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.
[All that is involved is a trigonometric calculation: the 24-satellite network of the "Global Positioning System" (GPS) uses somewhat similar principles, with the travel time of radio waves from a set of orbiting satellites pin-pointing positions on the ground. These satellites, at distances of 4.1 Earth radii, continually broadcast their precise locations, and these can be read by small portable receivers, relatively inexpensive. Using a built-in computer, these receivers then derive their own precise position on the ground, within 10-50 meters. Russia operates its own system, GLONASS, and European countries are planning a third one. For more details, see here.]
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. 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.
Locating the Sources of Bursts
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).
Since then, other sources were identified visually, some by amateurs.
(Update 11-24-04)
The riddle of the source of gamma ray bursts led astronomers to design and build a space observatory, designed to pin down and observe the source of bursts within seconds. That observatory was launched by NASA on November 20, 2004, under the name SWIFT, and at its heart if BAT, the Burst Alert Telescope, a gamma-ray detector covering 1/6 the sky and capable of deriving (within 10-20 seconds) the position of a gamma-ray burst, within 1-4 minutes of arc. It orbits about 600 km above the ground.
That position will be transmitted to ground observatories which--weather and position on the globe permitting--will immediately turn to the indicated location. Meanwhile SWIFT itself will orient itself, using momentum wheels, so that its two telescopes will also observe that spot--an X-ray telescope (XRT) and one in ultra-violet and visible light (UVOT). The X-ray detector will derive a spectrum in about 20 minutes, and will at other times conduct a survey of sources of "hard" (high-energy) X-rays, with a sensitivity 20 times of the best earlier observations. UVOT has a 30-cm wide telescope and sensitive detectors; using filters, it will go through a 2-hour cycle after an event. SWIFT can also be rotated on command to observe a gamma-ray burst seen by another satellite in a different part of the sky.
Magnetars and Bursts
(added 4 March 2005)
In addition to the "ordinary" variety of gamma ray bursts, originating in distant galaxies, there exist brief bursts which may originate closer to home, in our own galaxy. One of those, surprisingly powerful, reached Earth on December 27, 2004. It was intense enough to saturate most detectors aboard "Swift," described above, and it lasted about half a second, but its effect on the ionosphere above the Pacific Ocean interfered with communications for about and hour. About 15 satellites around Earth detected it.
The source of this burst was pulsar SGR 1806-20, already being monitored because of its strong magnetic field. Pulsars are remnants of supernova events, the collapse of massive stars which have used up all their nuclear fuel. Such a star, if is massive enough (our Sun apparently does not qualify) enters a rapid runaway nuclear reaction, which drains almost all their gravitational energy, a huge amount. It leaves behind a "neutron star" a few kilometers across, a rapidly rotating assembly of neutrons with a density like that of an atomic nucleus and a mass of the order of the Sun's.
In the compression process, any magnetic field present can get greatly amplified. As explained elsewhere, in a plasma which conducts electricity well (as it does in a collapsing star) magnetic field lines behave as if they were "frozen" into the material which they permeate. If that material gets compressed, the same lines occupy a smaller space at greater density, which means, the magnetic field becomes much more intense. For example, if the dimensions of the field decrease 10,000 fold, the cross-section of any "tube" formed by magnetic field lines shrinks 100 million times, and the magnetic field inside the tube becomes 100 million times more intense.
The star SGR 1806-20 apparently had a respectable magnetic field when it began collapsing, and as a result, it ended as a neutron star with an enormously intense magnetic field, a "magnetar". Such stars in our galaxy (about 10 are known) sometimes emit gamma ray bursts. This one had previously emitted small bursts, and two appreciable events were recorded in 1979 and 1998, but the latest one outdid them by about a factor of 100.
How the gamma rays were produced can only be guessed, but magnetic energy must be involved--it also seems to be associated with to the acceleration of particles on the Sun, and particle acceleration is probably essential to the production of gamma rays. Some believe that stressed magnetic field lines, twisted by rotation (which is also enormously amplified when a star collapses) managed to suddenly "unwind" to some extent, like an overly wound-up spring working loose. The star is about 50,000 light years from Earth, and astrophysicists are beginning to wonder whether some short gamma ray bursts, detected from distant galaxies, might not represent similar events there.
Note: This event was described in the "New York Times" on 2-20-2005 and on p.1178 of the issue of "Science" of 2-25-2005.
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, now identified as 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 8-10 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.