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Earth is under siege from outer space! In a way. We get peppered by speedy particles, called cosmic rays, all the time. Some come from the sun, some from supernovas and some via solar-style winds emanating from far-off massive stars. Anything that can accelerate a proton to nearly light speed does the trick.
But all cosmic rays are not created equal. The most energetic among them are single atomic nuclei that strike Earth's atmosphere with the energy of a well-thrown baseball. The source of these ultraenergetic cosmic rays is unclear. But, thanks to a new study, one of the leading candidates has probably been ruled out.
Gamma-ray bursts occur when a massive star collapses in on itself. It had been thought that these ultra-energetic bursts might produce super high-energy cosmic rays. But a giant Antarctic neutrino detector called IceCube has been looking for neutrinos that should accompany the high-energy cosmic rays from a gamma-ray burst. And it hasn't seen any, researchers announced in the journal Nature. [The IceCube Collaboration,"An absence of neutrinos associated with cosmic-ray acceleration in gamma-ray bursts"]
There is another possibility: that flaring black holes are what's hurling cosmic baseballs our way. That idea is looking better now that gamma-ray bursts have been called out.
—John Matson
[The above text is a transcript of this podcast.]
[Scientific American is part of Nature Publishing Group.]
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3 Comments
Add CommentWhat is meant by 'flaring' black holes? I haven't seen this term used before. Is it meant to describe the relativistic jets of active galactic nuclei, by chance? Besides GRBs, what other potential source for such
Reply | Report Abuse | Link to thisenergetic massive particle are there?
Google "black hole flare" without the quotes; they flare when stuff falls from their accretion disks into the hole, emitting gamma rays and energetic particles, apparently largely from their poles.
Reply | Report Abuse | Link to thisAs for other candidates, the line in the article "Anything that can accelerate a proton to nearly light speed does the trick" covers it, so any cosmological object or process that can manage it qualifies. Consider supernovae, GRBs in other galaxies, merging black holes, and so on. Identifying them by trying to associate cosmic ray events with neutrino bursts might not help much since particle paths can easily be altered by large-scale magnetic fields while neutrinos aren't.
Thanks - that's essentially the same effect produced by AGNs.
Reply | Report Abuse | Link to thisAs the article states, "The source of these ultraenergetic cosmic rays is unclear." As the
research report abstract states, "Very energetic astrophysical events are required to accelerate cosmic rays to above 1018 electronvolts."
The abstract goes on:
"In the GRB ‘fireball’ model, cosmic-ray acceleration should be accompanied by neutrinos produced in the decay of charged pions created in interactions between the high-energy cosmic-ray protons and [gamma]-rays.
"...Here we report an upper limit on the flux of energetic neutrinos associated with GRBs that is at least a factor of 3.7 below the predictions. This implies either that GRBs are not the only sources of cosmic rays with energies exceeding 1018 electronvolts or that the efficiency of neutrino production is much lower than has been predicted."
Critically, it's the shortfall between the number of "ultraenergetic" proton associated neutrino detections and the predictions of the "GRB ‘fireball’ model" that the researchers are using to conclude that GRBs are not producing all of the "ultraenergetic" cosmic rays.
An alternative possibility is simply that the "GRB ‘fireball’ model" is not correct. Unless the model has been definitively confirmed, IMO this is the most likely scenario.
Another alternative is that there is a problem in the neutrino detection process.
If the researchers' conclusions are correct, there is some other event(s) producing sufficient quantities of cosmic rays accelerated to energies exceeding 1018 electronvolts to explain observations that does not produce neutrinos.
As I understand, the observed GRBs are almost certainly produced in distant galaxies. Would supernovae or merging black holes occur in frequencies sufficient to produce the additional "ultraenergetic" cosmic rays observed? I think there would be very few remaining known cosmic ray sources that would qualify...