London, May 14 : A new study has cast doubts on the origins of cosmic rays, suggesting that they might not have come from a set of giant black holes in nearby galaxies, as was claimed earlier by scientists.
According to a report in New Scientist, ultra-high-energy cosmic rays, or UHECRs, are individual sub-atomic particles with energies up to about 1020 electron volts.
When they hit the Earth's atmosphere, they produce a shower of other particles, and the Pierre Auger Observatory in Argentina has spotted more of these events than any other detector.
In November 2007, an Auger team looked at the arrival directions of the 27 highest-energy cosmic rays, and found that they fit a suggestive pattern.
Most came from within 3 degree of the directions of nearby active galaxies, which hold supermassive black holes at their cores and emit many kinds of radiation. So, it seemed that the galaxies were emitting UHECRs too.
Now, researchers led by Igor Moskalenko of Stanford University in California, US, have looked more closely at these particular active galaxies, as well as others along the same line of sight, to conclude that they are an unremarkable bunch.
"The sample consists mainly of low-power active galaxies," said Moskalenko.
Such weak active galaxies are son common that astronomers expect to find several within 3 degree of any random direction on the sky.
"The correlation found by the Auger group is likely to be a chance coincidence," Moskalenko told New Scientist.
According to Moskalenko, small, weak active galaxies simply lack the firepower to generate the highest energy cosmic rays. For one thing, they show no signs of high-energy gamma-ray emission, which he believes should go along with cosmic-ray acceleration.
Instead, he suggests that UHECRs are more likely to come from the more energetic breeds of active galaxy, such as quasars and radio galaxies, especially those that squirt out high-speed jets of material, which are already known to emit gamma rays.
Some of the cosmic rays seen by Auger do coincide with examples of this tooled-up type of active galaxy. At least four of them could have been fired at us by Centaurus A, a radio galaxy only 12 million light years away.
Moskalenko and colleagues say the other UHECRs could also be from nearby active galaxies with jets, even if their arrival directions don't coincide with such sources.