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UCLA physicists help solve the mystery of Earth’s cosmic rays

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A state-of-the-art telescope array with the apt acronym VERITAS (the mythical goddess of truth) and located at the Fred Lawrence Whipple Observatory in southern Arizona has been seeking the truth about a mystery that has puzzled scientists for nearly 100 years: the origin of cosmic rays.
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UCLA Professor of Physics Rene Ong and his colleagues built several of the telescope array's electronic components,developed much of the software for the project and manage all the data in a huge archive here on campus.
Now, recent results presented by VERITAS (Very Energetic Radiation Imaging Telescope Array System) have provided evidence to support a long-held theory — that cosmic rays are formed by exploding stars and stellar “winds.” The findings were published in the Nov. 1 online issue of the journal Nature.
“Cosmic rays cause outages of cell phones and satellites and things like that, and they’re an important source of energy and particles in our galaxy,” said Rene Ong, UCLA professor of physics and spokesman for VERITAS’ international team of astronomers.
Despite general knowledge about them, Ong said, “for 90 years, we actually weren’t sure where these particles were created.”
Cosmic rays are charged subatomic particles, mostly protons, that zip through space at nearly the speed of light. They strike the Earth’s atmosphere violently and often, with the punch of a 98-mph fastball, but don’t do significant damage to humans because the atmosphere shields us.
Still, astronomers wondered what natural forces could accelerate these cosmic rays to such high speeds. They suspected that the rays were created by supernovae (star explosions) and stellar winds, but they couldn’t prove it because they lacked an instrument sensitive enough to see it.
VERITAS
The Very Energetic Radiation Imaging Telescope Array System, VERITAS, at Fred Lawrence Whipple Observatory in southern Arizona.
Along came the VERITAS telescopes, which have found evidence for the interactions of high-energy cosmic rays in the starburst galaxy M82, also known as the “Cigar Galaxy.” M82 sits about 2 million light-years from Earth, in the direction of the Ursa Major constellation.
VERITAS’ observations over a two-year period strongly support the theory that a starburst galaxy such as M82 — with its exceptionally high level of star formation — produces a high density of cosmic rays at a level greatly exceeding the density in our own galaxy, the Milky Way.
This information didn’t come easily. VERITAS couldn’t detect M82’s cosmic rays directly because they were too far away, locked inside the Cigar Galaxy. Instead, VERITAS looked for clues to the presence of cosmic rays in the form of gamma rays, which are the most energetic form of light. Created when cosmic rays interact with interstellar gas and radiation within M82, gamma rays can reach Earthbound detectors, appearing as a shower of particles.
“Our telescopes look up at the sky and can actually see this shower in the atmosphere,” Ong explained. “By looking at it in the atmosphere, they can figure out where the shower originated and where the gamma rays came in to start the whole process. The telescopes can point back and find out where the shower came from in space and can also measure the energy of the gamma ray.”
Image of M82
The starburst galaxy M82, also known as the “Cigar Galaxy,” sits about 2 million light-years from Earth, in the direction of the Ursa Major constellation.
Ong and his UCLA colleagues — Associate Professor Vladimir Vassiliev, two postdoctoral researchers and several graduate students — are supported by the National Science Foundation for their work with VERITAS. In addition to observing and analyzing the data, the UCLA scientists built various electronic components of the telescope array, which were fabricated here.
The VERITAS collaboration comprises an international team of more than 100 scientists from 24 different institutions in the United States, Canada, the U.K. and Ireland. Many of the scientists log on to UCLA every day to retrieve data —Ong and his colleagues also developed much of the software required for the project and are responsible for storing all of the data from VERITAS in one huge archive on the UCLA campus.
This discovery provides new proof that not all gamma rays are dependent upon the existence of a black hole. Prior to 2009, all of the known VHE (very high energy) gamma-ray sources from outside of our galaxy were associated with active galaxies that are powered by matter falling into a supermassive black hole. As matter falls into the black hole, it gives off energy; scientists believe that some of the energy released acts as a supercharging force that accelerates particles to high speeds, creating cosmic rays.
While our galaxy has a black hole, it’s not a very active one, said Ong.
“So these high-energy particles that are coming down to us as cosmic rays couldn’t have been created [in the Milky Way’s black hole]. They must have been created someplace else,” he added. “In the VERITAS observations of this nearby starburst galaxy M82, we have no evidence that there’s any activity due to a black hole.”
“This is the first evidence we’ve seen,” Ong said, “of another galaxy creating gamma rays through what we consider ‘ordinary’ processes that could also happen in our galaxy.”
For more information on VERITAS, click here.
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