Gamma rays from pulsar defy explanation

MCGILL U. (CAN) — Astrophysicists have detected pulsed gamma-ray emission from the Crab pulsar at energies far beyond what current theoretical models of pulsars can explain.

With energies exceeding 100 billion electron-volts (100 GeV), the surprising gamma-ray pulses were detected by the VERITAS (Very Energetic Radiation Imaging Telescope Array System) telescope array at the Whipple Observatory in Arizona and reported by an international team of scientists in a paper in Science.

Andrew McCann, a PhD candidate from McGill University in Montreal, along with corresponding authors Nepomuk Otte of the University of California, Santa Cruz, and Martin Schroedter of the Smithsonian Astrophysical Observatory performed most of the analytic work for the study, which involved nearly 100 scientists in the VERITAS collaboration.


VERITAS features four, 12-meter reflector dishes covered with 350 mirrors. All those mirrors direct light into cameras mounted in front of each dish. Each camera is about 7 feet across and contains 499 tube-shaped photon detectors or pixels. (Credit: Pascal Fortin)
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Frank Krennrich, an Iowa State professor of physics and astronomy and a co-author of the paper, says such high energies can’t be explained by the current understanding of pulsars.

Pulsars are compact neutron stars that spin rapidly and have a very strong magnetic field, Krennrich says. The spin and magnetism pull electrons from the star and accelerate them along magnetic field lines, creating narrow bands of “curvature radiation.”

Curvature radiation is produced when a high-energy charged particle moves along a curved magnetic field. But according to McCann, this mechanism cannot account for gamma rays with energies above 100 GeV. It is a discovery that came as a real surprise and researchers will need to characterize the very high-energy gamma-ray emission in much greater detail in order to gain more insight into the mechanisms behind it.

Gamma rays are a form of high-energy electromagnetic radiation. They have energies of one million to several trillion electron volts; the energy of visible light is one electron volt.

Even with their very high energies, gamma rays can’t penetrate the earth’s atmosphere. When they hit the atmosphere, they create showers of electrons and positrons that create a blue light known as Cerenkov radiation. Those showers move very fast. And they’re not very bright.

And so it takes a very sensitive instrument such as VERITAS to detect those rays. VERITAS features four, 12-meter reflector dishes covered with 350 mirrors. All those mirrors direct light into cameras mounted in front of each dish. Each camera is about 7 feet across and contains 499 tube-shaped photon detectors or pixels.

The research was funded by Natural Sciences and Engineering Research Council, the U.S. Department of Energy, NSF, the Smithsonian Institution, STFC (UK), and Science Foundation Ireland.

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