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In May 1998, Kouveliotou et al. reported the discovery of a very special neutron star SGR1806-20. This star emits X-ray pulses with a period of 7.47 s. From time to time, outbursts of low energy gamma-rays are also observed. Hence, observational astronomers sometimes call this type of stars "soft gamma ray repeaters". The periodic pulsation and low energy gamma-ray outbursts are two typical characteristics of a young energetic neutron stars. Most importantly, they observed that the rotational speed of this star slows down at a rate of 2.6 x10-3 s/yr. Since the stronger the magnetic field, the faster the slow down rate an object will be. Thus, they can use the slow rate rate data to infer the magnetic field strength of SGR1806-20. Quite surprisingly, the answer they found is 8 x 1014 G, some 80 times higher than all other known neutron stars! We now call this kind of super-strong magnetic field neutron stars magnetar, a term coined a few years ago by two theoretical astrophysicists Duncan and Thompson. (As expected, the work of Duncan and Thompson did not receive much attention in the astronomy community until Kouveliotou et al.'s discovery.) With such a high magnetic field, magnetar slows down very rapidly. Therefore soon after their supernova birth, their rotational speed will become so low that they stop to emit electromagnetic pulses. In other words, astronomers can only detect those extremely young energetic magnetars before they stop emittng electromagnetic pulses. Consequently, it is extremely hard for us to find them in the sky. In this respect, we are very lucky to find one (and still the only one at this moment) - SGR1806-20 - in the sky. Owing to its super-strong magnetic field, calculating various properties of magnetars are hard problems for astronomers. To give readers an example: the electron cloud of a hydrogen atom on Earth is essentially distributed spherically around the hydrogen nucleus. But interesting things occur when we place a hydrogen atom in a strong magnetic field of 8 x 1014 G. The magnetic field greatly distorts the electron cloud in such a way that the electron cloud looks like a cylinder instead of a sphere. The axis of this cylinder is parallel to the strong magnetic field. Consequently, heat conduction along the magnetic field lines on the surface of a magnetar is far more efficient than the heat conduction perpendicular to the magnetic field lines. This may lead to a profound effect on the thermal evolution as well as the shape of the X-ray pulse of a magnetar. Many of us are trying to predict their properties at this moment. Related Links
Photo courtesy:NASA |
Frequent
Space Museum visitors should know that neutron star is one of the
possible final products of stellar evolution. These visitors may have
also heard of a number of extreme conditions on a neutron star: its
density is about the same as an atomic nucleus, and its diameter is not
bigger than that of Hong Kong. Moreover, neutron stars have a huge
magnetic field ranging from about 108 to 1012G.
(As a comparison, the magnetic field on Earth is about 1G, and
artificial magnetic fields created in laboratories seldom exceed 106G.)
In fact, neutron star magnetic field is the strongest amongst all stars
we know of to date. Moreover, most astronomers believed in the above
magnetic field figures for neutron stars until recently.