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Spinning star with two personalities baffles astronomers

New observations of a highly variable pulsar using a satellite X-ray telescope combined with Earth-based radio telescopes are puzzling astronomers. The pulsar in question was discovered to flip between two states; one dominated by X-ray pulses, the other by a highly-organized pattern of radio pulses.

“There is a general agreement about the origin of the radio emission from pulsars: it is caused by highly energetic electrons, positrons and ions moving along the field lines of the pulsar’s magnetic field, and we see it pulsate because the rotation and magnetic axes are misaligned,” says Wim Hermsen from SRON, the Netherlands Institute for Space Research in Utrecht, The Netherlands, according to a news release from the European Space Agency. “How exactly the particles are stripped off the neutron star’s surface and accelerated to such high energy, however, is still largely unclear.”

Pulsars are spinning stars that are approximately the size of a city, according to a news release from the University of Manchester. They discharge oppositely directed beams of radiation from their magnetic poles. Some pulsars generate radiation across the entire electromagnetic spectrum. Despite being found more than four decades ago the exact mechanism by which pulsars shine is still unknown to scientists. While it is known that some radio pulsars can also be observed at X-ray frequencies, the X-ray signal is so weak that scientists know nothing of its variability.

To see if X-rays could flip like radio pulses scientists examined a pulsar called  PSR B0943+10. This particular pulsar has radio pulses which change in form and brightness every few hours.

“The behavior of this pulsar is quite startling, it’s as if it has two distinct personalities,” says Dr Ben Stappers from The University of Manchester’s School of Physics and Astronomy. “As PSR B0943+10 is one of the few pulsars also known to emit X-rays, finding out how this higher energy radiation behaves as the radio changes could provide new insight into the nature of the emission process.”

Astronomers used the ESA’s XMM-Newtown to observe the pulsar. The observations occurred over six separate sessions of approximately six hours each. Astronomers made several interesting discoveries:

  • X-rays change their behavior at the same time as the radio emission.
  • In the state where the radio signal is strong and organized the X-rays were weak
  • When the radio emissions switched to weak the X-rays got brighter

“To our surprise we found that when the brightness of the radio emission halved, the X-ray emission brightened by a factor of two,” says Hermsen. “Furthermore the intense X-rays have a very different character from those in the radio-bright state, since they seem to be thermal in origin and to pulse with the neutron star’s rotation period.”

“As well as brightening in the X-rays we discovered that the X-ray emission also shows pulses, something not seen when the radio emission is bright,” adds Dr Ben Stappers from The University of Manchester’s School of Physics and Astronomy. “This was the opposite of what we had expected. I’ve likened the changes in the pulsar to a chameleon. Like the animal the star changes in reaction to its environment, such as a change in temperature.”

Researchers hope to examine other objects which have similar behavior to see what happens to the X-ray emission.

The study’s findings will be described in detail in the journal Science.

Photo credit: ESA/ATG medialab.

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