Ultra-High-Energy Cosmic Rays Explained?

I have a bunch of science news sources in my RSS feeds, and every evening, I scan through the accumulated articles to try to figure out what physics-related stories there are to talk about. Sometimes, it's hard to find anything, but other days, you get stories that lead to four press releases at EurekAlert (one, two, three, four), a write-up at Backreaction, a Physics World news story, and a Dennis Overbye piece in the New York Times. I guess I really ought to say something about the new results from the Pierre Auger Observatory, published this week in Science (Science 318, 938 (2007), if you have access).

Some background: the Auger (pronounced "Oh-zhay," not like a tool one uses to drill holes, because it's French) Observatory is a collection of large particle detectors spread over a few thousand square kilometers in a remote part of Argentina. These detectors, plus a half-dozen telescopes at each site, are used to pick up the "particle showers" caused by high-energy cosmic rays striking the atmosphere.

The term "cosmic ray" is a little deceptive, as it suggests some form of light. In fact, these tend to be charged particles, many of them protons, that slam into the atmosphere at high speeds. When they hit, they interact with other particles in the atmosphere, and convert their kinetic energy into... stuff. They produce large numbers of new subatomic particles, which subsequenctly decay into other things, and collide with other particles creating new particles which decay, and so on. You get a shower of exotic particles, and their passage through the atmosphere creates faint flashes of light that can be picked up by the Auger detectors.

If you look at how much stuff is produced in these showers, you discover that some of these cosmic rays have an astonishing amount of energy, upwards of a joule. You're talking about a single proton with the kinetic energy of a baseball thrown by a moderately good pitcher. Or, if you'd like that in particle physics units, more than 10¹⁹ eV, or roughly a million times more than the energy that will be produced at the LHC.

These ultra-high-energy cosmic rays were noticed a long time ago, and nobody has really had any clue where they could be coming from, because that's a ridiculous amount of energy. The Auger observatory was built specifically to detect these UHECR's, and after three and a half years of operation, they think they've got it figured out.

The trick here is to trace the particles back to their source. This isn't quite as simple as it sounds, because we're not picking them up like light from a telescope, where we know exactly the direction they were heading. We see the UHECR's indirectly, through the showers they produce, and that makes it difficult to determine the source.

The Auger collaboration manages this through a combination of detector size and precise timing. The detector consists of about 1600 water tanks on the ground that serve as particle detectors, laid out in a triangular array with the detectors separated by about a kilometer and a half. As particles from a shower reach the ground, they are detected, and their arrival time is recorded. This allows the direction of the iniital particle to be reconstructed, and knowing where the Earth was at that particular instant lets them trace it back to a specific point in the sky.

They do amazingly well with this, stating that: "Events that triggered at least six surface stations have energies above 10 EeV and an angular resolution better than 1°" (1 EeV = 10¹⁸eV = 0.16 J). One degree is pretty big in astronomical terms-- astronomers like to talk about minutes and seconds or arc-- but given what they're working with, that's pretty impressive.

The trick, then, is to try to match these particles up with sources in the sky. One possible source for particles with these ridiculous energies would be "active galactic nuclei," which is to say, gigantic black holes in the center of galaxies, where the high temperatures and huge magnetic fields can possibly accelerate particles to the sort of energies seen in the UHECR's. They compare their set of detected UHECR's to a sky map of known active galactic nuclei, and find a significant correlation between the two: they're more likely to see high-energy particles coming from the general direction of known AGN's than not.

To be quantitative about it, they write that "Strong correlation signals occur for energy thresholds around 60 EeV and several combinations of the other parameters in the range 6°, and z_max 0.024 (D_max < 100 Mpc)." Or, to put that in English, most of the particles they detect at energies above 6 x 10¹⁹ eV appear to be coming from a point within 6 degrees of a known AGN that is within about 100 megaparsecs of Earth. The correlation is even better for the highest energy particles: for these, "12 events among 15 [were within 3.1 degrees of] the selected AGN, whereas only 3.2 were expected by chance if the flux were isotropic."

This is fairly convincing evidence that the ultra-high-energy cosmic rays we see are coming from (relatively) nearby galaxies containing giant black holes. Now, there are a couple of important caveats, here, chief among them being the fact that they're dealing with a tiny number of events-- they slice the data up a lot of different ways, with different lower energy limits, but the largest number I saw in the papaer was 81 particles. There were either 15 or 27 particles in their highest energy category (there are two different counts given, reflecting slightly different energy threholds, I think). It's possible-- not terribly likely, but possible-- that there's some statistical weirdness going on.

Of course, their AGN map isn't comprehensive, either, so it may be that they understate the correlation-- there might be AGN's that weren't included in the survey the used that would correlate with some of the particles that don't have obvious sources. Then again, some of the particles they see correlate with more than one source, so it's a little difficult to make a solid identification for any of them.

Most of these issues will be solved with more time. The data they have comes from a bit more than three years of operation of the observatory, but they can only really run on moonless nights, and there's been a lot of fiddling around with the detectors. They say toward the end of the paper that this really amounts to 1.2 years of full operation. As that number increases, they'll get more particles, and start to be able to make more sensitive tests of the correlation.

As preliminary measurements go, though, this is awfully good.

One final note: the distance cut-off in the statement quoted above is significant: there's a theory developed by Greisen, Zatsepin, and Kuzmin that says that particles at the highest measured energies should not be able to travel all that far without scattering off photons of the cosmic microwave background. This is discussed in some detail at Backreaction, but the basic idea is that there ought to be a sharp drop in the number of particles traveling more than about 100 megaparsecs above an energy of 10¹⁹ eV.

Their results are consistent with this theory:

90% of the protons arriving at Earth with energy exceeding 60 EeV originate from sources closer than 200 Mpc. This (somewhat arbitrarily defined) "GZK horizon" decreases rapidly with increasing energy and drops to 90 Mpc for energies exceeding 80 EeV.

"Consistent with" is a sort of scientific weasel phrase meaning "this agrees with the theory, but doesn't conclusively prove anything." They don't really have enough particles in their dataset to do the analysis that you really want to check this. To test the GZK theory accurately, you would want to see multiple detections from the same source, enough so you could make a stab at reconstructing the energy spectrum. Then you would compare spectra from sources at different distances, and hopefully see that the more distant sources produed fewer particles at the really high energies than the nearer sources. This, again, is just a matter of time.

So, in summary: big news from the Auger collaboration. They have fairly convincing evidence that ultra-high-energy cosmic rays are coming from active nuclei of other galaxies. They also see a hint of what might be the expected GZK cut-off. These are early results, and will only improve with time.

And, as a bonus, it's remarkably readable for a particle astrophysics paper. There are pretty pictures, too, so check it out if you have access to Science.