January 25th, 2013 by James Ayre
Pulsars have been something of a mystery since they were first discovered. A rapidly spinning star that is heavier than our Sun but smaller than a city? The concept was already somewhat alien seeming compared to the physics that are witnessed in daily life, on the surface anyways. But now, new research has found them to be even stranger than previously thought.
Researchers at the University of Vermont recently discovered a pulsar (PSR B0943+10) that very rapidly shifts between very different different types of emissions. One moment emitting radio waves, and the next emitting almost entirely X-rays and no radio waves. As the researchers note, the discovery “challenges all proposed pulsar emission theories,” and completely reopens the debate on the subject. There has long been some dissenting opinions on how these stars actually “work”.
To the eyes of astronomers, the stars essentially look like they are the universe’s lighthouses. “Pulsars shine beams of radio waves and other radiation for trillions of miles. As these highly magnetized neutron stars rapidly rotate, a pair of beams sweeps by, appearing as flashes or pulses in telescopes on Earth.”
“Using a satellite X-ray telescope, coordinated with two radio telescopes on the ground, the researchers observed a pulsar that was previously known to flip on and off every few hours between strong (or ‘bright’) radio emissions and weak (or ‘quiet’) radio emissions.”
By monitoring the pulsar simultaneously in X-rays and radio waves, the researchers observed that it shows the same behavior when it’s observed at X-ray and radio wavelengths, simply in reverse.
“This is the first time that a switching X-ray emission has been detected from a pulsar. Flipping between these two extreme states — one dominated by X-ray pulses, the other by a highly organized pattern of radio pulses — was very surprising,” says Joanna Rankin, an astrophysicist at the University of Vermont.
“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,” said Rankin. “This was completely unexpected.”
None of the currently proposed model of pulsars can account for this behavior. “All theories to date suggest that X-ray emissions would follow radio emissions. Instead, the new observations show the opposite.”
“The basic physics of a pulsar have never been solved,” Rankin says.
“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,” explains Wim Hermsen.
“How exactly the particles are stripped off the neutron star’s surface and accelerated to such high energy, however, is still largely unclear,” he adds.
“By studying the emission from the pulsar at different wavelengths, the team’s study had been designed to discover which of various possible physical processes take place in the vicinity of the magnetic poles of pulsars.”
“Instead of narrowing down the possible mechanisms suggested by theory, however, the results of the team’s observing campaign challenge all existing models for pulsar emission. Few astronomical objects are as baffling as pulsars, and despite nearly fifty years of study, they continue to defy theorists’ best efforts.”
Erratic behavior amongst pulsars isn’t that uncommon. There have been, roughly, 2,000 pulsars discovered so far, and a number of them have shown strange behavior. Showcasing “emissions that can become weak or disappear in a matter of seconds but then suddenly return minutes or hours later.”
B0943+10 is one of the erratic ones, “this star has two very different personalities,” says Rankin. “But we’re still in the dark about what causes this, and other pulsars, to switch modes. We just don’t know.”
“But the fact that the pulsar keeps memory of its previous state and goes back to it,” says Hermsen, “suggests that it must be something fundamental.”
More background on what exactly a pulsar is:
“A pulsar (portmanteau of pulsating star) is a highly magnetized, rotating neutron star that emits a beam of electromagnetic radiation. This radiation can only be observed when the beam of emission is pointing toward the Earth, much the way a lighthouse can only be seen when the light is pointed in the direction of an observer, and is responsible for the pulsed appearance of emission. Neutron stars are very dense, and have short, regular rotational periods. This produces a very precise interval between pulses that range from roughly milliseconds to seconds for an individual pulsar.”
“The precise periods of pulsars makes them useful tools. Observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of gravitational radiation. The first extrasolar planets were discovered around a pulsar, PSR B1257+12. Certain types of pulsars rival atomic clocks in their accuracy in keeping time.”
“The events leading to the formation of a pulsar begin when the core of a massive star is compressed during a supernova, which collapses into a neutron star. The neutron star retains most of its angular momentum, and since it has only a tiny fraction of its progenitor’s radius (and therefore its moment of inertia is sharply reduced), it is formed with very high rotation speed. A beam of radiation is emitted along the magnetic axis of the pulsar, which spins along with the rotation of the neutron star. The magnetic axis of the pulsar determines the direction of the electromagnetic beam, with the magnetic axis not necessarily being the same as its rotational axis. This misalignment causes the beam to be seen once for every rotation of the neutron star, which leads to the “pulsed” nature of its appearance. The beam originates from the rotational energy of the neutron star, which generates an electrical field from the movement of the very strong magnetic field, resulting in the acceleration of protons and electrons on the star surface and the creation of an electromagnetic beam emanating from the poles of the magnetic field. This rotation slows down over time as electromagnetic power is emitted. When a pulsar’s spin period slows down sufficiently, the radio pulsar mechanism is believed to turn off (the so-called “death line”). This turn-off seems to take place after about 10–100 million years, which means of all the neutron stars in the 13.6 billion year age of the universe, around 99% no longer pulsate. The longest known pulsar period is 9.437 seconds.”
“Though this very general picture of pulsars is mostly accepted, Werner Becker of the Max Planck Institute for Extraterrestrial Physics said in 2006, ‘The theory of how pulsars emit their radiation is still in its infancy, even after nearly forty years of work.'”
The new research was just published in the January 25th edition of the journal Science.
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