prove or disprove Einstein’s theory, and how can we detect those elusive gravitational waves? The answer comes from some intriguing objects known as pulsars.

Pulsars really are extreme. They have more mass than our Sun, but compressed into a star about the size of Sydney.

Pulsars have huge magnetic fields – about a million times stronger than the most powerful magnetic field ever achieved in a laboratory on Earth – and the north and south poles of this magnetic field accelerate particles to almost the speed of light. These particles emit narrow beams of radiation as they stream away from the pulsar.

Some pulsars spin so fast that their surface is moving close to the speed of light. As the pulsar rotates, its radiation beams sweep across our line of sight and are detected by radio telescopes as a series of pulses. For this reason, pulsars are often described as celestial lighthouses.

Pulsars can be studied for their own unique and exciting properties. However, since they produce a radio pulse every time they rotate, they can be used by astronomers as precise cosmic clocks.

In 1993, Russell Hulse and Joseph Taylor won the Nobel Prize for their discovery and analysis of the first pulsar in a binary system. The pulsar was orbiting another compact object, and they had therefore identified a celestial clock moving in a strong gravitational field – a perfect test for Einstein’s theory of relativity. They managed to confirm that all of their observations were in agreement with Einstein’s predictions, and noted that the orbit was losing energy at a rate that was consistent with the emission of gravitational waves. However, they still had not directly detected the gravitational waves.

Astronomers are currently trying to detect the gravitational waves passing the Earth by observing an array of pulsars distributed across the sky. As the waves pass, the pulses from each pulsar will seem to arrive slightly early or slightly late compared with their normal arrival times. Unfortunately, the expected deviation is tiny – less than 100 nanoseconds – so we need the largest and most sensitive radio telescopes to do this work.

We are currently combining data from CSIRO’s radio telescope in Parkes with observations from other telescopes around the world, yet we still have not detected the waves. Luckily the astronomy community is preparing to build even more sensitive telescopes.

The Square Kilometre Array (SKA) is an ideal pulsar telescope. With its huge collecting area and its location in the Southern Hemisphere, it will be able to first discover and then study the majority of the observable pulsars in the galaxy. We currently know of ~2500 pulsars, but with the SKA we should be able to study tens of thousands of them – and even pulsars in nearby galaxies.

A key project for the SKA is entitled “Strong Field Tests of Gravity Using Pulsars and Black Holes”. Its major goals are to:

  • Discover a pulsar orbiting a black hole. Currently we know of pulsars orbiting white dwarfs, neutron stars and main sequence stars. A holy grail of astronomy would be to find a pulsar–black hole binary system. When observed with a telescope as sensitive as the SKA, such a system would provide a unique laboratory for black hole research. It would allow us to test a prediction of general relativity that all black holes can be characterised by only three externally observable parameters: mass, electric charge and angular momentum.
  • Test theories of gravitation. Einstein’s theory is not the only game in town – there are other theories of space–time that deviate from Einstein’s predictions. Some have already been studied using known pulsar binary systems, but finding more extreme pulsar binary systems with the SKA will allow more stringent tests to be carried out on all theories of gravity.
  • Detect and study gravitational waves. The SKA telescope will enable astronomers to carry out regular, high-precision observations of many hundreds of pulsars over many years. These observations will be significantly more sensitive to gravitational waves than our existing observations, and it is extremely likely that the SKA will make a detection within a few years of observations. Over time these observations will allow us to carry out gravitational wave astronomy. Instead of simply noting that gravitational waves exist, we will be able to use their properties to pin-point supermassive binary black holes, understand the merger rate of galaxies in the distant past and perhaps even probe the universe less than a second after the Big Bang.

Part of the SKA will be built in South Africa, and consists of the primary set of antennas used for the work described here. However, there’s a problem.

The signals that we receive from the pulsar travel through the tenuous plasma of the interstellar medium, which causes unpredictable delays in the pulse’s arrival times. These delays are significantly greater than the delays expected from the gravitational waves, so they need to be measured and then removed.

Our work with current telescopes has shown us that this cannot be done using only the observing system proposed for the South African part of the SKA. We need lower frequency observations.

Therefore the same pulsars will be observed using a second part of the SKA that will be built in Western Australia and operate at a lower observing frequency. The Australian observations will be used to remove the effects of the interstellar medium from the African observations. Together these telescopes will allow us to probe the gravitational wave sky in incredible detail.

The invention of the telescope in the 17th century revolutionised our understanding of our place in the universe. The more recent era of radio, infrared, X-ray and gamma-ray telescopes has opened our view of the universe across the entire electromagnetic spectrum.

Nobody knows what the universe will look like with a gravitational wave telescope, but with the SKA we soon will!

George Hobbs is a Research Scientist at CSIRO Astronomy and Space Sciences.