Research

My research is in the area of radio pulsar astrophysics. Below is a list of some of the projects which I am currently involved with.

The Searching for Gravitational Waves using Pulsar Timing Arrays

The Pulsar Timing Arrays uses the most stable pulsars known to search for gravitational waves. These waves were predicted by Einstein in his General Theory of Relativity but have yet to be detected directly. Gravitational waves warp space-time very subtly changing the distance between two points. By timing the most stable pulsars down to an accuracy of 100 ns over 10 years it is hoped that gravitational waves will be detected in the delays and advances of pulses from the pulsars as space expands and contracts. Beyond gravitational waves, errors in the Earth time standard and Solar System ephemeris, and possibly new science will be found.

Measuring the mass of the planets

When timing pulsars the time-of-arrival of a pulse is converted to the centre of the Solar-System (the barycentre). This removes any affect of the Earth's orbit. To do this an ephemeris is used which determines the barycentre based on the positions and masses of the planets and their moons. If the masses are incorrect the barycentre position will be incorrect resulting in a signal in the pulsar timing data. This allows the masses of the planets to be determined using pulsar timing.

European Pulsar Timing Array

The European Pulsar timing array uses the Effelsberg (Max-Plank Institut für Radioastronomie), Nançay (Observatoire de Paris), Lovell (Jodrell Bank Centre for Astrophysics), Westerborg (ASTRON) and (once completed) Sardinia (Cagliari Astronomy Observatory) radio telescopes. The Large European Array for Pulsars (LEAP) project will combine the telescopes to produce the equivalent of a 200m telescope.

Parkes Pulsar Timing Array

The data for the Parkes Pulsar Timing Array comes from the Parkes radio telescope. This is the longest running of the pulsar timing array projects.

Interesting pulsars

Eccentric Millisecond Pulsar: PSR J1903+0327

PSR J1903+0327 was discovered in the p-ALFA survey for pulsars (see below). Its combination of very short rotational period (2.14 ms), long orbital period (95 days) and highly eccentric orbit (0.44) make this pulsar unique in the Galactic-plane.

Such a system is not predicted by current theories of millisecond pulsar evolution. The discovery of a main-sequence star much like our Sun at the position of the pulsar make it stranger still. A main-sequence companion could not have 'recylced' the system as would be expected.

It has a DM of nearly 300 pc/cm3 and a period of only 2.15 ms. This makes the most distant millisecond pulsar to be found in a blind survey.

For more information see the press release and Science paper in which we discuss the discovery in detail and suggest potential origins for the system.

Double neutron star system: PSR J1829+2456

PSR J1829+2456 was discovered in the Gregorian dome 430-MHz drift-scan survey undertaken at the Arecibo 305m dish in Puerto Rico in 1997. The unparallelled collecting area of the Arecibo dish makes it a very sensitive instrument for pulsar work.

This 41-ms pulsar was the 7th discovered of only 9 which are known to orbit another neutron star.

An extended timing campaign has allowed a determination of relativistic advance of periastron and has placed limits on the masses of the pulsar and it's companion. Timing continues to measure the gravitational redshift and relativistic time dilation parameter and any proper motion.

Pulsar surveys

High Time Resolution Legacy Survey

The High Time Resolution Legacy Survey uses the Effelsberg and Parkes radio telescopes to cover the whole sky (both northern and southern hemispheres) in search of pulsars. Both telescopes use multibeam receivers to cover the sky quickly at broad bandwidth, high time and frequency resolution. The survey uses long integrations to search deeply in the Galactic plane (where most pulsars are found) for exotic systems, shorter integrations are employed in the intermediate Galactic latitudes where very short period pulsars are expected. Finally the rest of the sky is covered by a shallow survey.

The p-ALFA survey for pulsars

In the hope of finding rare pulsars and pulsar systems a survey for pulsars using the Arecibo telescope began in 2004. This survey makes use of the new L-band multibeam receiver system, ALFA.

This survey is the deepest survey of the Galactic plane for pulsars and is expected to discover up to 1000 pulsars from the 1PB of data which will be taken. The time and frequency resolution mean that it is particularly sensitive to short period pulsars. Crude processing of the data as it is comes of the telescope has already resulted in the discovery of more than 45 pulsars.

Of the new pulsars one is in a relativistic system with the second largest measured advance of periaston. Timing of this double neutron star system continues.

The GBT 350-MHz Galactic plane survey

Although the p-ALFA survey is very sensitive, the Arecibo telescope cannot see most of the Galactic plane (due to it's position on the Earth and design). As survey using the Greenbank telescope is underway to cover the northern part of the Galactic plane.

The Arecibo 327-MHz survey

This is an excellent high Galactic latitude complement to the p-ALFA survey. The high Galactic latitude ensures that the dispersion measure is reduced allowing a low frequency to be used, where the pulsar's emission is strongest. The receiver is due to be cooled and the bandwidth increased to 50 MHz which will further improve the sensitivity of the system. If the survey is extended to the whole Arecibo sky it is expected to discover 100s of pulsars.