The result was made possible by applying a new analysis technique to data from two different observational teams using distinct approaches to finding and measuring powerful cosmic rays. The analysis will enable physicists to explore in a sustained and focused fashion how such energetic particles are produced.
The data upon which this work is based were obtained by the Akeno Giant Air Shower Array (AGASA) experimental collaboration in Japan and the National Science Foundation's High Resolution Fly's Eye Air Fluorescence Detector (HiRes) collaboration in Utah. The hints of possible clusters of cosmic ray occurrences detected by AGASA and HiRes individually were each too weak to be statistically convincing, and previous analysis methods were unsuitable for studying the combined dataset. In a paper in press (Astrophysical Journal), Farrar and the HiRes collaboration used the "method of maximum likelihood" to analyze the combined AGASA and HiRes datasets, revealing a tightly clustered group of four ultra-high energy events from one segment of the sky. The probability that such a well-correlated cluster of particles arises simply by coincidence is calculated to be a less than one in a hundred. Nevertheless, the authors caution that such coincidences do happen and note that a posteriori probability estimates are hazardous, so stress that more data are needed before firm conclusions can be drawn.
Farrar reported today on two follow-up studies. In one project, Farrar, Andreas Berlind, and David Hogg of NYU studied the environment of the source and the intervening region using data from the Sloan Digital Sky Survey• and information from NASA's Extragalactic Database. This work clarifies how the observed bundle of particles could have come so far through space without being jostled apart, and it identifies potential sources of the cosmic rays.
In the other project, Farrar extended the Maximum Likelihood analysis to infer the properties of the magnetic fields in space between the source and Earth. This enabled lower energy data to be included in the analysis, uncovering yet another event likely to come from the same source. The group of five events already teaches us a lot about what and where the source may be, according to Farrar.
Until now, the only way astrophysicists could examine the cosmos has been to study light (electromagnetic energy) emitted and absorbed by stars, gas, and dust in a variety of wavelengths from low energy radio waves to optical photons and high energy X-ray and gamma rays. The bundle of ultra-high energy cosmic particles announced today, assuming it is not a statistical fluke, is therefore a breakthrough for cosmic ray astronomy. This is the first time material particles from the distant universe have been identified which have propagated together from their origin in some cataclysmic event without straying apart and losing the memory of their source. Exploring them in greater detail and with more data will give a unique perspective on extreme events such as the creation of black holes.
Cosmic rays of ordinary or high energy are common and well-understood. However the nature and source of the highest energy cosmic rays--with energies above a few times 1019 electron Volts (few tens of Ecta-eV)--has been a mystery. The highest energy particles are extremely rare--just one hits the Earth per square kilometer per century--and only about 100 have ever been observed. Some have almost a trillion times more energy as ever achieved in the highest-energy man-made accelerators, or an energy equivalent to concentrating the kinetic energy of a bowling ball, with thousands of trillions of protons and neutrons into a single proton.
The very existence of such particles presents a quandary to physicists. Few if any astrophysical systems are capable of accelerating particles to such energies. The most likely candidates would be gamma ray bursts or active galactic nuclei also known as blazers and quasars. However, according to the laws of special relativity, the ultrahigh energy cosmic rays (UHECRs) produced by such phenomena should lose energy when they collide with the omnipresent cosmic microwave background radiation left over from the Big Bang.
Therefore, unless the standard laws of physics are violated at these otherwise inaccessible energies, the highest energy UHECRs we see must have originated within about 50-100 million light years of Earth. Yet no plausible source had been identified so nearby in the direction of the highest energy UHECRs previously observed. This leads some theorists to propose that the UHECRs are not produced by astrophysical accelerators, but are the by-products of the decay of invisible but super-heavy relics of the Big Bang waiting to be discovered in the extended halo of the Milky Way, or by some other exotic process. The new discovery should help clarify just what is the source of these ultra-high energy particles.
"Nature has been very, very kind in presenting us with this opportunity to unravel one of the real mysteries of physics," said Farrar. "If, as widely supposed, tumultuous magnetic fields fill most of the cosmos, then charged particles such as these ought to be deflected when traveling to Earth and would not point back to their sources. The discovery of this bundle of cosmic rays seems to tell us that there is at least one direction in which the fields are sufficiently weak that the particles are not dispersed - just as one can sometimes see a patch of blue sky through a break in the clouds."
"By incredible luck, the source of this handful of ultra-high energy particles is in just the right direction to be seen clearly," she said. "Thanks to this, we will be able to simultaneously study the nature of the magnetic fields between galaxies and the nature of the source. The experimental teams and the pioneers before them deserve tremendous praise for developing the techniques needed to do this science."
The High Resolution Fly's Eye is the only UHECR observatory presently in operation in the northern hemisphere. The Pierre Auger Observatory in Argentina--an international collaboration of several hundred physicists supported in part by the National Science Foundation--is just coming online. It has 10 times greater sensitivity than previous experiments and will report its first results next summer. Given its location in the southern hemisphere, the Auger Observatory cannot see the new source reported here (which is in the vicinity of the Big Dipper), but it will view the center of the Milky Way Galaxy and several nearby superclusters of galaxies. Astrophysicists are waiting eagerly to know whether other identifiable bundles of UHECRs will be seen in those regions.
An intriguing note is that the energy distribution of particles arriving at Earth during a few-year period is completely different if UHECRs are created in a "burst"--as in the collapse of a massive star to a black hole--than if they are accelerated more slowly in a continuous process such as when matter accretes onto a pre-existing supermassive black hole. Farrar points out that when her analysis technique is applied to the lower energy AGASA data that have not yet been released, no new events would be expected for a burst, such as a gamma ray burst, while a number of new events should be discovered if the source is continuous such as an AGN. Interestingly, the information so far is consistent with either a GRB or AGN as possible source.
While the cluster of events studied in this work are not yet statistically compelling, they auger well for the future of cosmic ray astrophysics. Ten times more data, which would become available if the northern hemisphere extension of the Pierre Auger Observatory were constructed, would allow unambiguous confirmation or rejection of the hypothesis these events are from a single source, and would allow detailed study of the nature of the source and the magnetic fields along the particles' trajectory to Earth.
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