Iowa State researchers help detect very-high-energy gamma rays from Crab pulsar

AMES, Iowa - Iowa State University astrophysicists are part of an international team that unexpectedly discovered very-high-energy gamma rays from the already well-known Crab pulsar star.

The team's findings are published in the Oct. 7 issue of the journal Science.

"This is the first time very-high-energy gamma rays have been detected from a pulsar - a rapidly spinning neutron star about the size of the city of Ames but with a mass greater than that of the sun," said Frank Krennrich, an Iowa State professor of physics and astronomy and a co-author of the paper.

The discovery was the work of three post-doctoral researchers - including Martin Schroedter, who left Iowa State last year for a position at the Fred Lawrence Whipple Observatory near Amado, Ariz.

The researchers' finding was a surprise, said Amanda Weinstein, an Iowa State assistant professor of physics and astronomy. Astrophysicists started looking for very-high-energy gamma rays from the Crab pulsar decades ago and had never found them with energies greater than 25 billion electron volts.

This time, using the $20 million Very Energetic Radiation Imaging Telescope Array System (VERITAS) in southern Arizona, the researchers discovered pulsed gamma rays from the Crab pulsar that exceeded energies of 100 billion electron volts.

Krennrich said 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 said. The spin and magnetism pull electrons from the star and accelerate them along magnetic field lines, creating narrow bands of "curvature radiation."

Krennrich and Weinstein said curvature radiation doesn't explain the very-high-energy gamma rays reported in the Science paper. And so astrophysicists need to develop new ideas about pulsars and how they create gamma rays.

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.

All those detectors were built in a laboratory on the fourth floor of Iowa State's Zaffarano Physics Addition. The assembly took about $1 million and a lot of work by a team of Iowa State researchers.

Weinstein, then working as a post-doctoral researcher at the University of California, Los Angeles, helped design and build the VERITAS array trigger. The trigger is an electronics system that works in real-time to determine which telescope observations contain useful data that should be recorded for analysis.

Researchers believe a better understanding of gamma rays could help them explore distant regions of space, help them look for evidence of dark matter, determine how much electromagnetic radiation the universe has produced, answer questions about the formation of stars and help explain the origins of the most energetic radiation in the universe.

The three lead authors of the Science paper are Schroedter; Andrew McCann of McGill University in Montreal; and Nepomuk Otte of the University of California, Santa Cruz and now at the Georgia Institute of Technology in Atlanta. Iowa State co-authors are Krennrich; Weinstein; Matthew Orr, a post-doctoral research associate in physics and astronomy; Arun Madhavan, a doctoral student in physics and astronomy; and Asif Imran, a former Iowa State doctoral student who's now at Los Alamos National Laboratory in New Mexico.

The research project was supported by the U.S. Department of Energy Office of Science, the National Science Foundation, the Smithsonian Institution, the National Sciences and Energy Research Council of Canada, Science Foundation Ireland, and the Science and Technology Facilities Council in the United Kingdom.

There's more than a gamma-ray discovery in this particular research paper, Weinstein said. There's also a lesson about scientific discovery.

"Because this was something people didn't expect, it took courage to pursue this study," she said. "The lesson is you keep making your instruments better and you keep looking."

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