Icecube and the Source of Cosmic Rays
by John G. Cramer
A major breakthrough in cosmic ray physics was recently achieved by the IceCube Neutrino Observatory, a major astrophysics project funded by the U. S. National Science Foundation and constructed between 2004 and 2010 at the Amundsen-Scott South Pole Station in Antarctica. IceCube uses 85 detector strings spaced 125 meters apart in a hexagonal pattern, each string containing sixty light-sensing digital optical modules that send the data from detected light flashes to the surface where the data processing equipment is located. The top module of each string is located at 1,450 meters below the surface of the South Pole, and the bottom module is at a depth of 2,450 meters, just above the Antarctic bedrock. The IceCube Collaboration consists of about three hundred scientists from 12 countries. During the Antarctic summer between November 1 and February 15 about one hundred people work at the IceCube site for maintenance, improvements, and upgrades, but only two lucky individuals remain there at the South Pole over the hostile Antarctic winter to maintain operations.
* * *
The new IceCube discovery sheds light on one of the major unsolved problems of contemporary astrophysics: “What is the origin of ultra-energetic cosmic rays?” In the July/August 1999 issue of Analog, just before the end of the twentieth century, I wrote an AV column entitled “What We Don’t Understand.” That column, written over 18 years ago, listed what in my opinion at the time were the top seven major unsolved problems in physics and astrophysics. These seven, in no particular order, were: (1) What are dark matter and dark energy?; (2) What’s behind particle physics’ Standard Model?; (3) What’s the origin of gamma ray bursts?; (4) What’s the origin of ultra-energetic cosmic rays?; (5) Why do we detect only one third of the Sun’s neutrinos?; (6) Why does the Universe contain more matter than antimatter?; and (7) What is the origin of the arrow of time?
As of last week, problem (5), the solar neutrino problem (see AV 123), and problem (3), the origin of gamma ray bursts (see AV 142), had both been satisfactorily solved. A good but incomplete start was also made at solving problem (4), the origin of ultra-high energy cosmic rays (see AV 142). That came from work by the Pierre Auger Collaboration, which operates the Auger Observatory, an array of 1,600 large particle detectors spaced 1.5 kilometers apart and occupying three thousand square kilometers of the Argentine pampas. Active galactic nuclei (AGN) are super-active galaxies probably powered by very large central black holes. The group compared the sky coordinates of their 27 most energetic cosmic ray events (all with energies greater than the GZK Limit, the energy at which cosmic ray protons should interact and lose energy from collisions with the photons of the cosmic microwave background) with the positions of known AGNs that are within about 240 million light years from Earth. The Auger Group demonstrated an excellent correlation between known AGN positions and the incoming directions of these highest energy cosmic rays, suggesting that these highest energy particles are coming from AGNs.
But this correlation with AGNs raised questions. There is no known physical mechanism that could accelerate particles to such high energies. Further, the only explanation of how the GZK limit is being breached across such large separation distances is that the 27 detected particles with these extreme energies must have been super-energetic iron nuclei rather than protons. However, that remains a hypothesis that has never been verified. The new IceCube result sheds more light on the question of whether AGNs are indeed a source of ultra-high-energy cosmic rays.
* * *
Neutrinos are among the most elusive stable particles presented to us by nature. They are spin ½ fermions, leptons with no electric charge, their three flavors (ne, nm, and nt) all with the smallest of non-zero rest masses (around 0.001 electron-volts), and they interact with other particles of matter only through the weak and gravitational forces. If a ne electron-neutrino with a kinetic energy of one million electron-volts was passing through solid lead, its range (i.e., average distance traveled before an interaction) would be about one light-year. Therefore, the detection of neutrinos presents a very formidable experimental challenge, usually requiring huge detectors with tons of active detector material and subject to very low counting rates.
The neutral Z0 and the charged W± are the mediating particles of the weak interaction. Typically, a neutrino is detected either by a Z0-exchange scattering from an electron or nucleus, or by a charge-changing W±-exchange interaction in which the electrically neutral neutrino is converted to its electrically charged lepton twin. For example, in an interaction with a nucleus, a nm neutrino might be converted to a m lepton.
The most energetic particles in nature are ultra-high-energy cosmic rays, some of which have been observed with kinetic energies of over 1020 electron-volts. These highest energy objects may be charged particles, particularly protons and iron nuclei, but they may also be neutrinos. The study of neutrino cosmic rays is particularly interesting because, having no electric charge, neutrinos will not be deflected in flight by galactic magnetic fields, as charged particles would be. Therefore, their flight paths point directly back to the site of their origin and can be used to identify their source.
* * *
The most ambitious attempt to detect cosmic ray neutrinos is the previously mentioned IceCube Neutrino Observatory. The ice near the surface in Antarctica is rather cloudy due to tiny included air bubbles, but it was discovered that if one drills down a kilometer or so into the ice cap, the ice becomes crystal clear. This has allowed the IceCube Collaboration to construct a huge detector one kilometer on a side in which five thousand light-sensitive photomultiplier tubes are embedded in the ice about a kilometer and a half below the surface. Effectively, the IceCube detector has an active volume of one billion cubic meters and an active mass of one trillion tons of ice and is the largest and most massive particle detector ever constructed.
Why is ice a good detector medium for neutrinos? In ice, refraction effects slow light to 76.3% of its speed in vacuum, so any high-energy charged particle that is moving through ice at near vacuum lightspeed will be traveling at much greater than the local speed of light in the medium and will produce the electromagnetic equivalent of a sonic-boom shock wave called Cerenkov radiation. The blue Cerenkov photons form a cone-shaped shock wave centered on the path of the particle and propagating outward, and these photons are efficiently detected by IceCube’s photomultipliers, allowing the parent particle’s trajectory to be accurately reconstructed. Therefore, if any incoming neutrino has an interaction that produces a fast charged particle, IceCube will detect it and measure its energy and direction.
In particular, if an incoming mu-neutrino (nm) has a charge-exchange interaction in the ice that converts it into a m-lepton, that particle will have a very long, straight flight path through the ice, making Cerenkov light all the way. This is exactly what happened on September 22, 2017, when an event given the name IceCube-170922A occurred. A cosmic ray mu-neutrino from the northern sky having a kinetic energy of about 2.9 x 1011 electron-volts passed all the way through the Earth before it was converted to a charged m-lepton within the IceCube detector volume. The long range of the produced m-lepton allowed the determination of the direction of origin within a small circle in the northern sky spanning half a degree of arc and located at right ascension 77.33° and declination +5.72°. With these coordinates, IceCube’s automatic notification system went into action, notifying ground-based and space-based observatories around the world of their estimate of the coordinates of the event. Within a few seconds the Fermi Gamma-ray Space Telescope turned to examine this location and detected an outburst of x-rays and gamma rays coming from the so called “Texas Blazar” TXS 0506+056, an object that fell within the circle determined by IceCube. Optical astronomers have also observed increased activity from the “Texas” blazar TXS 0506+056 near the time of the IceCube detection.
What is a blazar? Blazars are the brightest objects in the sky. They are active galactic nuclei powered by large central black holes. The gravitational energy liberated by the black hole is observed to create jets of relativistic particles that are beamed out perpendicular to the plane of the AGN, probably because of magnetic and rotational angular momentum effects. But there is another requirement for an AGN to qualify as a blazar: the AGN must have one of its relativistic beams pointing directly at the Earth. The probability of such an accidental alignment is small, so there are only a few dozen known blazars. The name is derived from the first blazar ever recognized, an AGN object with the name BL Lacertae. That identifier suggested the generic name “blazar.”
After the discovery that event IceCube-170922A can be tracked back to the Texas blazar, the IceCube team went back in their set of accumulated data to look for other neutrino events that pointed back to the same source, and they found about 13 previous neutrino detection events that matched their criteria. In particular, there seems to have been a previously unnoticed strong burst of neutrinos coming from the Texas blazar around the beginning of the year 2015.
* * *
So we now know that at least one blazar had beamed ultra-high-energy neutrinos in our direction more than once. The implication of this work is that AGNs in our Universe are probably the source of all ultra-high-energy cosmic rays. However, this conclusion is not without its own problems. We do not presently understand how such extremely high-energy particles could be created from the gravitational energy released by a black hole. In other words, the basic acceleration mechanism remains a mystery and a challenge for astrophysical theory. Therefore, we cannot yet say that problem (4), the source of ultra-high-energy cosmic rays, has been completely solved, but only that significant progress has been made.
Watch this column for further developments.
IceCube UHE Neutrino Detection: “Neutrino emission from the direction of the blazar TXS 0506-056 prior to the IceCube-170922A alert,” IceCube Collaboration, Science 361, 147-151 (2018).
John G. Cramer’s 2016 nonfiction book describing his transactional interpretation of quantum mechanics, The Quantum Handshake—Entanglement, Nonlocality, and Transactions, (Springer, January 2016) is available online as a hardcover or eBook at: http://www.springer.com/gp/book/9783319246406.
SF Novels by John Cramer: eBook editions of hard SF novels Twistor and Einstein’s Bridge are available from the Book View Café co-op at: http://bookviewcafe.com/bookstore/?s=Cramer.
Alternate View Columns Online: Electronic reprints of over 180 “The Alternate View” columns by John G. Cramer, previously published in Analog, are available online at: http://www.npl.washington.edu/a
Copyright © 2018 John G. Cramer