1.1 Theoretically Predicted Neutron Stars
5
neutron stars in the universe had generally been doubted for a long time. There are
two reasons for the undiscovered neutron stars: one is that the physical features of
neutron stars, such as the rapid rotation, radio radiation, pulsing emission and so on,
are all unknown completely; another is that both the exposure time and the energy
integration time, for the optical observation and the radio astronomy, are required so
longer that the shorter periodic signals are usually smoothed out.
1.2 The Discovery and Identification
1.2.1 Observations of Interplanetary Scintillations
and Quasars
When we look carefully at the clear night sky in summer, it can be seen that all
visible stars are scintillating. It is also commonly known as “twinkling of stars”.
In simple terms, the twinkling of stars is a view phenomenon resulting from the
variation of the light intensity, while the lights from the distant stars pass through
different layers of a turbulent atmosphere. Its root cause is due to the irregularity and
disturbance of the air density in the atmosphere of the Earth. Similarly, there are also
scintillations in radio astronomy. It is usually called “radio twinkling”, but occurs
in the radio band rather than in the visible band of the electromagnetic spectrum.
The scintillations of radio waves from celestial sources also occur due to random
refraction in the terrestrial ionosphere, the ionized interplanetary gas of the solar
system and the ionized interstellar gas of the Milky Way Galaxy. In all three regions,
the radio waves from distant celestial sources traverse a medium with fluctuations
of refractive index sufficient to deviate radio signals into paths which cross before
they reach the radio telescope, giving rise to interference and hence to variations in
the radio signal strength.
The scintillation of radio waves due to the terrestrial ionosphere was observed
as early as 1951 by Antony Hewish (1924−), a radio astronomer at the University
of Cambridge in the United Kingdom. And then, Hewish focused on investigating
the intensity fluctuation from extragalactic radio sources. In 1964, while observing
several bright sources from celestial radio waves, Hewish’s research team discovered
the unusual fluctuations of intensity in a few of the sources. Analyzing the observational data in detail, they believed that the fluctuation resulted from irregularities in
the density of the plasma associated with the solar wind, which is called interplanetary scintillation. The solar wind is a stream of charged particles with supersonic
speed released from the upper atmosphere of the Sun, also called solar corona. The
solar wind is essentially the plasma composed mostly of electrons, protons and alpha
particles, with thermal energy between 1.5 and 10 keV. Its speed can reach the range
from 200 to 800 km/s, and continue to affect the whole solar system. As the radio
waves emitted from celestial sources cross through the solar wind, the phenomenon
of radio scintillation will appear.
5
neutron stars in the universe had generally been doubted for a long time. There are
two reasons for the undiscovered neutron stars: one is that the physical features of
neutron stars, such as the rapid rotation, radio radiation, pulsing emission and so on,
are all unknown completely; another is that both the exposure time and the energy
integration time, for the optical observation and the radio astronomy, are required so
longer that the shorter periodic signals are usually smoothed out.
1.2 The Discovery and Identification
1.2.1 Observations of Interplanetary Scintillations
and Quasars
When we look carefully at the clear night sky in summer, it can be seen that all
visible stars are scintillating. It is also commonly known as “twinkling of stars”.
In simple terms, the twinkling of stars is a view phenomenon resulting from the
variation of the light intensity, while the lights from the distant stars pass through
different layers of a turbulent atmosphere. Its root cause is due to the irregularity and
disturbance of the air density in the atmosphere of the Earth. Similarly, there are also
scintillations in radio astronomy. It is usually called “radio twinkling”, but occurs
in the radio band rather than in the visible band of the electromagnetic spectrum.
The scintillations of radio waves from celestial sources also occur due to random
refraction in the terrestrial ionosphere, the ionized interplanetary gas of the solar
system and the ionized interstellar gas of the Milky Way Galaxy. In all three regions,
the radio waves from distant celestial sources traverse a medium with fluctuations
of refractive index sufficient to deviate radio signals into paths which cross before
they reach the radio telescope, giving rise to interference and hence to variations in
the radio signal strength.
The scintillation of radio waves due to the terrestrial ionosphere was observed
as early as 1951 by Antony Hewish (1924−), a radio astronomer at the University
of Cambridge in the United Kingdom. And then, Hewish focused on investigating
the intensity fluctuation from extragalactic radio sources. In 1964, while observing
several bright sources from celestial radio waves, Hewish’s research team discovered
the unusual fluctuations of intensity in a few of the sources. Analyzing the observational data in detail, they believed that the fluctuation resulted from irregularities in
the density of the plasma associated with the solar wind, which is called interplanetary scintillation. The solar wind is a stream of charged particles with supersonic
speed released from the upper atmosphere of the Sun, also called solar corona. The
solar wind is essentially the plasma composed mostly of electrons, protons and alpha
particles, with thermal energy between 1.5 and 10 keV. Its speed can reach the range
from 200 to 800 km/s, and continue to affect the whole solar system. As the radio
waves emitted from celestial sources cross through the solar wind, the phenomenon
of radio scintillation will appear.