16
1 Introduction to the Pulsars
extremely high density, it can be regarded as a point source of the Roche lobes, and
the whole Roche lobes will be surrounded by the atmosphere from its companion
star. Therefore, the matter from the companion star easily gets into the Roche lobe
of the neutron star through intersection, and then it is controlled and captured by the
neutron star. Using the abundant materials from the companion star, an accretion disk
can also be built around the neutron star. Similar to the high-mass companion stars,
the materials within the accretion disk will fall onto the magnetic poles along the
magnetic field lines, and continuously impact on the solid crust of the neutron star
nearby the poles, so that high-energy particle streams including X-rays are brought
up ultimately.
In X-ray binary systems, pulsars continuously accrete the matter from the
companion stars, and thereby their rotations are accelerated and their pulse periods
gradually shortened from a typical 10 s order to dozens of milliseconds. In this way,
the ordinary pulsars can slowly be evolved into millisecond pulsars. Furthermore,
due to the shielding effect of the accretion disk, the magnetic field strength of the
millisecond pulsars is usually four magnitudes lower than that of the ordinary pulsars.
So, many observational features that the millisecond pulsars have, such as the older
age, shorter period and weaker magnetic field, all may be interpreted reasonably.
It should be noted that although pulsars within binary systems can produce significant amounts of X-ray photons, the pulsations of the type of pulsars are more complex.
The complexity results from the combined effects of both the rotating neutron star
and the orbit of the star around its companion. Moreover, along the line-of-sight
to an observer, the neutron star is probably eclipsed by its companion, so that its
signals can not be received. The eclipsing binary systems introduce additional signal
processing complexity.
In addition to the above X-ray binaries, the first radio binary pulsar, PSR
B1913+16, was discovered by Russell A. Hulse and his supervisor Joseph H. Taylor
using a 305-m radio telescope at Arecibo Observatory in 1974. PSR B1913+16
together with another neutron star are in elliptical orbits around a common center of
mass and they form a binary star system. The period of the orbital motion is 7.75 h,
and the two neutron stars are believed to be nearly equal in mass, about 1.4 solar
masses. Up to now, radio emissions have been observed from only one of the two
neutron stars. Most of radio binaries consist of a neutron star and a white dwarf, and
the masses of companion stars are typically 0.08−0.87 solar masses and their orbital
periods longer. For example, the companion star of PSR B0820+02 is a white dwarf
and their orbital period is about 1232 days; another pulsar PSR J2051–0827 with its
companion both are neutron stars and their orbital period is only 0.099 days.
The AX pulsars are the neutron stars powered by the decay of their extremely
powerful magnetic fields, and their magnetic field strengths get to 10
13
−10
15 Gauss,
while that of the Sun is only about 50 Gauss. The AX pulsars are widely believed
to be a kind of magnetars which are young, isolated and highly magnetized neutron
stars, and characterized by soft high-energy bands, faint luminosities and slow rotation periods of about 10 s. In general, the rotational energy loss of pulsars can be
determined by observing their rotational period changes. It is not sufficient from
the rotational energy loss of the AX pulsars to provide their X-ray radiation energy.
1 Introduction to the Pulsars
extremely high density, it can be regarded as a point source of the Roche lobes, and
the whole Roche lobes will be surrounded by the atmosphere from its companion
star. Therefore, the matter from the companion star easily gets into the Roche lobe
of the neutron star through intersection, and then it is controlled and captured by the
neutron star. Using the abundant materials from the companion star, an accretion disk
can also be built around the neutron star. Similar to the high-mass companion stars,
the materials within the accretion disk will fall onto the magnetic poles along the
magnetic field lines, and continuously impact on the solid crust of the neutron star
nearby the poles, so that high-energy particle streams including X-rays are brought
up ultimately.
In X-ray binary systems, pulsars continuously accrete the matter from the
companion stars, and thereby their rotations are accelerated and their pulse periods
gradually shortened from a typical 10 s order to dozens of milliseconds. In this way,
the ordinary pulsars can slowly be evolved into millisecond pulsars. Furthermore,
due to the shielding effect of the accretion disk, the magnetic field strength of the
millisecond pulsars is usually four magnitudes lower than that of the ordinary pulsars.
So, many observational features that the millisecond pulsars have, such as the older
age, shorter period and weaker magnetic field, all may be interpreted reasonably.
It should be noted that although pulsars within binary systems can produce significant amounts of X-ray photons, the pulsations of the type of pulsars are more complex.
The complexity results from the combined effects of both the rotating neutron star
and the orbit of the star around its companion. Moreover, along the line-of-sight
to an observer, the neutron star is probably eclipsed by its companion, so that its
signals can not be received. The eclipsing binary systems introduce additional signal
processing complexity.
In addition to the above X-ray binaries, the first radio binary pulsar, PSR
B1913+16, was discovered by Russell A. Hulse and his supervisor Joseph H. Taylor
using a 305-m radio telescope at Arecibo Observatory in 1974. PSR B1913+16
together with another neutron star are in elliptical orbits around a common center of
mass and they form a binary star system. The period of the orbital motion is 7.75 h,
and the two neutron stars are believed to be nearly equal in mass, about 1.4 solar
masses. Up to now, radio emissions have been observed from only one of the two
neutron stars. Most of radio binaries consist of a neutron star and a white dwarf, and
the masses of companion stars are typically 0.08−0.87 solar masses and their orbital
periods longer. For example, the companion star of PSR B0820+02 is a white dwarf
and their orbital period is about 1232 days; another pulsar PSR J2051–0827 with its
companion both are neutron stars and their orbital period is only 0.099 days.
The AX pulsars are the neutron stars powered by the decay of their extremely
powerful magnetic fields, and their magnetic field strengths get to 10
13
−10
15 Gauss,
while that of the Sun is only about 50 Gauss. The AX pulsars are widely believed
to be a kind of magnetars which are young, isolated and highly magnetized neutron
stars, and characterized by soft high-energy bands, faint luminosities and slow rotation periods of about 10 s. In general, the rotational energy loss of pulsars can be
determined by observing their rotational period changes. It is not sufficient from
the rotational energy loss of the AX pulsars to provide their X-ray radiation energy.