2.9 Observability of X-ray Pulsars
83
The Crab Nebula was ever observed and investigated by an X-ray astronomical
research team of the U.S. Naval Research Laboratory (NRL) in 1963. However, the
spatial resolutions of X-ray detectors were so low at that time that it could not be
decided by direct observations whether the radiating source was exactly a point or not
a surface. And thus, a particular observation of celestial phenomena was sufficiently
utilized by the research team. The Moon coincidently passed through the direction
of the Crab Nebula in July 1964. Supposed that X-rays come from the extended Crab
nebula, they will gradually be shaded by the Moon, and this duration will continue
about 12 min; supposed that X-rays come from a central point source, all of them
will be shaded instantaneously. It was shown from the observational experiment that
the flux of X-rays was only changed slowly during the Moon passing through the
Crab Nebula, and there were 90% of X-rays emitted from the extended radiation
source for the Crab Nebula. Although it was possible from this conclusion that there
was 10% of X-rays emitted from the central point source (in fact, only 4% confirmed
later.), nobody focused on it so seriously that the opportunity to discover the neutron
stars was lost as easily as that.
There was a neutron star with pulsating frequency of 33 Hz at the center of the
Crab Nebula, discovered by radio astronomy observation in 1969. And thus, the point
source of X-ray scintillation was searched in the Crab Nebula by using the method
of X-ray scintillation detection, and separated from the extended radiation source.
With launching the Uhuru in 1970s, two X-ray sources, Centaurus X-3 and Hercules
X-1, were discovered successively. Both Centaurus X-3 and Hercules X-1 radiate
periodic X-ray pulsating signals, and their periods are 4.84 s and 1.24 s, respectively. They belong to a new type of celestial objects, which have the observational
features of periodic eclipses and the eclipsing durations are usually very short. It is
shown reasonably that the two X-ray pulsars are located, respectively, in the binary
systems with short-period orbits. In other words, the close binary systems are formed
by the modes. The X-ray radiations are produced by accreting the materials from
the companions in the close binary systems, and the spinning rates of pulsars are
increased further to form the millisecond pulsars.
2.9.2 Flux Analysis of X-ray Pulsars
With launching and using a series of X-ray astronomy satellites, such as the Einstein
Observatory, ROSAT and so on, a large amount of X-ray sources were discovered
successively. Particularly, the millisecond pulsars with periodic X-ray radiations were
also discovered. In general, many rotation-powered pulsars like the Crab pulsar are
located at the centers of the supernova remnants whose outer layers are the extended
radiation sources. In order to get the central point radiation sources, it is required to
compare the flux intensities of X-ray pulsars with those of the nebulas around them.
Moreover, there is also a large amount of diffuse X-rays in space, whose intensity
distributions are not uniform in different energy ranges. Therefore, the observabilities
of X-ray pulsars mainly depend on the ratio between the flux intensities of the pulsars
83
The Crab Nebula was ever observed and investigated by an X-ray astronomical
research team of the U.S. Naval Research Laboratory (NRL) in 1963. However, the
spatial resolutions of X-ray detectors were so low at that time that it could not be
decided by direct observations whether the radiating source was exactly a point or not
a surface. And thus, a particular observation of celestial phenomena was sufficiently
utilized by the research team. The Moon coincidently passed through the direction
of the Crab Nebula in July 1964. Supposed that X-rays come from the extended Crab
nebula, they will gradually be shaded by the Moon, and this duration will continue
about 12 min; supposed that X-rays come from a central point source, all of them
will be shaded instantaneously. It was shown from the observational experiment that
the flux of X-rays was only changed slowly during the Moon passing through the
Crab Nebula, and there were 90% of X-rays emitted from the extended radiation
source for the Crab Nebula. Although it was possible from this conclusion that there
was 10% of X-rays emitted from the central point source (in fact, only 4% confirmed
later.), nobody focused on it so seriously that the opportunity to discover the neutron
stars was lost as easily as that.
There was a neutron star with pulsating frequency of 33 Hz at the center of the
Crab Nebula, discovered by radio astronomy observation in 1969. And thus, the point
source of X-ray scintillation was searched in the Crab Nebula by using the method
of X-ray scintillation detection, and separated from the extended radiation source.
With launching the Uhuru in 1970s, two X-ray sources, Centaurus X-3 and Hercules
X-1, were discovered successively. Both Centaurus X-3 and Hercules X-1 radiate
periodic X-ray pulsating signals, and their periods are 4.84 s and 1.24 s, respectively. They belong to a new type of celestial objects, which have the observational
features of periodic eclipses and the eclipsing durations are usually very short. It is
shown reasonably that the two X-ray pulsars are located, respectively, in the binary
systems with short-period orbits. In other words, the close binary systems are formed
by the modes. The X-ray radiations are produced by accreting the materials from
the companions in the close binary systems, and the spinning rates of pulsars are
increased further to form the millisecond pulsars.
2.9.2 Flux Analysis of X-ray Pulsars
With launching and using a series of X-ray astronomy satellites, such as the Einstein
Observatory, ROSAT and so on, a large amount of X-ray sources were discovered
successively. Particularly, the millisecond pulsars with periodic X-ray radiations were
also discovered. In general, many rotation-powered pulsars like the Crab pulsar are
located at the centers of the supernova remnants whose outer layers are the extended
radiation sources. In order to get the central point radiation sources, it is required to
compare the flux intensities of X-ray pulsars with those of the nebulas around them.
Moreover, there is also a large amount of diffuse X-rays in space, whose intensity
distributions are not uniform in different energy ranges. Therefore, the observabilities
of X-ray pulsars mainly depend on the ratio between the flux intensities of the pulsars