Iron-ic twist deepens cosmic ray puzzle

BLOIS, France — In the genteel surroundings of the Blois chateau, home to 17th century French royalty, a controversial finding about the highest-energy cosmic rays has landed with a thud. If confirmed, a new report could spark a revolution in the way astronomers think about these speedy but rare charged particles, which carry as much oomph as a big league pitcher’s fastball.

Scientists have generally assumed that the most energetic cosmic rays are primarily protons. That’s true even though heavier nuclei such as iron are more easily accelerated to high energies because of their greater electric charge. Heavy nuclei, however, are also more fragile and the extraordinarily violent processes that rev them up to enormous energies can also cause these nuclei to fragment. Collisions with photons left over from the Big Bang or with intense infrared radiation from stars, for example, can easily break massive nuclei into lighter particles. Even if nuclei managed to leave their region of origin intact, they are still susceptible en route to Earth.

“Ask anybody what are the highest-energy [cosmic ray] particles, and they’d say ‘protons,’ ” says physics Nobel laureate James W. Cronin of the University of Chicago. But, as he announced June 22 at the Windows on the Universe meeting, the Pierre Auger Observatory in Malargüe, Argentina, has identified an abundance of iron nuclei at some of the highest energies its cosmic ray detectors can record.

From just above 10 million trillion electronvolts to three times that energy, the number of iron nuclei appears to rise steeply, with heavy nuclei ultimately dominating the cosmic ray population, Cronin reported.

“We have been sitting on this data for two years trying to be sure it’s correct,” Cronin said. He and his colleagues have posted these and other new Auger findings online (arxiv.org/abs/0906.2319 and arxiv.org/abs/0906.2189).

“It’s a surprise,” says theorist Todor Stanev of the University of Delaware in Newark.

The data are particularly puzzling because it’s unclear what the source of the iron could be, notes Pierre Sokolsky of the University of Utah in Salt Lake City. The disks of material that surround and feed supermassive black holes at the center of galaxies are a likely source for the generation of high-energy cosmic rays. But those disks consist primarily of protons and some helium, not iron.

Supernovas forge iron, and shock waves from these exploded stars could rev up the heavy nuclei to high energies. Still, “It’s hard to see how you get iron when you don’t have that much [of it] in the first place,” Sokolsky says.

Moreover, Sokolsky reported at the meeting that a final analysis of data from a separate cosmic ray experiment called the High Resolution Fly’s Eye at the U.S. Army Dugway Proving Ground in Utah shows only protons at high energies. Sokolsky also noted that Fly’s Eye, which stopped operating in 2006, found that energetic cosmic rays are distributed randomly across the sky. That’s also in contrast to the Auger results, which suggest that about 40 percent of the highest energy particles detected can be traced back to galaxies housing giant black holes (SN Online: 5/4/09).

One explanation for why some of the cosmic rays detected by Auger are associated with galaxies is because the southern sky, which Auger scanned, contains Centaurus A, the nearest galaxy known to house a supermassive black hole. The northern sky, which Fly’s Eye scanned, has no such nearby counterpart.

But “before we start talking about north and south differences, we ought to be sure that the data are OK,” Sokolsky says.

Because energetic cosmic rays striking Earth’s atmosphere create a cascade of other particles, astronomers can only indirectly fingerprint the rays’ composition. But astronomers can generally distinguish between heavy and light cosmic rays by examining the altitude at which the rays collide with nuclei in the atmosphere and generate a shower of other particles.

Iron, because of its greater mass and charge, tends to interact with the nitrogen in Earth’s atmosphere at higher altitudes than do protons. The range of altitudes at which different iron nuclei decay is also narrower than it is for lighter nuclei.

It’s easy for errors to creep in that make the range of altitudes appear broader, but difficult to generate errors that would make the range narrower, as seen in the Auger observations, Cronin said. The Auger observations that indicate iron combine data from two detectors. Telescopes record the height at which nitrogen in Earth’s atmosphere fluoresces in response to an incoming cosmic ray. Then, water tanks on the ground record secondary particles created by the cosmic rays. 

In contrast, Fly’s Eye used pairs of telescopes to generate stereo observations of cosmic rays but did not have a ground-based water tanks.

Stanev says that Auger’s equipment is “quite a bit better” at assessing fluctuations than Fly’s Eye. He also says he’s hoping that further observations with Auger will continue to show a correlation between energetic cosmic rays and the location of particular galaxies with black holes. “Otherwise, we’re lost” in trying to trace the origin of these most energetic of particles, he says.

In addition to understanding the location of the source of cosmic rays, determining the composition of these energetic particles are essential to figuring out their origin, Sokolsky says.