FEB 08, 2017 10:05 AM PST

Why Some Pulsars Emit Gamma Rays and No Radio Waves, and Vice-Versa

Pulsars are one of the things astronomers have a hard time understanding. We have a pretty good idea of what they are, but yet they’re so far away that they’re difficult to observe in detail. We have a lot of questions about their physics and the conditions that they generate that still need to be answered.

What we do know is that pulsars are remnants of star supernovae and are basically rapidly-spinning neutron stars that get left behind from the gravitational collapse that happens afterwards. These objects are very dense, and can have a diameter of no more than 18 miles while still containing just as much mass as several stars. 

On the other hand, one of the things that seems to make no sense is why some pulsars seem to give off lots of radio waves and no gamma rays, while others seem to give off lots of gamma rays and no radio waves. This is a mystery astronomers have been trying to figure out for a very long time, but always seem to come to a dead-end on.

An artist's rendition of Geminga, a distant pulsar that's active in terms of gamma ray activity.

Image Credit: Nahks TrEhnl/PSU

In a new study, published in The Astrophysical Journal, astronomers from Pennsylvania State University (more colloquially known as Penn State), compare two pulsars that appear to be very different in the way they express themselves.

Each of them have a name; the first is Geminga and the second is B0355+54. While the former has a name that sticks to memory more easily, both are equally as important to scientific research.

Geminga is important because it’s a pulsar that astronomers say emits high amounts of gamma rays, but is dead in terms of radio wave activity. B0355+54, on the other hand, is a pulsar that astronomers say emits high amounts of radio waves, but it as dim as can be in terms of gamma ray activity.

Because there was a lot of discrepancy between the two kinds of observations we can get when we look at pulsars, it was long theorized that perhaps there were a such thing as two different classes of pulsars. On the other hand, this new research seems to point in another direction.

Rather than there being two different kinds of pulsars, based on their emissions, it seems that all pulsars give off both gamma rays and radio waves. On the other hand, you might only see one or the other, depending on how you’re looking at it. That’s because, to certain planes, one or the other might not be visible.

It would seem that pulsars emit their gamma rays in a perpendicular fashion, compared to their radio waves, and it just depends which one is facing us that determines which we are going to see. In the case of Geminga, we have a visible angle of the gamma rays, but the radio waves are being emitted at an invisible angle. The opposite can be said for B0355+54 It’s very much an optical illusion.

In images captured by NASA’s Chandra Space Telescope, we can see each of the pulsars compared with an artist’s renditions, which gives us a greater understanding of just what it is that we’re actually looking at here.

Image Credit: (Top left and right: NASA/CXC/PSU/B.Posselt et al; Infrared, NASA/CXC/GWU/N.Klinger et al; Infrared), Bottom illustrations: Nahks Tr'Ehnl)/PSU

The “tails” that are given off by the emissions can tell us a lot about the pulsars, including how we’re looking at them. These tails are influenced by the pulsar’s spin and magnetic field, which are also important factors when we consider what’s visible to us from here on Earth.

"For Geminga, we view the bright gamma ray pulses and the edge of the pulsar wind nebula torus, but the radio beams near the jets point off to the sides and remain unseen," Bettina Posselt, a co-author of the study said.

"For B0355+54, a jet points nearly at us so we detect the bright radio pulses while most of the gamma-ray emission is directed in the plane of the sky and misses the Earth," said another co-author of the study, Oleg Kargaltsev. "This implies that the pulsar's spin axis direction is close to our line-of-sight direction and that the pulsar is moving nearly perpendicularly to its spin axis."

With this understanding now under the belts of astronomers, we can apply our greater understanding of the universe and continue our observations to learn more about what’s out there and help predict why certain things are the way they are. It’s a big universe out there, and we’ve barely even scratched the tip of the iceberg.

Source: PSU

About the Author
  • Fascinated by scientific discoveries and media, Anthony found his way here at LabRoots, where he would be able to dabble in the two. Anthony is a technology junkie that has vast experience in computer systems and automobile mechanics, as opposite as those sound.
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