Washington: An international team of astrophysicists has spotted pulsed gamma ray emissions coming from the Crab Pulsar with energies far higher than expected, surpassing what current theoretical models of pulsars can explain.
The emissions were detected by the VERITAS (Very Energetic Radiation Imaging Telescope Array System) array of four 12-meter Cherenkov telescopes in Arizona.
The astrophysicists, including a group from the Department of Physics in Arts and Sciences at Washington, say that the pulsed gamma rays had energies between 100 and 400 billion electron volts (Gigaelectron volts, or GeV), far higher than 25 GeV, the highest energy radiation from the nebula previously detected.
A 400 GeV photon is 11 orders of magnitude -- almost a trillion times -- more energetic than a visible light photon.
“We presented the results at a conference and the entire community was stunned,” said VERITAS member and Washington University physicist Henric Krawczynski.
The Crab Nebula is the spectacular remains of a massive star that became a supernova in the year 1054 and was brilliant enough that its flaring was recorded by Chinese and Arab astronomers.
The collapsed core of the defunct star, a pulsar discovered only in 1969, is only 30 km in diameter but pumps out enormous amounts of energy, making the nebula 75,000 times brighter than the Sun.
The dance of the high-energy particles spewed by the star and its strong magnetic field fill the inner nebula with phantasmagoric, ever-changing shapes like expanding rings made up flickering knots and turbulent high-speed jets.
Jets of particles stream out of the pulsar’s magnetic poles, producing powerful beams of light. Because the magnetic field and the star’s spin axis are not aligned, these beams sweep out a circle in space, crossing the line of sight from Earth at regular intervals, so that the emission appears to pulse.
“The pulsar in the center of the nebula had been seen in radio, optical, X-ray and soft gamma-ray wavelengths,” says Matthias Beilicke, PhD, research assistant professor of physics at Washington University.
“But we didn’t think it was radiating pulsed emissions above 100 GeV,” he added.
The VERITAS team turned the telescope to the pulsar for 107 hours over a period of four years. To everyone’s surprise, they found a very high energy pulsed emission.
They can be absolutely certain the high-energy beam is coming from the pulsar because it has exactly the same period as the pulsed radio and X-ray emissions that have long been observed.
Models of pulsar emission worked out to explain these earlier observations can’t explain the VERITAS result without major adjustments.
The research is published in the Oct. 7 issue of Science.
First Published: Friday, October 07, 2011, 16:34