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 an 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.
Further Notes: 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.
A more detailed and technical discussion of this event is in the article "Record Gamma-Ray Flare Is Attributed to a Hypermagnetized Neutron Star in Our Galaxy" by Bertram Schwartzschild on page 19 of the May 2005 isssue of "Physics Today."
Scientists will find interesting information in 5 articles in the 28 April 2005 issue of "Nature, p. 1098-1114.
The December event is also discussed here.
For an earlier discussion of magnetars in our galaxy, see here, including references to a previous event on 27 August 1998, originating at an estimated distance of 20,000 light years.
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