Pulsars are like cosmic lighthouses sending out sweeping beams that blink at us across the galactic expanse. Now scientists have spotted a wacky pulsar that doesn't behave exactly like its fellows: Instead of circling a white dwarf star, this one orbits a sun-like star along an oval path.
All other known pulsars that rotate as quickly as this one seem to have picked up speed by pulling off mass from a companion star that has reached the advanced stage of red giant, when its gaseous layers bloat out prior to the end-stage of life as a very compact, dim, white dwarf.
"The fact that it's around a sun-like star is fascinating because if that is the companion to this pulsar, then it certainly didn't accrete matter from that star — it hasn't been a red giant yet," said David Champion, an astronomer at Canada's McGill University.
To account for this odd duck, called PSR J1903+0327, scientists have concocted a few new ideas, including the possibility that the pulsar originated in a globular cluster with a different companion, but was kicked out by a near-miss with another star.
"The reason why we're so excited about this is the impact it might have on our understanding of where the pulsars that we look at are coming from," Champion told SPACE.com. "We've never seen anything like this before."
Champion and his colleagues detail their findings in the May 15 issue of the journal Science.
Quite a surprise
Pulsars are thought to form when a massive star reaches the end of its life and explodes in a supernova. The remnants of these stars sometimes collapse into neutron stars, so-called because they are so dense that the protons and electrons that formed the star's atoms have been squashed into neutrons (if the original star was even more massive, it would collapse into a black hole).
Not only is the star's matter tightly-packed after all this squashing, but the star's magnetic field is compressed into a tiny space as well. Scientists think this powerful field accelerates charged particles around the star, causing them to emit radiation that is focused into a beam by the magnetic field lines.
As these neutron stars rotate, so too do their light beams. If a neutron star happens to be shooting out its jet in our direction, we call it a pulsar, because we see a pulse every time the rotating beam reaches us.
Usually, pulsars slow down in their rotation over time as they lose energy. When one attains a speed as fast as PSR J1903+0327, which rotates every 2.15 milliseconds, scientists think it has "recycled" itself by sucking up mass from a companion red dwarf. When this happens, scientists usually see a quickly spinning pulsar orbiting around a white dwarf (the end stage of a red giant) in a circular orbit (the red giant's tidal forces stabilize the pulsar's orbit into a circle).
"This new pulsar is quite a surprise," Champion said, referring to the new object's oblong orbit around a sun-like star.
The scientists speculate that the strange pulsar may have started out in a globular cluster, where stars are much closer to each other and interact more often than in the rest of the galaxy. In this case, it would have originally gone through the normal recycling process, but lost its aged red giant companion when a younger sun-like star came flying in and knocked it away.
Another hypothesis is that the pulsar originated in a triple star system, but its main red giant companion was destroyed.
The team hopes further study of this unique object will help solve the mystery and teach us more about how pulsars form.
"It's always the most unusual objects that advance our understanding the most in these cases," Champion said.