Lovell telescope helps scientists solve gamma-ray mystery

09 Jul 2009

Scientists from the University of Manchester have worked with an international team of astronomers using NASA's Fermi Gamma-ray Space Telescope to get a new look at the spinning cosmic lighthouses known as pulsars.

In two studies published in Science Express, the team has analyzed gamma-rays from two dozen pulsars - including eight of the most rapidly rotating pulsars.

These detections were only possible thanks to the contributions of radio telescopes like the Lovell Telescope at The Jodrell Bank Centre for Astrophysics, which is owned and operated by The University of Manchester.

A pulsar is the rapidly spinning and highly magnetized core left behind when a massive star explodes. Most of the 1,800 catalogued pulsars were found through their periodic radio emissions. Astronomers believe these pulses are caused by narrow, lighthouse-like radio beams emanating from the pulsar's magnetic poles.

The arrival times of the pulses of more than a third of all known pulsars are monitored on a regular basis with the Lovell radio telescope at Jodrell Bank, in what is the largest and longest running project of its type,

And this work has proved to be essential for the new studies presented in Science Express.

As well as emitting at radio wavelengths, it was known that a handful of pulsars also emit gamma-rays. However they are extremely faint. Even with the greatly increased sensitivity of the new Fermi Large Area Telescope (LAT), astronomers only see about one gamma-ray photon every two minutes from the brightest gamma-ray emitting pulsar, the Vela pulsar.

"That's about one photon for every thousand Vela rotations," said Marcus Ziegler at the University of California, Santa Cruz, a member of the team reporting on the new pulsars. "From the faintest pulsar we studied, we see only two gamma-ray photons a day."

Like spinning tops, pulsars slow down as they lose energy. Eventually, they spin too slowly to power their characteristic emissions and become undetectable. But pair an old, retired pulsar with a normal star, and a stream of stellar matter from the companion can spill onto the pulsar and spin it up. Rotating between 100 and 1,000 times a second, ancient pulsars can be revitalised and resume the activity of their youth.

The extreme faintness of the gamma-ray emission from these so-called reborn pulsars means that in order to correctly add up the signal in line with their extreme rotation periods, accurate models of how they rotate are needed.

"It is only dedicated monitoring observations with telescopes like the Lovell dish that make it possible to see these remarkable objects in gamma-rays" said The University of Manchester's Andrew Lyne.

"Despite their vastly different rotation rates, ages and magnetic field strengths it seems that these reborn pulsars and the normal pulsars share something in the way they emit gamma-rays" said Ben Stappers, also of the University of Manchester.

The radio beam carries only a tiny fraction, a few parts per million, of the total power emitted by a pulsar, whereas its gamma rays can account for around 10 percent of the energy budget. In order to do so, pulsars must be able to accelerate particles to speeds near that of light. These particles emit a broad beam of gamma rays as they arc along curved magnetic field lines.

Discovering the previously unseen faint emission of these broad beams allowed the new pulsars to be discovered as part of a comprehensive search for periodic gamma-ray fluctuations using five months of Fermi LAT data and new computational techniques.

"Before launch, some predicted that Fermi might uncover only a handful of new pulsars during its mission," Ziegler added. "To discover 16 in its first five months of operation is really beyond our wildest dreams.”

NASA's Fermi Gamma-ray Space Telescope is an astrophysics and particle physics partnership, developed in collaboration with the US Department of Energy, along with important contributions from academic institutions and partners in France, Germany, Italy, Japan, Sweden, and the US.

Notes for editors

For more information about Jodrell Bank’s contribution to this work, please contact Alex Waddington, Media Relations Officer, The University of Manchester, 0161 275 8387 / 07717 881569.

For Images related to this press release please visit

The UK Science and Technology Facilities Council (STFC) support the pulsar work with the Lovell Telescope through a rolling grant.