Crab Pulsar Dazzles Astronomers with Bright Gamma Rays

About 1000 years ago, a star in the constellation Taurus exploded, astonishing observers from Arabia to North America to China, who watched the slow detonation even in daylight. The remaining heart of that star, now know as the Crab Pulsar, continues to confound scientists today by emitting some of the most energetic gamma rays ever observed. A better understanding of these emissions ultimately could help reveal evidence of dark matter, determine how much electromagnetic radiation the universe has produced, and explain the origins of the most energetic radiation in the universe.

The pulsar, centered in the Crab Nebula — the name given to the gaseous remnants of a former sun located 6500 light-years from Earth — is churning out gamma rays with energies above 10¹¹ eV. The extremely high energy of the gamma rays has been described by investigators affiliated with VERITAS (Very Energetic Radiation Imaging Telescope Array System), a ground-based observatory for gamma-ray astronomy located at the Fred Lawrence Whipple Observatory in southern Arizona and operated by a collaboration of more than 100 scientists from about two dozen institutions. The group’s findings are reported in the Oct. 7 issue of Science.

Professor Jamie Holder (center) with graduate students Dana Saxon and Sajan Kumar in their lab at the University of Delaware. The black tube that Saxon is holding is one of the photodetectors the group is assembling and testing for the new VERITAS telescope cameras. (Photo: University of Delaware)

Astronomers observe very high energy gamma rays with ground-based Cerenkov telescopes. Gamma rays from celestial sources are absorbed in Earth's atmosphere, where they create a shower of subatomic particles. Cerenkov telescopes detect the faint, extremely short flashes of blue light that these particles emit (named Cerenkov light) using extremely sensitive cameras. The images can be used to infer the initial energy of the gamma rays as well as the direction from which they came.

This technique is used by gamma-ray observatories throughout the world, and was pioneered at the 10-m Cerenkov telescope at Whipple Observatory. Comprising four 12-m Cerenkov telescopes, VERITAS continues the tradition of Whipple's single 10-m telescope.

Close-up of an existing VERITAS telescope camera. (Photo: University of Delaware)

"This is a really exciting and unexpected result," said Jamie Holder, assistant professor at the University of Delaware. Holder's group helped construct the VERITAS telescopes, and it collected a portion of the data for this study and developed some of the software used in the analysis.

"Existing theories of gamma rays from pulsars predict a sharp cutoff in the emission at high energies, around 10 thousand million electron volts (1010 eV),” Holder said. “Our data show gamma rays with energies at least 20 times this, implying that the gamma rays are being produced in a different place, and probably by a different mechanism, than expected."

Holder added that the small flashes of blue light caused when a gamma ray hits the atmosphere last only a few billionths of a second. The VERITAS cameras take 200 photographs a second. He and his team developed software that would sift out the gamma rays from all of the background noise, representing about one-tenth of the images.

The Crab Pulsar emits light energy above 10¹¹ eV. (Image: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics/NASA/José Francisco Salgado, Adler Planetarium, European Space Agency. Based on images by M. SubbaRao, S. Criswell, B. Humensky and J.F. Salgado)

One of the study’s lead authors, Nepomuk Otte of the University of California, Santa Cruz, said that some researchers had told him he was crazy to even look for pulsar emission in this energy realm.

"It turns out that being persistent and stubborn helps," Otte said. "These results put new constraints on the mechanism for how the gamma-ray emission is generated."

An x-ray image of the Crab Nebula and pulsar. (Image: Chandra X-ray Observatory/NASA/Smithsonian Astrophysical Observatory/F. Seward)

"If you asked theorists a year ago whether we would see gamma-ray pulses this energetic, almost all of them would have said, 'No.' There's just no theory that can account for what we've found," said co-author Martin Schroedter of the Harvard-Smithsonian Center for Astrophysics.

The University of Delaware’s Holder said that he and his colleagues will keep observing the Crab Pulsar for the next few years as the spinning star continues to wind down. They currently are building 2000 photodetectors for new cameras destined for the VERITAS telescopes.

"The new photodetectors collect 50 percent more light than our existing ones, which will make us more sensitive to gamma rays, particularly in the energy range where the Crab Pulsar emits," Holder said.

An artist's conception of the pulsar at the center of the Crab Nebula, with a Hubble Space Telescope photo of the nebula in the background. Researchers using the VERITAS telescope array have discovered pulses of high-energy gamma rays coming from this object. (Image: David A. Aguilar, Harvard-Smithsonian Center for Astrophysics/NASA/European Space Agency)

Some possible scenarios to explain the data have been put forward, but it will take more data, or even a next-generation observatory, to really understand the mechanisms behind these gamma-ray pulses.

Pulsars spin rapidly and have a very strong magnetic field. The spin and magnetism pull electrons from the star and accelerate them along magnetic field lines, creating narrow bands of "curvature radiation." This curvature radiation doesn't explain the very high energy gamma rays reported in the paper, said Frank Krennrich of Iowa State University in Ames, another co-author of the paper. So astrophysicists need to develop new ideas about pulsars and how they create gamma rays.

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