#34.     High Energy Particles

Cosmic rays are not the only sign of high energy particles in the distant universe. Additional evidence comes (like most astronomical data) from visible light and other types of electromagnetic waves, e.g. x-rays and radio waves.

Such waves usually arise in one of two ways--by "photon processes" and by processes resembling the broadcasting of radio waves.

Photon Processes

Photon processes are related to the quantum nature of light, discovered only in the 20th century, by which light or any other electromagnetic wave is only created or absorbed in definite "energy packets" called photons. The shorter the wavelength, the more energetic the photon--for instance, since blue light has a shorter wavelength than red light, its photons are more energetic. Certain black-and-white films can be safely handled in photographic darkrooms illuminated by deep red light, because the red photons do not carry enough energy to initiate the chemical changes that darken the film.

Photons of visible light have about 2 electron volts (ev), while medical x-ray photons may have energies of 50,000 ev and those of gamma rays reach 1,000,000 ev and even more. The arrival of (say) 50,000 ev x-ray photons from space is evidence of particles with at least that much energy, often much more, since each photon comes from a single particle; it is not possible that, for instance, ten electrons with 5000 ev each combine their energy to create a single photon of 50,000 ev.

Celestial x-rays cannot be observed from the ground, because the atmosphere quickly absorbsthem. They do however reach satellite observatories orbiting above the atmosphere, and several of those have mapped the x-ray sky--the small Uhuru and the larger Einstein observatory, both launched by NASA, and more recently the very successful European ROSAT whose name (Roentgen-satellite) honors the discoverer of x-rays. Some x-ray sources seem associated with strange binary stars and black holes, others still puzzle us, but all suggest a source of high-energy particles.

(More about space missions observing high-energy photons)

Gamma Ray Bursts

Of all the high-energy photons beamed at us by the universe, probably none are more puzzling than those emitted in gamma ray bursts. In the 1960s the US launched a series of spacecraft with accurately timed gamma-ray detectors, to monitor nuclear tests in space and later to enforce the international ban on such tests. The idea was that by having several well-separated satellites note the exact arrival times of the radiation (gamma rays travel at the speed of light) the sources of radiation could be pin-pointed.

The spacecraft indeed observed brief bursts of gamma rays, but the timing suggested that they came not from Earth but from deep space. Later some fairly accurate "fixes" were obtained for a few events and powerful telescopes were trained on the indicated locations, but they saw nothing remarkable there.

There exists no generally accepted explanation for gamma ray bursts.s Some promising theories were abandoned when NASA's Compton Gamma Ray Observatory satellite (CGRO) found in 1991 that they seemed to occur equally in all directions. Had they originated in our own galaxy, they would have probably been concentrated in the direction of the Milky Way, where most of our galaxy's stars are found (the galaxy is a flattened disk, and when we look at the Milky Way we see it edge-on). The new evidence suggests that they could come instead from distant galaxies, and if so, their sources must be incredibly powerful.

Added note: On March 2, 1997, the Dutch-Italian satellite BeppoSAX ("Beppo" was the nickname of the late Italian physicist Ochialini, after whom the orbiting observatory was named) reported a gamma-ray burst, and turned its x-ray telescope to the region. The X-ray telescope reported a continuing source of X-rays, and NASA's orbiting Hubble telescope (as well as the Keck Observatory on the ground) observed a visible "star" at the appropriate location, probably a distant galaxy. So far no definite conclusions have emerged (see Nature, 17 April 1997, p. 650).

Radio Waves

The other mode resembles the broadcast of radio waves from an antenna. A radio antenna carries a rapidly alternating current which flows back-and-forth along it, and the back-and-forth motion (viewed from the side) of an energetic particle, when it spirals around a magnetic field line, acts the same way. ("Photon laws" apply here too, but because the photons are quite small, the "antenna viewpoint" may be used.)

Radio waves from space were discovered accidentally in 1932 by Karl Jansky, a radio engineer with the Bell Labs. Since then many radio telescopes have scanned the skies and have discovered remarkable sources of radio and microwaves. Often they seem to indicate high-energy particles; for instance, some sources associated with distant galaxies suggest particles trapped in enormous magnetic structures. Some come from the center of our own galaxy, where linked radio telescopes thousands of miles apart have pinpointed an extremely compact source, possibly a giant black hole.

Perhaps the best known sources of this class are pulsars, sources of radio pulses whose repetition rate is extremely regular. They seem to be "neutron stars," collapsed remnants left behind by supernova explosions, stars as massive as the Sun but as dense as the atomic nucleus, no larger than 6-8 miles across. The collapse also greatly amplifies any existing magnetic field and speeds up enormously the star's rotation, creating compact stars which rotate about once a second, sometimes faster, with extraordinary strong magnetic fields.

It is believed that the radio pulses come from particles spiraling in those fields and that they are beamed in directions dictated by magnetic field lines. Thus as the pulsar rotates its radio beam, like the light-beam of a lighthouse, sweeps again and again past the Earth. The pulsing rate has been observed to decrease very slowly, suggesting processes which gradually slow the rotation down.

The Crab Nebula

The most recent supernova in the Earth's part of the galaxy was observed in China in 1054. It left behind it a peculiarly looking glowing cloud, the Crab Nebula, whose central star was recently revealed as a very rapid pulsar, with a radio signal pulsing about 30 times a second; it also pulses in visible light and in x-rays. The light of the nebula itself is polarized (vibrating in a certain ordered way), again suggesting electrons of very high energy spiraling in a magnetic field, and the nebula also contains many bright filaments (picture), which might well be magnetic in origin.

Closer to Home

High-energy electrons in the magnetosphere also emit x-rays and radio waves, in their own style. Positive ions, being heavier,tend to move more slowly and to radiate less efficiently.

To produce x-rays or gamma rays, electrons must collide with something. In a doctor's x-ray machine, for instance, they are shot onto a target, inside a vaccum tube (electrons hitting the screen of a TV picture tube also produce x-rays, but these are absorbed by the glass). Out in space collisions are very few, but x-rays are produced when beams of auroral electrons hit the atmosphere. In 1957 instruments of the University of Minnesota, carried by a balloon to the upper fringes of the atmosphere, detected x-rays emitted by auroral electrons many tens of miles above them. The recent "Polar " satellite carries an x-ray imager, highlighting regions in which auroral electrons are particularly energetic. The pictures produced are much less detailed than the ones in visible and ultra-violet light, from the other auroral imagers on "Polar." These latter images are particularly useful when the satellite is far from Earth, because their images then cover the entire polar cap; but rather detailed x-ray pictures have been obtained from the other end of the orbit, when "Polar" sweeps down, close to the other polar cap.

Many different types of radio emissions are generated by ions and electrons trapped in the magnetosphere, but few can be detected from the Earth's surface, because the ionosphere, at 100-300 km above our heads, usually reflects them back into space, just as it reflects back to Earth broadcasts of short-wave radio stations. However, in July 1962, after a high-altitude nuclear test by the US created a dense temporary radiation belt of fast electrons, radio noise from the new belt could be observed on the ground.

Even earlier, in 1955, strange radio signals were found to come from the planet Jupiter, greatly puzzling radio astronomers. The source turned out to be the planet's immense radiation belt. The fact that some of the emissions were found to be controlled by the position of the satellite Io is probably related to the electrical currents linking Io to Jupiter. The space probes which have visited Jupiter--Pioneers 10 and 11, Voyagers 1 and 2, Ulysses and most recently Galileo--have observed at close hand many types of radio waves, beamed in interesting modes which still defy explanation. The first four went on to Saturn, and Voyager 2 continued to Uranus and Neptune, all of which were found to be magnetized, have radiation belts and emit radio waves. The solar system thus has magnetosphere beyond the Earths, waiting to be explored, differing from ours by the presence of moons and rings and by other features.

Satellites orbiting outside the Earth's ionosphere record a veritable "zoo" of radio emissions, not all of them understood. Most intense is the "auroral kilometric radiation" originating above the aurora ("kilometric" is the order of magnitude of its wavelength, below the AM radio band). The "antenna processes" by which these waves originate are profoundly affected by the surrounding plasma and by the way it interacts with the magnetic field. Such waves therefore provide valuable information about magnetospheric plasmas.

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Last updated March 13, 1999