Astronomers have long thought that supernovas are the source of cosmic rays, but there's a troubling discrepancy between theory and measurements. Could a balloon flight over Antarctica have shed new light on the mystery?

by Dr Tony Phillips

Hold out your hand for 10 seconds. A dozen electrons and muons just zipped unfelt through your palm. The ghostly particles are what scientists call "secondary cosmic rays" - subatomic debris from collisions between molecules high in Earth's atmosphere and high-energy cosmic rays from outer space.

Cosmic rays are atomic nuclei and electrons that streak through the Galaxy at nearly the speed of light. The Milky Way is permeated with them. Fortunately, our planet's magnetosphere and atmosphere protects us from most cosmic rays. Even so, the most powerful ones, which can carry a billion times more energy than particles created inside atomic accelerators on Earth, produce large showers of secondary particles in the atmosphere that can reach our planet's surface.

Where do cosmic rays come from? Scientists have been trying to answer that question since 1912, when Victor Hess discovered the mysterious particles during a high altitude balloon flight over Europe. Galactic cosmic rays shower our planet from all directions. There's no definite source astronomers can pinpoint, although there is a popular candidate.

"Most researchers are betting that cosmic rays come from supernova explosions," says Jim Adams of the NASA/Marshall Space Flight Centre. When massive stars explode they blast their own atmospheres into space. The expanding shock waves can break apart interstellar atoms and accelerate the debris to cosmic ray energies. Cosmic rays are subsequently scattered by interstellar magnetic fields -- they wander through the Galaxy, losing their sense of direction as they go.

"It takes an awful lot of power to maintain the galactic population of cosmic rays," says Adams. "Cosmic rays that lose their energy or leak out of the Galaxy have to be replenished. Supernovae can do the job, but only if one goes off every 50 years or so." The actual supernova rate is unknown. Observers estimate that one supernova explodes somewhere in the Galaxy every 10 to 100 years - just enough to satisfy the energy needs of cosmic rays.

But there could be a problem with the supernova theory, says Adams.

"A supernova blast blows a bubble in the interstellar medium that grows until the shock wave runs out of energy," he explained. "They can accelerate particles up to some point, about 10¹⁴ electron volts (eV) per nucleon, but not beyond that. Below that level all of the different cosmic ray species - protons, helium nuclei, etc; should have the same kind of energy spectrum: a power law with index around -2.7."

A "power law" spectrum is one that looks like a straight line on a piece of log-log graph paper. In the energy range ~10¹⁰ eV to 10¹⁴ eV, the supernova theory of cosmic ray acceleration predicts that the power law spectrum of protons should have the same slope as the power law spectra of heavier nuclei (about -2.7).

The problem is when scientists compare the energy spectra of protons and helium nuclei, the two don't resemble one another as much as they should. Both are power laws, as expected, but "existing data indicate a possible spectral index difference between protons and helium of about 0.1," says Eun-Suk Seo, a cosmic ray researcher at the University of Maryland. "The [slope of the] proton spectrum is close to -2.7, but the energy spectra of helium and heavier nuclei seem to be flatter. The difference is small and it might not be statistically significant." If there is a genuine discrepancy, she added, it could signal trouble for supernova models of cosmic ray acceleration.

To find out if the supernova theory is indeed in peril, a team of scientists led by John Wefel ( Louisiana State University) and Eun-Suk Seo, and aided by personnel from the National Science Balloon Facility, launched a helium-filled balloon from McMurdo, Antarctica on Dec. 28, 2000. The payload, which soared to120,000 feet above Earth's surface, included a NASA-funded cosmic ray spectrometer known by its builders as the Advanced Thin Ionisation Calorimeter or "ATIC" for short.

"ATIC is sensitive to cosmic rays with energies between ~10¹⁰eV and 10¹⁴eV," says Wefel. By covering such a wide range of energies with a single modern spectrometer, the team hopes to measure the proton and helium cosmic ray spectra with better precision than ever before.

 

The ATIC payload hangs from a launch vehicle while the helium balloon is being filled in the background by personnel from the National Science Balloon Facility.

The higher energy cosmic rays are rare," he continued. "For example, each day ATIC collects no more than ~10 cosmic rays with energies exceeding 10¹³ eV. That's why we have to fly the balloon for such a long time, to gather enough particles for a statistically significant result." By the time ATIC landed in mid-January 2001, the spectrometer had been in the stratosphere counting cosmic rays for nearly two full weeks.

The long flight time, more than any other reason, is why the researchers chose to fly the balloon over Antarctica. "We would be happy to fly this payload over North America," says Adams. "The problem is that we needed the spectrometer to be aloft for a long time. Antarctica has two advantages: It's international territory, so we don't need to apply for lots of overflight permissions, and the Antarctic Vortex (a circulating weather system around the south pole) keeps the balloon confined to airspace over the continent."

"If there is a difference between the proton and the helium spectra - and that's not certain, by the way - it won't necessarily kill the supernova model," continued Wefel. "But a discrepancy would cause problems." Theorists may have to consider the progress of supernova shock fronts in greater detail. "Every supernova explosion is an individual work of art," says Adams. "We use mathematical models that assume the explosions are spherical, but they are not. Within the blast wave itself you can see irregularities. There are bright knots, for example, where shock waves run into interstellar clouds. In crowded groups of massive stars ('OB associations') where supernovae can occur in quick succession, blast waves collide with other blast waves." It can get a little messy! Modelling such details might affect any necessary reconciliation between the theory and the data.

The ATIC balloon payload.

 

And what if the supernova model can't be rescued? "There are other possibilities," says Wefel, "but not a lot of good ones. We'll really have to look hard to find something other than supernovae that can meet the cosmic ray energy requirement."

The analysis team led by Eun-Suk Seo hope that the new particle count will be the most accurate to date in Alice's energy range, and could shed new light on the decades-old mystery of cosmic rays.