The precise, regular bursts of radio energy emitted by pulsars were initially thought to be signals from extraterrestrial aliens. They turned out to be a natural phenomenon, one important enough to merit the Nobel Prize for the supervisor of the woman who actually discovered them in 1967. Pulsars are superdense neutron stars, the remnants of massive stars that exploded as supernovae. Pulsars emit powerful beams of radio waves and light. As the neutron star spins, the beam sweeps through space like the beam of a lighthouse. When such a beam sweeps across the Earth, astronomers see a pulse from the pulsar. One of the most studied is the pulsar at the center of the Crab Nebula, a cloud of glowing debris from a star more than 6,000 light-years from Earth that was seen to explode on July 4, 1054 by Chinese and Native American skywatchers. The Crab pulsar spins some 33 times every second and is one of only three pulsars known to emit superstrong “giant” pulses. “Giant” pulses, second only to the Sun in their radio brightness in the sky, occur occasionally among the steady but much weaker “normal” pulses coming from the neutron star. The origin of the giant pulses has remained a mystery – until now.
As reported in the March 12 issue of Nature, astronomers at New Mexico Tech and the National Radio Astronomy Observatory in New Mexico have discovered that some of the “giant” pulses contain subpulses that last no longer than two nanoseconds. That means, they say, that the regions in which these subpulses are generated can be no larger than about two feet across, the distance that light could travel in two nanoseconds. The size of a beachball, these objects are the smallest ever to be detected outside the Solar System. This fact, the researchers say, is critically important to understanding how the powerful radio emission is generated. A pulsar’s magnetosphere — the region above the neutron star’s magnetic poles where the radio waves are generated — is “the most exotic environment in the Universe,” said Kern. In this environment, matter exists as a plasma, in which electrically charged particles are free to respond to the very strong electric and magnetic fields in the star’s atmosphere. The very short subpulses the researchers detected could only be generated, they say, by a strange process in which density waves in the plasma interact with their own electrical field, becoming progressively denser until they reach a point at which they “collapse explosively” into superstrong bursts of radio waves. The team believes that a process known as “plasma turbulence” – the conversion of kinetic energy in the pulsar’s magnetic atmosphere to radio energy – could explain how such short radio pulses are produced. “None of the other proposed mechanisms can produce such short pulses,” researcher Jean Eilek said. “The ability to examine these pulses on such short time scales has given us a new window through which to study pulsar radio emission,” she added.
Beachball-sized pulsars. Coming soon to a fireworks stand near you in time for your first summer pool party.