1.4 The Physical Mechanisms and Characteristics
23
magnetic power. However, what are the mechanisms used to transform all kinds
of powers into the radiant energy and how are the pulsating signals also brought
up? It is a fundamental issue on the radiation mechanisms of pulsars. It is shown
by theoretic studies that there are mainly five possible radiation mechanisms for
pulsars, the curvature radiation from magnetic poles, synchrotron radiation from
magnetic equators, high-energy radiation from hot spots, accreting matter radiation
and cyclotron radiation. Whatever is any kind of these radiation mechanisms, they
will produce the wave beams with continuous radiation spectrum at a certain region
on the surfaces of pulsars, and the beams point to various directions.
1.4.2.1 Curvature Radiation
In the extremely strong magnetic field of the pulsar’s magnetosphere, a relativistic
electron may be constrained to follow the path of a magnetic field line very closely,
with pitch angle nearly zero; the field line will generally be curved, so that the electron
will be accelerated transversely to its motion and then radiated; the radiation is usually
called curvature radiation. The so-called relativistic electron refers to a type of
electron whose velocity is approximately to the light velocity, and the electron whose
velocity is less than the light velocity is called non-relativistic electron.
Why do the signals emitted from pulsars always appear to a beam shape? It results
from the extremely strong magnetic fields of pulsars. A highly rapid rotation and
magnetization neutron star radiate its energy in the way of electromagnetic waves at
the same rotation frequency. It is shown by studying the Crab Pulsar that only if the
magnetic field strength on the surface of a pulsar gets to an order of 10
12 Gauss, the
radiant energy of electromagnetic wave will possibly equal to the rotational energy
of the pulsar losing. It seems hard to believe that there is so strong magnetic field on
the surface of a pulsar. In fact, it can easily be interpreted according to the theory that
neutron stars come from massive star evolutions. Supposed that a star with a radius
of 10
6 km evolves to the late term and collapses into a neutron star, its radius is only
about 10 km. Since all atoms inside the star are ionized and magnetic induction lines
frozen in the plasmas, the magnetic flux of the collapsed star is constant. That is,
C = 4π R
2 B,
(1.6)
where C is the magnetic flux of the collapsed star, R is the radius of the star, and B
is the magnetic field strength on the surface of the star.
It is clearly known from formula (1.6) that the magnetic field strength B will
increase 10
10 times, if the magnetic flux C keeps a constant and the radius R is
decreased to 10
–5 of its original value. On the basis of the observation data of the
rotation slowdown rates of radio pulsars, it is obtained by the calculation that the
magnetic field strengths of most pulsars are 10
11
−10
13 Gauss, while those of few
pulsars are weak, only 10
8
−10
10 Gauss.
23
magnetic power. However, what are the mechanisms used to transform all kinds
of powers into the radiant energy and how are the pulsating signals also brought
up? It is a fundamental issue on the radiation mechanisms of pulsars. It is shown
by theoretic studies that there are mainly five possible radiation mechanisms for
pulsars, the curvature radiation from magnetic poles, synchrotron radiation from
magnetic equators, high-energy radiation from hot spots, accreting matter radiation
and cyclotron radiation. Whatever is any kind of these radiation mechanisms, they
will produce the wave beams with continuous radiation spectrum at a certain region
on the surfaces of pulsars, and the beams point to various directions.
1.4.2.1 Curvature Radiation
In the extremely strong magnetic field of the pulsar’s magnetosphere, a relativistic
electron may be constrained to follow the path of a magnetic field line very closely,
with pitch angle nearly zero; the field line will generally be curved, so that the electron
will be accelerated transversely to its motion and then radiated; the radiation is usually
called curvature radiation. The so-called relativistic electron refers to a type of
electron whose velocity is approximately to the light velocity, and the electron whose
velocity is less than the light velocity is called non-relativistic electron.
Why do the signals emitted from pulsars always appear to a beam shape? It results
from the extremely strong magnetic fields of pulsars. A highly rapid rotation and
magnetization neutron star radiate its energy in the way of electromagnetic waves at
the same rotation frequency. It is shown by studying the Crab Pulsar that only if the
magnetic field strength on the surface of a pulsar gets to an order of 10
12 Gauss, the
radiant energy of electromagnetic wave will possibly equal to the rotational energy
of the pulsar losing. It seems hard to believe that there is so strong magnetic field on
the surface of a pulsar. In fact, it can easily be interpreted according to the theory that
neutron stars come from massive star evolutions. Supposed that a star with a radius
of 10
6 km evolves to the late term and collapses into a neutron star, its radius is only
about 10 km. Since all atoms inside the star are ionized and magnetic induction lines
frozen in the plasmas, the magnetic flux of the collapsed star is constant. That is,
C = 4π R
2 B,
(1.6)
where C is the magnetic flux of the collapsed star, R is the radius of the star, and B
is the magnetic field strength on the surface of the star.
It is clearly known from formula (1.6) that the magnetic field strength B will
increase 10
10 times, if the magnetic flux C keeps a constant and the radius R is
decreased to 10
–5 of its original value. On the basis of the observation data of the
rotation slowdown rates of radio pulsars, it is obtained by the calculation that the
magnetic field strengths of most pulsars are 10
11
−10
13 Gauss, while those of few
pulsars are weak, only 10
8
−10
10 Gauss.