|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
emittedby the pulsar, and hence in the shape of the
pulse, or tick, as the beam crosses a radio telescope.
(Credit: Michael Kramer, University of Manchester)
This important advance, led by scientists at The University of Manchester and appearing June 24 in the journal Science Express, could improve the search for gravitational waves and help studies into the origins of the universe.
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."