Making good time with pulsars

Decade-long observations of pulsars using the Lovell radio at Jodrell Bank could help astronomers track down long sought-after gravitational waves.

Schematic view of a pulsar, the most precise natural cosmic clocks known in the Universe. Image: Kramer, MPIfR.

Pulsars are highly magnetic rotating neutron stars that emit a beam of radiation, which can only be observed when directed towards the Earth. As they rotate, their emission sweeps across our line of sight in pulses, just like a lighthouse beam. The interval between pulses is extremely regular, ranging from about 1.4 milliseconds to 8.5 seconds, making them the most precise natural cosmic clocks known in the Universe. However, recent evidence suggests that over time pulsars 'spin-down', reducing their usefulness as precision tools.

“Mankind’s best clocks all need corrections, perhaps for the effects of changing temperature, atmospheric pressure, humidity or local magnetic field,” says team leader Andrew Lyne of the University of Manchester. “Here, we have found a potential means of correcting an astrophysical clock.

By training the 76-metre Lovell Telescope at the UK's Jodrell Bank on the radio signals of known pulsars, the astronomers have found that the changes to the pulsars' spin relate to the shape of the pulse. They also reveal that there are not one but two spin-down rates, and that the pulsars can switch between the two methods quickly and with no warning. The two states appear to differ by the amount of charged particles flowing from the surface out into space, and is visible as a change in the rotation such that the pulsar 'brakes' faster when the currents are large and slower when the currents are weak.

The 76m Lovell Telescope at Jodrell Bank. Image: Jodrell Bank Centre for Astrophysics, University of Manchester.

Precision measurements of the pulse shape at any particular time therefore indicate exactly what the slowdown rate is and allow the calculation of a 'correction' factor. It is anticipated that this new insight will allow astronomers to use the fastest spinning pulsars to attempt the first direct detection of gravitational waves, the 'ripples' in the fabric of space time that have as yet never been observed, but are predicted by Einstein. As these waves pass over pulsars they are predicted to inflict a change, but until now these changes may have been disguised by the irregular tick of the pulsars.

“Many observatories around the world are attempting to use pulsars in order to detect the gravitational waves that are expected to be created by supermassive binary black holes in the Universe,” says Professor Ingrid. “With our new technique we may be able to reveal the gravitational wave signals that are currently hidden because of the irregularities in the pulsar rotation.”