論文の公開元へX-ray Study of Stellar-Wind Accretion on a Highly-Magnetized Neutron Star in a Binary System
概要
Stars are spherical objects which are supported by some type of inner pressure against
gravitational compression. A difference of the inner pressure, which depends on stellar
equations of state, determines their strictures. We can employ four types of pressure
for a supporting power of the stars, i. e. Coulomb repulsion, electron degeneracy
pressure, classical gas pressure, and neutron degeneracy pressure. In the case of
planets like the Earth and Jupiter, the pressure which works against the gravitational
pressure is the Coulomb repulsion. For main sequence stars like the Sun and Vega,
the classical gas pressure plays a role of repulsive force. The electron degeneracy
pressure is mainly working in brown dwarfs and white dwarfs. Neutron stars (NSs),
which are protagonists of this thesis, are supported by the degeneracy pressure of
neutrons.
In 1934, Baade and Zwicky predicted the existence of NSs following a discovery
of uncharged particles, i. e. neutrons, by James Chadwick in 1932. A first discovery
of an NS was made in 1967 by Antony Hewish and Jocelyn Bell. This was observed
as periodic radio pulses with the period of ∼1.3 s. So far, about 3000 NSs were
catalogued on the pulsar catalog of the Australia Telescope National Facility (ATNF).
Current understandings of the NS’s natures are followings.
• Most of the NSs are found in the Galactic plane.
• Typical radius: 10−12 km
• Typical mass: ∼1.4 M⊙ (Theoretical upper limit ∼ 3M⊙ )
• Spin period: 1.5 ms−107 ms
• Surface Magnetic field: 108 − 1015 G
• There are roughly two types of emission mechanisms, i. e. rotation-powered
and accretion-powered.
• The emissions are aligned by their strong magnetic fields (MFs) and a misalignment of the NS’s spin and magnetic axes causes the periodic signals.
However, the extreme conditions, e. g. the strong MFs, fast spin period, and
strong gravitational field, block the way to understand the properties of the NSs.
We still have a lot of mysteries for their birth, characteristics, radiation process, and
gravitational wave. One of the most mysterious characteristics of NSs is their MFs.
NSs with the strong MFs of ≳ 1014 G are generally called Magnetars.
Interestingly, magnetars are only found as isolated systems (not in binary systems)
and the reason is still unknown. ...
