University of Vermont

The College of Arts and Sciences

Department of Physics

Probing a Pulsar's Personality

Astronomer to lead international study

A star just ten miles wide, pulsars are surrounded by a magnetic field of nearly unbelievable power. These super-dense spheres shoot out rotating beams of radiation like lighthouses to the universe. But just how they do this remains mysterious. Astrophysicist Joanna Rankin will be turning space-based telescopes toward one peculiar pulsar, looking for answers. (Image courtesy of NASA)

UVM astrophysicist Joanna Rankin would like to know what makes pulsar B0943+10 so moody. “It has beautiful orderly pulses in one mode and it’s pretty chaotic in another,” she says, “and we don’t know why.”

This spring she may get closer to an answer. Rankin has been selected to lead a new international effort to study the strange star using a combination of both radio and x-ray telescopes. She traveled to the Netherlands in February to develop plans with her collaborators there. Other team members came from Britain and India.

The work will proceed with 36 hours of x-ray observation on the European Space Agency’s X-ray Multi-Mirror Mission (XMM-Newton) satellite-based telescopes, “probably in May,” Rankin says. This x-ray data will be correlated to radio observations of the pulsar’s two modes, likely collected at the Giant Metrewave Radio Telescope in Pune, India.

Rankin’s work is supported by the U.S. National Science Foundation.

At the limit of Maxwell’s equations

For more than four decades, astronomers have wondered how pulsars do their pulsing. These bizarre hyper-dense neutron stars send forth beams of radio waves and other radiation like a rotating lighthouse. Pulsars are more massive than our sun, but only the size of Manhattan. A sugar cube of pulsar would weigh more than all the people on Earth and the power of their emissions test the outer limits of our understanding of electromagnetism.

Pulsars, thus, serve as a kind of space-based laboratory of extreme physics.

But some pulsars are stranger than others. Like B0943+10.

“Why should a pulsar do one thing for hours and then another thing for hours? Something really fundamental has got to change,” Rankin says. “And we don’t know what that fundamental something is.”

Hot under the cap

By looking at the combination of x-ray and radio emissions from B0943+10, about three thousand light years distant, Rankin hopes that the cause of the pulsar’s distinct “bright” and “quiescent” modes will become clearer.

“We know that the magnetic polar caps of pulsars are hot because that is what produces the x-ray emissions,” she says, “but it might be that the region has a varying temperature,” she says. And this varying temperature might be the on/off switch for the pulsar’s two modes.

 Under the extreme conditions of a pulsar, particles streaming off the surface are ripped into their constituent atoms and their electrons are peeled away too. In other words, they form a fourth state of matter called a plasma. And it’s this plasma above the star that ultimately sends forth the radio signals that have made pulsars famous as cosmic lighthouses. (When the first pulsar was observed in 1967 there was some speculation that it was aliens signaling and so it was, wryly, labeled LGM-1 for “little green men.”)

But the massive outpouring of radio waves and other radiation must have an effect back on the surface of the pulsar. “If you have electrons going out, you have positrons coming back—and those heat the surface,” Rankin says. “So, part of the star’s radiation energy gets put into surface heating and maybe it’s the surface heating that changes and puts the pulsar in a different state.”

If this is true, then the varied heating should show up in telescope images as a difference in the “brightness” of the x-rays between the two modes. But this can’t be measured on Earth’s surface; it has to be measured in space, outside our atmosphere.

The rare and highly sought opportunity to use the satellite-based x-ray telescopes on XMM-Newton will allow Rankin and her colleagues to put this idea to the test.

“If there really is a difference it will be a major new direction,” she says, “This is something that nobody has ever had occasion to do before.”

Rankin’s primary colleagues in the venture are x-ray astronomer Wim Hermsen from SRON, the Netherlands Institute for Space Research; Dipanjan Mitra with the National Center for Radio Astrophysics in Pune, India; and Joeri van Leeuwen, with the Dutch Radio Astronomy Center, ASTRON.