This important advance, led by scientists at The University of Manchester and appearing June 24th in the journal Science Express, could improve the search for gravitational waves and help studies into the origins of the universe.
Pulsars appear to be able to switch between two states which differ in the current of charged particles flowing from the surface into outer space. This change in current results in a change of slow-down in their rotation rate, such that the pulsar 'brakes' faster (upper panel) when the currents are large and 'brakes' less fast when the currents are weak (lower panel). These currents also result in a change in the shape of the beam emitted by the pulsar, and hence in the shape of the pulse, or tick, as the beam crosses a radio telescope. These processes are illustrated in this mpeg simulation.
Credit: Michael Kramer, University of Manchester.
Gravitational waves are mysterious, powerful ripples which have not yet been directly observed, although are known to exist and were predicted by Einstein.
The direct discovery of gravitational waves, which pass over cosmic clocks and cause them to change, could allow scientists to study violent events such as the merging of super-massive black holes and help understand the universe shortly after its formation in the Big Bang.
The scientists made their breakthrough using decades-long observations from the 76-m Lovell radio telescope at The University of Manchester's Jodrell Bank Observatory to track the radio signals of extreme stars known as pulsars.
Pulsars are spinning collapsed stars which have been studied in great detail since their discovery in 1967. The extremely stable rotation of these cosmic fly-wheels has previously led to the discovery of the first planets orbiting other stars and provided stringent tests for theories of gravity that shape the Universe.
However, this rotational stability is not perfect and, until now, slight irregularities in their spin have significantly reduced their usefulness as precision tools.
The team, led by the University of Manchester's Professor Andrew Lyne, has used observations from the Lovell telescope to explain these variations and to demonstrate a method by which they may be corrected.
Professor Lyne explains: "Mankind's best clocks all need corrections, perhaps for the effects of changing temperature, atmospheric pressure, humidity or local magnetic field. Here, we have found a potential means of correcting an astrophysical clock".
The rate at which all pulsars spin is known to be decreasing very slowly. What the team has found is that the deviations arise because there are actually two spin-down rates and not one, and that the pulsar switches between them, abruptly and rather unpredictably.
These changes are associated with a change in the shape of the pulse, or tick, emitted by the pulsar. Because of this, precision measurements of the pulse shape at any particular time indicate exactly what the slowdown rate is and allow the calculation of a "correction". This significantly improves their properties as clocks.
The results give a completely new insight into the extreme conditions near neutron stars and also offer the potential for improving already very precise experiments in gravitation.
It is hoped that this new understanding of pulsar spin-down will improve the chances that the fastest spinning pulsars will be used to make the first direct detection of ripples, known as gravitational waves, in the fabric of space-time.
The University of Manchester team worked closely on the project with Dr George Hobbs of the Australia Telescope National Facility, Professor Michael Kramer of the Max Planck Institute for Radioastronomy and Professor Ingrid Stairs of the University of British Columbia.
The research was funded by the Science and Technology Facilities Council.
Their Director of Science, Professor John Womersley, said: "Astronomy is unlike most other sciences, as we cannot go out and measure directly the properties of stars and galaxies.
“They have to be calculated based on our understanding of how the Universe works - which means that something as significant as being able to use pulsars as cosmic clocks, a new standard for time measurement, will have far-reaching consequences for advancing science and our understanding of the Universe."
Many observatories around the world are attempting to use pulsars in order to detect the gravitational waves that are expected to be created by super-massive binary black holes in the Universe.
With the new technique, the scientists may be able to reveal the gravitational wave signals that are currently hidden because of the irregularities in the pulsar rotation.
Head of the Pulsar Group at The University of Manchester Dr Ben Stappers said: “These exciting results were only possible because of the quality and duration of the unique Lovell Telescope pulsar timing database”.
A pulsar is a neutron star, which is the collapsed core of a massive star that has ended its life in a supernova explosion. Weighing more than our Sun, yet only 20 kilometres across, and hence only the size of a city like Manchester, these incredibly dense cosmic fly-wheels produce beams of radio waves which sweep around the sky like a lighthouse, often hundreds of times a second. Radio telescopes receive a regular train of pulses as the beam repeatedly crosses the Earth so that the object is observed as a pulsating radio signal.
The clock-like nature of the arrival times of these pulses at the Earth means that pulsars have been used for some of the most precise studies of our understanding of the General Relativity theory of gravity. The best pulsars are the fastest rotating, called millisecond pulsars, which keep time to better than a millionth of a second over a year. These sources rotate up to 700 times a second and are currently being used to try and make the first ever detection of gravitational waves. This new insight provides an opportunity to achieve the necessary precision.
Further information on pulsars can be found on the Jodrell Bank Centre for Astrophysics Pulsar Group pages at http://www.jodrellbank.manchester.ac.uk/research/pulsar/.
The Jodrell Bank work was supported by funding from the UK Science and Technology Facilities Council (STFC). Jodrell Bank Centre for Astrophysics is part of the School of Physics and Astronomy at The University of Manchester. Jodrell Bank is home to the Lovell Radio Telescope and the MERLIN/VLBI National Facility which is operated by the University on behalf of STFC.
STFC is the UK's strategic science investment agency. It fundsresearch, education and public understanding in four areas of science - particle physics, astronomy, cosmology and space science. STFC is government funded and provides research grants and studentships to scientists in British universities, gives researchers access to world-class facilities and funds the UK membership of international bodies such as the European Southern Observatory (ESO), the European Laboratory for Particle Physics (CERN), and the European Space Agency. It also contributes money for the UK telescopes overseas on La Palma, Hawaii, and in Chile, the UK Astronomy Technology Centre at the Royal Observatory, Edinburgh and the MERLIN/VLBI National Facility, which includes the Lovell Telescope at Jodrell Bank observatory.
Contacts and sources:
Further information on pulsars can be found at The Jodrell Bank Centre for Astrophysics Pulsar Group