The Royal Swedish Academy of Sciences has awarded this year’s Nobel Prize for Physics to three astrophysics pioneers. Raymond Davis Jr and Masatoshi Koshiba share one half of the award “for pioneering contributions to astrophysics, in particular for the detection of cosmic neutrinos”. The second half goes to Riccardo Giacconi “for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources”.
Neutrinos were postulated by Wolfgang Pauli in 1930 and first detected by Frederick Reines and Clyde Cowan in the mid-1950s using a detector placed close to a nuclear reactor. Soon after, Ray Davis proposed building an underground detector to look for neutrinos coming from the Sun. The majority of reactions in the Sun generate neutrinos with energies too low to be detected with the technology of the 1950s, but one relatively rare reaction – the decay of boron-8 – produces neutrinos of up to 15 MeV. This is high enough to be detected by the technique elaborated by Bruno Pontecorvo and Luis Alvarez, who had suggested in the 1940s that such neutrinos could interact with chlorine atoms to produce a radioactive isotope of argon with a half-life of around 35 days. By 1967, Davis had installed a tank filled with 615 tonnes of the common cleaning fluid tetrachloroethylene in the Homestake gold mine in South Dakota, US. His background in chemistry had allowed him to devise the techniques for extracting the argon atoms every few weeks and counting their number – a feat equivalent to finding a particular grain of sand in the Sahara desert.
What Davis discovered came as a surprise – he detected only about half the number of neutrinos expected from the standard solar model. This meant the experiment was wrong, the standard solar model was wrong, or something was happening to the neutrinos on their way from the Sun.
Davis’s experiment ran continuously from 1967 to 1994, and was joined in 1987 by the huge Kamiokande water Cerenkov detector, built in Japan by a team led by Koshiba. This provided a confirmation that Davis’s experiment was right, and focused attention on the hypothesis proposed by Pontecorvo and Vladimir Gribov in 1968 – one year after Davis’s first results – that neutrinos oscillate, or change flavour on their way from the Sun. Both the Homestake and Kamiokande experiments are sensitive only to electron-type neutrinos. Kamiokande was also able to trace the direction of incoming neutrinos, confirming that they came from the Sun.
Koshiba went on to build the larger Superkamiokande experiment, which saw evidence for neutrino oscillation in neutrinos produced in the atmosphere by cosmic rays. Solar neutrino oscillation has since been confirmed by the Sudbury Neutrino Observatory in Canada.
It was not until 1949 that X-ray astronomy got off the ground. X-rays from cosmic sources are almost entirely absorbed by the Earth’s atmosphere, and it was only in the 1940s that rocket technology had advanced sufficiently to allow Herbert Friedman and colleagues to launch detectors to a sufficiently high altitude to make significant measurements. These experiments showed that X-ray radiation comes from areas on the surface of the Sun with sunspots and eruptions, and from the surrounding corona. Their observations were, however, confined to the solar system.
When in June 1962, Giacconi’s group launched an instrument consisting of three Geiger counters equipped with windows of varying thickness aboard a rocket, they became the first to record a source of X-rays beyond the solar system. Designed to see whether the Moon could emit X-rays under the influence of the Sun, the experiment instead located a source at a far greater distance, and observed an evenly distributed X-ray background. These discoveries gave an impetus to the development of X-ray astronomy.
Giacconi’s pioneering efforts in X-ray astronomy have changed our view of the universe.
The first source to be identified with a visible object was Scorpio X-1; other important sources were the stars Cygnus X-1, X-2 and X-3. Most proved to be binary systems in which a visible star orbits around a dense compact object such as a neutron star or a black hole. Detailed studies using short flights on rockets were, however, difficult, so Giacconi initiated construction of the UHURU satellite, which was launched in 1970. This was 10 times more sensitive than the rocket experiments, and was itself succeeded by the Einstein X-ray observatory – the first X-ray telescope capable of generating sharp images at X-ray wavelengths. Giacconi’s most recent accomplishment is the Chandra observatory, named for astrophysicist Subrahmanyan Chandrasekhar. Initiated by Giacconi in 1976, Chandra was launched in 1999 and has provided remarkable images of the X-ray universe.
Giacconi’s pioneering efforts in X-ray astronomy have changed our view of the universe. Some 50 years ago, the universe appeared largely to be a system in equilibrium. Today, we know that it is also the scene of extremely rapid developments in which enormous amounts of energy are released in processes lasting less than a second, in connection with objects not much larger than the Earth. Studies of these processes, and of the central parts of active galaxy cores, are largely based on data from X-ray astronomy.