Super Wallops: Tracking the origin of cosmic rays

When it comes to revving up subatomic particles, the heavens leave the biggest particle accelerator on Earth in the dust. Our galaxy abounds with charged particles carrying energies that are thousands to millions of times as high as those that the most powerful atom smashers can muster. Known as cosmic rays, the particles–mostly protons–heat and ionize the interstellar medium, profoundly altering its chemical composition.

ULTIMATE POWERHOUSE? Hosting a dormant quasar and a supermassive black hole, this galaxy may be a source of ultra-high-energy cosmic rays. NASA/HST/T. Hamilton

Two reports this week shed light on the longstanding mystery of where these particles come from.

One study focuses on lower-energy cosmic rays that originate within our galaxy and have energies up to 1,000 trillion electronvolts. The findings support the popular notion that the particles are generated by shock waves from supernovas, the explosive death of massive stars.

That scenario has been difficult to prove because magnetic fields in the Milky Way divert these cosmic rays from their original paths. Even so, researchers had previously demonstrated that supernova remnants–expanding shells of jettisoned material–can accelerate electrons to cosmic ray energies. But there was no evidence that protons are accelerated by the same mechanism.

In the April 25 Nature, Ryoji Enomoto of the University of Tokyo in Kashiwa, Japan, and his colleagues report that they have for the first time associated a supernova remnant with cosmic ray protons. When high-energy protons collide with atoms and molecules in space, they create a short-lived subatomic particle called a neutral pion. Its decay produces gamma rays with energies of a trillion electronvolts. When those slam into Earth’s atmosphere, they generate showers of visible-light photons. On several occasions, Enomoto’s group detected photon showers emanating from the patch of sky that contains a supernova remnant called RX J1713.7-3946. Their spectra indicated that they were generated by protons.

If the findings are confirmed and astronomers can demonstrate that the supernova remnant is typical, “the production of cosmic rays within our galaxy could be conclusively linked to the aftermath of supernovas,” says Felix Aharonian of the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, in a commentary accompanying the Nature report.

Another study examines the much rarer ultra-high-energy cosmic rays, which originate outside our galaxy and rank as the most energetic particles known. Packing as much punch as a major league baseball pitch, these particles have such high energies–up to a million trillion electronvolts–that our galaxy’s magnetic field can’t divert them.

The source of these cosmic rays has proven elusive. Researchers now report the first experimental links between ultra-high-energy cosmic rays and possible sources.

Analyzing data from high-energy cosmic ray detectors in Japan and England, Elihu Boldt of NASA’s Goddard Space Flight Center in Greenbelt, Md., and his colleagues have traced the trajectories of several of the particles to four galaxies known to surround dead or dormant quasars and suspected to contain supermassive black holes.

Boldt reported the findings at a joint meeting of the American Physical Society and the American Astronomical Society in Albuquerque.

The finding fits with a scenario in which a spinning, supermassive black hole acts like a giant battery. Magnetic field lines in close contact with the rotating hole would generate a billion trillion volts, which accelerate charged particles to ultrahigh energies. In this theory, the quasar must be dormant. If the cosmic rays revved up by the black hole were to collide with intense radiation from an active quasar, their energy would be drained away.

The findings “are the first indication of a correlation between candidate objects and actual events,” says Michael L. Cherry of Louisiana State University in Baton Rouge.

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