1.3 The Name, Classification and Distribution
17
The powered sources by decaying the magnetic fields are only one of theoretical
interpretations. Besides, another interpretation is the accretion model, similar to the
AP pulsars, but accreting matter can not provide X-ray radiation energy so sufficient
that a part of the radiant energy is still obtained by rotating energy loss. At present,
a population of the discovered AX pulsars is dozens, but their counterparts in the
infrared, visible or gamma-ray bands have never been observed.
Furthermore, according to different pulse periods, pulsars are also divided into
two types, ordinary pulsars and millisecond pulsars, in which the former period
range is typically 0.05−8.5 s and the latter mostly 1−10 ms. The millisecond pulsars
have been detected in radio, X-ray and gamma-ray portions of the electromagnetic
spectrum. In 1982, the American astrophysicist Donald C. Backer (1943−2010),
et al. discovered the first millisecond pulsar, PSR B1937+21, with a spinning period
of 1.558 ms [13]. The rotating period of PSR B1937+21 is only about 1/20 of the
Crab Pulsar, while its age gets to 400 million years and is more five orders than the
Crab Pulsar; its magnetic field strength is smaller and about 10
8 Gauss; its periodic
change rate gets to 1.049 × 10
−19 s/s. The PSR B1937+21 is a X-ray binary system
and the best probe to explore the globular cluster. Its discovery started a pioneer to
search the planetary system outside the solar system.
It is well known that most of neutron stars are older stars with an age of about one
billion years. Why can these older stars rapidly rotate yet? The leading theory for the
origin of millisecond pulsars is that they are old, rapidly rotating neutron stars which
have been spun up through accretion of matter from a companion star in the close
binary system. For this reason, it is considered that the millisecond pulsars are related
to the LMXB systems. In the binary systems, the companion stars are usually too faint
to be observed. The matter accreted from the companion stars continuously impact on
the accretion disks and produce an impact force; the kinetic energy will be transferred
onto the surface of neutron stars, and hence the neutron stars are accelerated to rotate.
However, there has been recent evidence that the standard evolutionary model fails to
explain the evolution of all millisecond pulsars, especially young millisecond pulsars
with relatively high magnetic fields, like PSR B1937+21. It is shown that different
millisecond pulsars must form by at least two distinct processes, while the nature of
other process remains a mystery [14]. The millisecond pulsars have extremely stable
periodicity and their periodic change rates get to 10
–19
−10
–21 s/s. In other words, the
millisecond pulsars, which can be used to time with high precision, are better clocks
than atomic clocks [15]. Currently, there are approximately 200 millisecond-pulsars
that have been discovered and catalogued.
Particularly, it is worth noting that a Polish astronomer Aleksander Wolszczan
(1946−) discovered a millisecond pulsar with a rotating period of 6.22 ms, PSR
B1257+12, by using Arecibo radio telescope on February 9, 1990. It was found to
have anomalies in the pulse period, which led to investigations as to the cause of the
irregular pulses. And thus, Wolszczan and Dale Frail published a paper on the first
confirmed discovery of the planet outside the solar system. By using refined methods,
more planets were found again orbiting this pulsar in 1994. The millisecond pulsar
is located in the constellation of Virgo, and 2300 light years away from the Sun.
It has a planetary system with three known extra-solar planets respectively named
17
The powered sources by decaying the magnetic fields are only one of theoretical
interpretations. Besides, another interpretation is the accretion model, similar to the
AP pulsars, but accreting matter can not provide X-ray radiation energy so sufficient
that a part of the radiant energy is still obtained by rotating energy loss. At present,
a population of the discovered AX pulsars is dozens, but their counterparts in the
infrared, visible or gamma-ray bands have never been observed.
Furthermore, according to different pulse periods, pulsars are also divided into
two types, ordinary pulsars and millisecond pulsars, in which the former period
range is typically 0.05−8.5 s and the latter mostly 1−10 ms. The millisecond pulsars
have been detected in radio, X-ray and gamma-ray portions of the electromagnetic
spectrum. In 1982, the American astrophysicist Donald C. Backer (1943−2010),
et al. discovered the first millisecond pulsar, PSR B1937+21, with a spinning period
of 1.558 ms [13]. The rotating period of PSR B1937+21 is only about 1/20 of the
Crab Pulsar, while its age gets to 400 million years and is more five orders than the
Crab Pulsar; its magnetic field strength is smaller and about 10
8 Gauss; its periodic
change rate gets to 1.049 × 10
−19 s/s. The PSR B1937+21 is a X-ray binary system
and the best probe to explore the globular cluster. Its discovery started a pioneer to
search the planetary system outside the solar system.
It is well known that most of neutron stars are older stars with an age of about one
billion years. Why can these older stars rapidly rotate yet? The leading theory for the
origin of millisecond pulsars is that they are old, rapidly rotating neutron stars which
have been spun up through accretion of matter from a companion star in the close
binary system. For this reason, it is considered that the millisecond pulsars are related
to the LMXB systems. In the binary systems, the companion stars are usually too faint
to be observed. The matter accreted from the companion stars continuously impact on
the accretion disks and produce an impact force; the kinetic energy will be transferred
onto the surface of neutron stars, and hence the neutron stars are accelerated to rotate.
However, there has been recent evidence that the standard evolutionary model fails to
explain the evolution of all millisecond pulsars, especially young millisecond pulsars
with relatively high magnetic fields, like PSR B1937+21. It is shown that different
millisecond pulsars must form by at least two distinct processes, while the nature of
other process remains a mystery [14]. The millisecond pulsars have extremely stable
periodicity and their periodic change rates get to 10
–19
−10
–21 s/s. In other words, the
millisecond pulsars, which can be used to time with high precision, are better clocks
than atomic clocks [15]. Currently, there are approximately 200 millisecond-pulsars
that have been discovered and catalogued.
Particularly, it is worth noting that a Polish astronomer Aleksander Wolszczan
(1946−) discovered a millisecond pulsar with a rotating period of 6.22 ms, PSR
B1257+12, by using Arecibo radio telescope on February 9, 1990. It was found to
have anomalies in the pulse period, which led to investigations as to the cause of the
irregular pulses. And thus, Wolszczan and Dale Frail published a paper on the first
confirmed discovery of the planet outside the solar system. By using refined methods,
more planets were found again orbiting this pulsar in 1994. The millisecond pulsar
is located in the constellation of Virgo, and 2300 light years away from the Sun.
It has a planetary system with three known extra-solar planets respectively named