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Telescopes and space missions

Telescopes and space missions

Pulsar timekeepers measure up to atomic clocks

24 Aug 2012 Hamish Johnston
Keeping time: using an array of pulsars to look for gravitational waves

An international team of astronomers has come up with a new way of keeping track of time by observing a collection of pulsars – rapidly rotating stars that emit radio pulses at very regular intervals. Although the ultimate goal of the research is to use pulsar timing to detect gravitational waves, the group has shown that the pulsar-based timescale can also be used to reveal inconsistencies in timescales based on atomic clocks.

Pulsars are neutron stars that rotate at very high speeds and appear to emit radio pulses at extremely regular intervals. The pulses are actually all we see of a radio beam that is focused by the star’s magnetic field and swept around like a lighthouse beacon. Using a radio telescope, astronomers can measure the arrival times of successive pulses to a precision of 100 ns over a measurement time of about an hour. While this level of precision is significantly less than that offered by an atomic clock, pulsars could in principle be used to create timescales that are stable for decades, centuries or longer. This could be useful for identifying fluctuations in Earth-based timekeepers such as atomic or optical clocks, which normally do not operate over such long periods.

The team, which is led by George Hobbs at CSIRO Astronomy and Space Sciences in Australia, looked at data from the Parkes Pulsar Timing Array (PPTA) project. Using the Parkes radio telescope in Australia, the project aims to use a set of about 20 pulsars in different parts of the Milky Way to detect gravitational waves. The idea is that when a gravitational wave passes through our galaxy, its presence warps space/time such that the millisecond gaps between the pulses arriving from various pulsars are affected in a very specific way.

Extremely precise timescale

In developing the PPTA, Hobbs and colleagues in Australia, Germany, the US and China realized that the timing data from a number of pulsars could be combined to create an extremely precise timescale stretching back to the mid-1990s. A timescale is a sequence of marks in time, each separated by a defined time interval. The most precise timescales available today are generated by atomic or optical clocks, which operate using the frequencies of certain atomic transitions.

The team made a timescale based on 19 pulsars by first correcting the data from each pulsar for a number of different things that can affect the measurement of the gap between pulses. These include instrumental effects, the motion of the Earth within the solar system and the effects of interstellar plasma. Also, the frequency of a pulsar drops slowly with time as rotational energy is radiated away, and this must be corrected for.

The team then combined the data from the 19 pulsars to create the Terrestrial Time PPTA11 or TT(PPTA11) timescale, where 11 signifies that the most recent data used are from 2011. To show how their new timescale could be used to evaluate timescales generated by atomic clocks, the researchers compared it with Terrestrial Time (International Atomic Time) – TT(TAI). This is a timescale that is created by combining the results of several hundred atomic clocks worldwide. TT(TAI) is never revized, and therefore provides a historical record of the performance of atomic clocks. Instead, the atomic-clock timescale is gently “steered” towards better timekeeping through revision and reanalysis of the time standard.

Looking for deficiencies

If the new pulsar timescale is indeed precise, it should be able to reveal historical deficiencies in the atomic-clock timescale – and this is exactly what the team was able to do. The researchers compared the two timescales going back to about 1994 and found a distinct departure at around 1998. The team also did a similar comparison between the atomic-clock timescale and a corrected version of Terrestrial Time that is produced annually by the International Bureau of Weights and Measures – TT(BIPM11). The researchers saw the same distinct departure at around 1998, which suggested that, like TT(BIPM11), the pulsar-based timescale is capable of revealing inconsistencies in atomic-clock-based timescales.

The similarity between TT(PPTA11) and TT(BIPM11) also allowed the team to conclude that there are no large unexpected errors in TT(BIPM11). Furthermore, the results corroborate previous research, which concluded that the TT(TAI) timescale is not sufficiently precise to be used for pulsar-timing applications such as the detection of gravitational waves, and that TT(BIPM11) should always be used in such applications.

Team member David Champion at the Max Planck Institute for Radioastronomy in Bonn told that the next step in developing the timescale is to incorporate pulsar data from other radio telescopes that were obtained over the same time period.

Proof of principle

Setnam Shemar of the Time and Frequency Group at the UK’s National Physical Laboratory described the work as “proof of principle that PPTA data can be used to find anomalies in some present-day atomic timescales”. While he thinks it is possible that a pulsar-based timescale could outperform the best present-day atomic timescale over long times, Shemar says that it is too early to tell. Indeed, he points out that if improvements in atomic and optical clock technologies outpace improvements in pulsar timing, as he expects to be the case, a pulsar-based timescale may in future be more useful in a search for gravitational waves than a means for checking atomic timescales.

The research will be published in Monthly Notices of the Royal Astronomical Society and a preprint is available on arXiv.

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