New findings from VERITAS Collaboration challenge theoretical models of pulsar emission
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 telescope array at the Whipple Observatory in Arizona and reported by an international team of scientists in a paper in the October 7 issue of 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. It is a discovery that McCann has been pursuing for close to seven years.
The Crab pulsar is a rapidly spinning neutron star, the collapsed core of a massive star that exploded in a spectacular supernova in the year 1054, leaving behind the brilliant Crab Nebula, with the pulsar at its heart. It is one of the most intensively studied objects in the sky. Rotating about 30 times a second, the pulsar has an intense, co-rotating magnetic field from which it emits beams of radiation. The beams sweep around like a lighthouse beacon because they are not aligned with the star's rotation axis. So although the beams are steady, they are detected on Earth as rapid pulses of radiation.
Scientists have long agreed on a general picture of what causes pulsar emission. Electromagnetic forces created by the star's rapidly rotating magnetic field accelerate charged particles to near the speed of light, producing radiation over a broad spectrum. But the details remain a mystery.
Prior to these new results, a phenomenon known as curvature radiation was the leading explanation for the Crab's pulsed gamma-ray emission. 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.
VERITAS (Very Energetic Radiation Imaging Telescope Array System) is a ground-based observatory for gamma-ray astronomy located at the Fred Lawrence Whipple Observatory in southern Arizona. It is an array of four 12-meter optical reflectors, used to record Cherenkov radiation created by high-energy gamma-ray showers in the atmosphere. These imaging Cherenkov telescopes have maximum sensitivity in the energy range from 100 GeV to 10 TeV (10,000 GeV or 10 trillion electron volts).
The 95 coauthors of the Science paper on the Crab pulsar include scientists from 26 institutions involved in the VERITAS collaboration.
The research was funded by Natural Sciences and Engineering Research Council, the U.S. Department of Energy, NSF, the Smithsonian Institution, STFC (U.K.) and Science Foundation Ireland.