A ‘Tail’ of Two Pulsars

Not all pulsars rocketing through space are alike. Using NASA images, researchers including Physics’ Noel Klingler are discovering why some stars shine with different lights.

The Geminga pulsar, 800 light years from earth
The Geminga pulsar, 800 light years from earth. (Photo courtesy Penn State University)
February 09, 2017

For 50 years, astronomers have watched pulsars—neutron stars born in supernova explosions—sweep through space with bursts of energy. But the spectacular stellar phenomenon have provided scientists with as many puzzles as wonders, including unanswered questions about the nature of these fast-moving remnants of massive exploded stars.

In two studies published in The Astrophysical Journal—including one authored by physics PhD candidate Noel Klingler—international teams of astronomers used images from NASA’s Chandra X-ray Observatory of two separate pulsars to shine a light one of the greatest pulsar riddles: Why do some of them pulse only in radio waves, others only in gamma-rays and still others both?

Super-dense pulsars rhythmically rotate as they rocket through space at speeds reaching hundreds of kilometers a second. While firing across the universe, many emit lighthouse-like radio beams from their poles. More recent research has found that pulsars can also produce cones of high-energy gamma rays, believed to be released only near their equators. But the scientific community had no clear answers for why the two emission patterns differ so drastically.

The answer may have as much to do with our perspective from earth as it does the pulsar’s distinctive emission signatures.

The two studies—one lead by Klingler, the other by Penn State’s Bettina Posselt, with contributions by Assistant Professor of Physics Oleg Kargaltsev and other scientists—used images of two separate pulsars, respectively named B0355+54 and Geminga. Both are about half a million years old and rotate at a speed of about five times per second. Like many young pulsars, both B0355 and Geminga leave trails of energetic particles—known as a pulsar wind nebulae—in their wake as they shoot through space. Most of the wind particles from these pulsars are either launched from their axis of rotation, forming jets, or radially from above their equators in an expanding ring-like manner. They are then swept back by the trace amounts of gas in space and blend together to form tails.

Geminga—one of the closest pulsars at only 800 light years from earth—clearly shows two jets and the equatorial outflow. The two streams of particles spewing out of each pole stretch for more than half a light year, longer than 1,000 times the distance between the sun and Pluto. The equatorial outflow stretches for only a fraction of that distance.

But a much different wind nebula can be seen in the X-ray image of B0355, the subject of Klingler’s research.  B0355 is about 3,300 light years away from earth. Its nebula head has a bullet-shaped cap of emission, followed by a narrower expanding outflow that extends almost five light years away from the star.

The two objects also exhibit vastly different pulses: Geminga radiates in the gamma ray spectrum, but is radio-quiet; it emits no signs of radio waves. B0355 is one of the brightest radio pulsars, but shows no gamma ray emission.

The explanation appears to lie in the tails—and our line-of-sight from earth. “The pulsations we see depend upon the angle at which we're viewing the pulsar,” Klingler said. In the case of Geminga, we see the pulsar edge-on—essentially at its equator where the bright gamma ray pulses are released. The radio beams near the jets point off to the sides where, from the edge-on angle, they cannot be seen. We see B0355 from a top-down perspective—above one of the poles—making the radio beams visible from the earth. If we look at pulsars from angles in between, we would see both radio and gamma-rays, Klingler explained.

In this simulation, radio beams (green) are emitted from the pulsar's poles while gamma-ray (purple) are emitted near its equator. (Courtesy NASA/Fermi/Cruz deWilde)

“In short, pulsations from pulsars vary, and whether we see radio, gamma rays or both depends upon the angle between the pulsar's spin axis and the earth, because they don't emit radiation in all directions uniformly,” Klingler said.

The insights on pulsar geometry will allow researchers to better estimate the total number of exploded stars in the galaxy. With more accurate models of pulsar properties, astronomers can now delve into the mysteries of how the objects actually produce radio beams, jets and gamma-rays, and how they accelerate particles to high energies.

In addition to Kargaltsev, the co-authors on Klingler’s study are Penn State University Senior Scientist George Pavlov, Stanford University Professor of Physics Roger Romani and Patrick Slane, astrophysicist at the Harvard-Smithsonian Center for Astrophysics.