|Messengers from Distant Galaxies
High-energy cosmic rays reveal mystery of their origin
Every now and then subatomic particles slam into the Earth’s atmosphere packing energy 100 million times greater than can be achieved in the world’s most powerful particle accelerators. Scaled up to the size of a pea, these particles would have energy equivalent to 270,000 billion Boeing 747s traveling at 600 mph. For decades these powerful cosmic rays have remained an alluring and paradoxical mystery to astrophysicists. Although their extraordinarily high energies are evidence that they originate from some of the most extreme events in the universe, their source has eluded astronomers and astrophysicists since their discovery more than 40 years ago.
However, an international team of scientists that includes Lawrence Wiencke and Fred Sarazin of the Colorado School of Mines Physics Department has linked these unusual particles to a particular class of nearby galaxies that have active nuclei in their centers. Active galactic nuclei (AGN) are thought to be powered by massive and tumultuous black holes that consume stars and gas, and may emit jets of plasma far into intergalactic space. The team’s findings are reported in a paper published in the November 9, 2007 edition of Science.
Making the link between high-energy cosmic rays and AGN was not easy. Made up of single protons or atomic nuclei, almost all cosmic rays carry a small electrical charge. As these charged particles travel through the magnetic fields that permeate space, their paths are altered. While scientists can now calculate the trajectory of certain cosmic rays that collide with the Earth’s atmosphere, the information is meaningless if they haven’t come in a straight line from their origin. However, a small class of the most powerful cosmic rays (those with energies over 4 x 1019 electron volts, or 40 EeV) are much less subject to deflection. They are also extremely rare—a few per square mile per century.
To collect a reasonable amount of data on such a rare phenomenon, the Pierre Auger Southern Observatory was constructed in Argentina to span 1,200 square miles. Not surprisingly, it is the largest observatory ever constructed. But even with an aperture of this size, the observatory records the passage of just a few dozen ultra-high-energy cosmic rays per year—80 events with energies over 40 EeV have been detected at Auger since 2004. However,the data from these events make such a compelling case that scientists on the team are sharing their findings and telling the fascinating story of how they were obtained.
The trajectories and energy of cosmic rays are actually extrapolated by recording the arrival of secondary particles created by the impact of the cosmic ray. When a cosmic ray slams into our atmosphere,it creates a fluorescent shower of subatomic particles, mostly muons, that fan out to cover about a 15-square-mile area on the Earth’s surface.
The observatory measures these secondary particles with two types of detectors. A grid of 1,600 individual particle detectors spaced one mile apart records a particle shower’s arrival to a tenth of a microsecond (1/10,000,000 of a second). By also recording the level of luminescence created when the particle shower passes through a large tank of water, these same detectors collect information on the intensity of the shower. As an incoming shower will be registered at several stations, the minute differences between arrival times provide adequate data to calculate the trajectory of the original cosmic ray to within one or two degrees—the more powerful the cosmic ray, the more accurate the estimate.
The second type of detector records the amount of fluorescence light emitted by the impact of a cosmic ray, but it will only do so on clear moonless nights. Although most nights aren’t clear and moonless, the minority of events that are captured provide relatively precise information about the energy level of the incoming cosmic ray. As such, this data can be used to calibrate the much more plentiful information captured day and night by the array of particle detectors. “Energy measurement is especially important,” says Wiencke, who came to Mines as an associate professor in August, 2007. “Our detectors record many more cosmic rays at lower energies, but only higher-energy rays provide meaningful directional information. Making this distinction is critical.”
The exciting finding of this study came when the trajectories of the 27 most powerful cosmic-ray events recorded since 2004 were analyzed. Of the group, 20 point to within a degree or two of known AGN. Of the remaining seven, several have trajectories that pass through the relatively opaque plane of our own galaxy, where optical surveys of AGN are incomplete and magnetic bending effects are strongest.
Wiencke, who is responsible for the calibrated laser test-beam facilities that simulate the optical signature of high-energy cosmic rays, says, “The discovery is exciting and fundamental. We don’t know whether the sources are AGN or something related in the vicinity, but this is an important step in understanding the origin of cosmic rays. The likelihood that these findings are coincidental is very slim.”
Scientists think that most galaxies have black holes at their centers, with masses ranging from a million to a few billion times the mass of our sun. The black hole at the center of our Milky Way galaxy weighs about three million solar masses, but it is not an AGN. Galaxies that have an AGN seem to be those that suffered a collision with another galaxy or some other massive disruption in the last few hundred million years. The AGN swallows the mass coming its way while releasing prodigious amounts of radiation.
All of these AGN are relatively nearby in cosmological terms—within 200 million light years. Wiencke says this is to be expected. Cosmic rays with over 60 EeV lose energy to the radiation left over from the Big Bang that permeates all of space. The only cosmic rays reaching Earth with energies over 60 EeV are those from nearby sources.
To complement data from the Argentine observatory, Auger scientists are developing blueprints for a Northern Hemisphere observatory to be located in Southeast Colorado, close to Lamar. Planned to be seven times larger than the Argentine facility, the project is being promoted by the Colorado Coalition for Cosmic-Ray Research (C3R2), a collaboration representing Mines, Colorado State University’s Fort Collins and Pueblo campuses, Lamar Community College and Colorado Southeast Enterprise Development.
“Hosting such a world-class scientific experiment constitutes a fantastic opportunity for Colorado,” said Fred Sarazin, assistant professor of physics at Mines and chair of C3R2. “We will need strong support from the state to secure federal and international funding.” If completed, the Northern Pierre Auger Observatory would have exposure to skies not seen by its sister facility and a capability of recording considerably more data from these cosmic messengers from some of the most tumultuous corners of the universe.