ASTRON astronomers are part of an international team that is discovering the exotic stars known as "millisecond pulsars" at an astonishing rate. Whereas in the last thirty years only sixty millisecond pulsars have been identified in the disk of our Galaxy, seventeen new millisecond pulsars have been found in just the last three months by using large radio telescopes to target sources of high-energy gamma-rays recently found with NASA's Fermi Gamma-ray Space Telescope.

This sudden jump in the known population of these rare stars offers the opportunity to better understand their formation and evolution, and increases the chances of using an ensemble of millisecond pulsars as the lever arms of an immense gravitational wave detector. These discoveries were announced January 5th, 2010 in Washington, DC at the annual meeting of the American Astronomical Society.

Pulsars are the stellar remnants of massive stars that have ended their lives in a supernova. They are rapidly rotating, super-dense, highly magnetized neutron stars that emit beams of radiation from their magnetic poles.

As the star turns, it is sometimes possible to observe these beams sweeping past the line of sight, creating a pulsating effect similar to that of a lighthouse. Millisecond pulsars, as their name implies, are pulsars that spin with rotational periods of only a few milliseconds - as fast as a kitchen blender.

These are the fastest-spinning stars known and are formed when a neutron star is "spun-up" by the transfer of angular momentum from a companion star. Their pulsations are detectable with large, ground-based radio telescopes, and in some cases they also emit light at the other end of the electromagnetic spectrum: high-energy gamma-rays, detectable only from space.

Finding the millisecond pulsars in our Galaxy has always been a computationally demanding and time-consuming process, partly because their extreme properties require very precise measurements, but also simply because we don't know where to look for them a priori. This has changed recently, with the Fermi Gamma-ray Space Telescope identifying hundreds of sources that are potentially pulsars.

"With Fermi leading the way, we now know where to concentrate our search efforts for millisecond pulsars", says Jason Hessels, an ASTRON Staff Astronomer who recently discovered four of the new batch of millisecond pulsars using ASTRON's "DROP" computer cluster to perform the required heavy calculations, "it's like having a treasure map to guide us," added Hessels.

The Fermi satellite has been observing the sky at gamma-ray energies (> 100 MeV) since August 2008, and its Large Area Telescope instrument has been pin-pointing faint sources of gamma-rays like never before possible.

A first year catalog of sources will soon be released and will contain more than 1000 sources. Many hundreds of these are not yet associated with known sources seen at other wavelengths. So far 130 of these sources have been searched for radio pulsations using many of the world's largest radio telescopes.

ASTRON is specifically involved in a pilot survey of 50 faint gamma-ray sources using the 100-m Green Bank Telescope, the world's largest steerable radio dish. This survey was initiated by Mallory Roberts (Eureka Scientific).

"We knew that there was a good chance that at least a fraction of these newly identified gamma-ray sources are associated with pulsars. Detecting radio pulsations tells us firmly that these sources are pulsars and opens a huge realm of possibilities for further scientific studies," says Roberts.

This sudden sharp increase in the number of known millisecond pulsars is exciting news for astronomers who want to use them as a means to directly detect gravitational waves. By measuring their rate of pulsation, millisecond pulsars can be used as precise clocks, whose long-term stability is comparable to that of man-made atomic clocks.

With a sizable array of known millisecond pulsars spread over the sky, astronomers will attempt to measure correlated timing changes among these clocks in order to make the first ever direct detection of the gravitational wave background of the Universe - an important consequence of Einstein's theory of General Relativity.

"Finding so many more millisecond pulsars that can potentially be used as precision clocks increases our chances of directly detecting gravitational waves," says Hessels.