February 19, 2013 | 1
The cosmos is full of surprises—not a week goes by without some group of astronomers announcing a perplexing new discovery that upends theory or expectation. But equally important is the difficult and time-consuming research required to firmly pin down what astronomers think they already know.
Take, for instance, a new study on the origins of cosmic rays in our galaxy. These high-energy particles, mostly protons, bombard Earth from all directions. Astrophysicists have long suspected that expanding shock waves from ancient supernovas—stars that exploded many thousands of years ago—accelerate protons to high speed, launching them into the galaxy to eventually collide with Earth. But it took a team of researchers four years on one of NASA’s premier space-borne observatories to back that suspicion up with hard evidence.
Stefan Funk and Yasunobu Uchiyama of Stanford University, Takaaki Tanaka of Kyoto University in Japan and their colleagues used an instrument on the Fermi Gamma-Ray Space Telescope to monitor two supernova remnants, known as IC 443 and W44, which exploded some 10,000 years ago relatively nearby in the Milky Way. Gamma rays are the highest-energy variety of photons, packing millions or even billions of times as much energy as a photon of visible light.
“Using gamma rays that we detect with the Fermi Large Area Telescope, we have shown that cosmic rays are accelerated in supernova remnants,” Funk said at a press briefing Webcast from the American Association for the Advancement of Science conference in Boston. “The gamma rays we detect from two supernova remnants have a unique smoking gun signature that now for the first time provides incontrovertible evidence that they are accelerating protons.” The researchers published their findings in the February 15 issue of Science.
The “smoking gun,” as Funk put it, was a deficit of low-energy gamma rays compared to their more energetic counterparts in the spectrum of photons emitted by the supernova remnants. That signals a gamma-ray origin from the decay of particles called neutral pions, which are produced when high-energy protons (from a supernova shock wave, say) collide with more pedestrian protons in dense clouds of interstellar gas. The production of neutral pions at the two supernova remnant sites thus signals that the objects have indeed accelerated protons to tremendous speeds.
Astrophysicists must rely on observational proxies such as neutral pions and the gamma rays they produce because the cosmic rays themselves—the high-energy protons—carry an electric charge and are therefore deflected by magnetic fields as they race through the galaxy. “They don’t point back to where they come from,” Funk said of cosmic rays. “We therefore have to turn to neutral messengers.”
Therein lies the appeal of gamma-ray photons, which carry no electric charge. “These gamma rays can be produced by energetic protons and then travel in straight lines and tell us where the protons are accelerated, where the cosmic rays are produced,” Funk added.
The objects that Funk and his colleagues studied both glow brighter in gamma rays than any other supernova remnants, which made them obvious targets for the search. But even still, parsing the gamma-ray output from the two glowing shells of material surrounding the sites of now-deceased stars took some time. “The problem is that the signature we are looking for is at the very bottom end of the energy range of the detector,” Funk said. “And at these low energies the gamma rays don’t leave a lot of information in the detector.”