6
1 Introduction to the Pulsars
Generally, a radio source with a large angular diameter can be regarded as a
complex consisted of many sources with a small angular diameter. For the same
observation time, since the intensities of many sources with small angular diameters
vary randomly, the fluctuating variation of average observation has not discrepancy
in overall effect. Therefore, the angular sizes of radio sources can be measured by
observing the interplanetary scintillation. From Hewish’s investigation results, it is
shown that the interplanetary scintillation is related to the radio wavelength; only if
the angular diameters of radio sources are less than 1 arc-second in the meter band,
the interplanetary scintillations appear obviously.
A quasi-stellar radio source with no corresponding visible object was first discovered in the late 1950s and then named “quasar” by a Chinese-American astrophysicist
Hong-Yee Chiu (1932−) in 1964. The quasar is an active galactic nucleus with very
high luminosity and has the most distant and highest energy. Moreover, the angular
diameters of the quasars are usually less than 1 arc-second. The observation of the
interplanetary scintillation became an effective approach to investigating the quasars
in the late 1950s and the early 1960s. With the discovery of the first quasar, a craze for
searching the quasars in all-sky radio surveys was set off widely in radio astronomy
at that time.
Just under this background, in order to improve further the capability of observing
the interplanetary scintillations and searching the faint quasars, Hewish’s research
team at the University of Cambridge decided to build a large array radio telescope
at the Mullard Radio Astronomy Observatory in 1965, known as Interplanetary
Scintillation Array. The radio telescope consists of 1 central receiver and a 2048dipole antenna array. The antenna array is rectangular arrangement over almost five
acres (approximately 20,235 m
2 ) on the ground, the west–east 470 m long and the
north–south 45 m wide. It includes 16 rows and has 128 dipoles in each row. The radio
telescope is operated at a wavelength of 3.7 m (corresponding frequency 81.2 MHz),
and at a time resolution of about 0.1 s. This high time resolution set it apart from
many other radio telescopes of the time, and it amounts to a parabolic antenna radio
telescope with an aperture of 165 m and has the capability recording rapid fluctuation
signals. Since astronomers did not expect emission from a radio source to feature
such rapid variation, the ordinary receivers only got the average signals with an
integration time of several minutes to smooth out the random fluctuation noises, and
thereby did not detect the rapid variation signals, with periods of less than 1 s. The
radio telescope designed by Hewish’s research team was intended originally to survey
the interplanetary scintillation in the sky. Considering the frequency of interplanetary
scintillation is as higher as the order of 1 Hz, the high time resolution of 0.1 s for
the radio telescope is demanded to accurately record the signal intensity variation.
So, it creates an advantage condition to capture the short-period signals from radio
pulsars. Just as Andrew Lyne and Francis Graham-Smith said in their monograph,
Pulsar Astronomy [5], “it was an investigation of interplanetary scintillation that led
to the discovery of pulsars, even though the discovery was a by-product rather than
the purpose of the investigation”.
1 Introduction to the Pulsars
Generally, a radio source with a large angular diameter can be regarded as a
complex consisted of many sources with a small angular diameter. For the same
observation time, since the intensities of many sources with small angular diameters
vary randomly, the fluctuating variation of average observation has not discrepancy
in overall effect. Therefore, the angular sizes of radio sources can be measured by
observing the interplanetary scintillation. From Hewish’s investigation results, it is
shown that the interplanetary scintillation is related to the radio wavelength; only if
the angular diameters of radio sources are less than 1 arc-second in the meter band,
the interplanetary scintillations appear obviously.
A quasi-stellar radio source with no corresponding visible object was first discovered in the late 1950s and then named “quasar” by a Chinese-American astrophysicist
Hong-Yee Chiu (1932−) in 1964. The quasar is an active galactic nucleus with very
high luminosity and has the most distant and highest energy. Moreover, the angular
diameters of the quasars are usually less than 1 arc-second. The observation of the
interplanetary scintillation became an effective approach to investigating the quasars
in the late 1950s and the early 1960s. With the discovery of the first quasar, a craze for
searching the quasars in all-sky radio surveys was set off widely in radio astronomy
at that time.
Just under this background, in order to improve further the capability of observing
the interplanetary scintillations and searching the faint quasars, Hewish’s research
team at the University of Cambridge decided to build a large array radio telescope
at the Mullard Radio Astronomy Observatory in 1965, known as Interplanetary
Scintillation Array. The radio telescope consists of 1 central receiver and a 2048dipole antenna array. The antenna array is rectangular arrangement over almost five
acres (approximately 20,235 m
2 ) on the ground, the west–east 470 m long and the
north–south 45 m wide. It includes 16 rows and has 128 dipoles in each row. The radio
telescope is operated at a wavelength of 3.7 m (corresponding frequency 81.2 MHz),
and at a time resolution of about 0.1 s. This high time resolution set it apart from
many other radio telescopes of the time, and it amounts to a parabolic antenna radio
telescope with an aperture of 165 m and has the capability recording rapid fluctuation
signals. Since astronomers did not expect emission from a radio source to feature
such rapid variation, the ordinary receivers only got the average signals with an
integration time of several minutes to smooth out the random fluctuation noises, and
thereby did not detect the rapid variation signals, with periods of less than 1 s. The
radio telescope designed by Hewish’s research team was intended originally to survey
the interplanetary scintillation in the sky. Considering the frequency of interplanetary
scintillation is as higher as the order of 1 Hz, the high time resolution of 0.1 s for
the radio telescope is demanded to accurately record the signal intensity variation.
So, it creates an advantage condition to capture the short-period signals from radio
pulsars. Just as Andrew Lyne and Francis Graham-Smith said in their monograph,
Pulsar Astronomy [5], “it was an investigation of interplanetary scintillation that led
to the discovery of pulsars, even though the discovery was a by-product rather than
the purpose of the investigation”.