Magnetars
Neutron stars have extremely strong magnetic fields. However, recently a new class of spinning neutron
stars with abnormally large magnetic fields, even for a neutron star, have been discovered. These have
been called magnetars.
Soft Gamma Repeaters
In these rotating neutron stars,
it has been proposed that
the enormous magnetic fields act as a kind of brake, slowing the rotation of
the star. This slowing of the rotation disturbs the interior structure of the neutron star and "starquakes"
in the star release energy into the surrounding gases that cause emission of bursts of gamma rays.
Observationally, these are called soft gamma ray repeaters (SGR). In this designation, "soft"
means that the gamma rays are of relatively low energy (in fact, they lie more in the X-ray portion of the
spectrum), and "repeater" means that the bursts of gamma rays
can repeat, unlike ordinary gamma ray bursts,
which have not been observed to repeat (see Chapter 26). The
top right animation illustrates this model of a magnetar as a soft gamma-ray repeater.
|
Typical Magnetic Field Strengths
|
| Object |
Strength (Gauss)
|
| Earth's magnetic
field |
0.6
|
| Simple iron bar
magnet |
100
|
Strongest sustained
laboratory
fields |
4 x 105
|
| Strongest man-made fields
(millisecond duration) |
107
|
| Maximum field for ordinary
stars |
106
|
| Typical fields for radio
pulsars |
1012
|
| Magnetars |
1014 - 1015
|
|
|
|
|
|
The Strongest Magnetar Known
The following image illustrates in galactic coordinates the location of the strongest
magnetar yet found.
The magnetar SGR 1900+14 is estimated to have
the strongest magnetic field known in the galaxy. It is approximately
fifteen powers of ten larger than the magnetic field of the Earth
(see the adjacent table, which is repeated from an earlier section).
If a magnet that strong were
placed halfway to the Moon, its influence on Earth would be so strong
that it could pull a metal pen out of your pocket!