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2 Observations of Radio and X-ray Pulsars
2.8 Development of X-ray Astronomy
2.8.1 Opening X-ray Astronomical Observations
During more than 50 years after X-rays discovered by Wilhelm Röntgen, X-rays were
widely applied in industrial manufacture, medical diagnosis and scientific research.
However, nobody knew anything about the X-rays from space. The X-rays can only be
able to be detected in outer space as they cannot pass through the Earth’s atmosphere.
And thus, progress on cosmic X-ray research is very slow. American astronomers
first detected the X-ray radiations from the Sun by sounding rockets until 1949. By
utilizing three Geiger counters carried with the sounding rocket, an Italian-American
astrophysicist Riccardo Giacconi (1931–2018) was leading a research team to detect
lunar fluorescence X-rays which resulted from the Sun in 1962, but they unexpectedly
found a strong X-ray source from Scorpius (Sco X-1), and cosmic X-ray diffusion
background. And thus, the prelude for human beings to investigate X-ray astronomy
was opened.
The cosmic X-ray radiations mainly come from four aspects: (1) the electron
transitions of atoms in the rarefied gas whose temperature is more than an order of 10
5
K, like the coronae and accretion disks; (2) the cyclotron radiations of non-relativistic
electrons in the extremely strong magnetic fields, like the accretion columns of
neutron stars; (3) the synchrotron radiations of relativistic electrons in the extremely
strong magnetic fields, like the Crab nebula; (4) the blackbody radiations in the dense
gas whose temperature gets to an order of 10
6 K, like the neutron stars’ surface and
accretion disks.
For detecting the high-energy radiations from celestial bodies, there are unique
advantage and important significance by using X-rays. On the one hand, there are
the extensive ground-state neutron atoms in interstellar space, whose energies are
more than 13.6 eV (ultraviolet frequency band); except Lyman series, these neutron
atoms have continuous absorption effects, which gradually weaken with increase in
frequency; the interstellar gas widely absorbs the ultraviolet rays whose energies
are higher than 13.6 eV, but are very transparent for the X-rays whose energies
are higher than 0.1 keV; thereby, X-rays can pass through interstellar space and
bring information on the Milky Way and extra-galactic objects. On the other hand,
for cosmic high-energy radiations, the number of photons gradually decreases with
radiation energy increasing; therefore, a large amount of information from cosmic
objects mainly concentrates on observable low-energy photons, which are the X-ray
frequency bands in electromagnetic spectrum.
In earlier cosmic X-ray detecting activities, besides using the sounding rockets,
hot air balloons are also utilized to carry large-scale detection equipment into the
upper atmosphere in order to detect the X-rays. Although the flight altitude of the
balloons is usually lower than that of the sounding rockets, the resident space time
of the balloons is so long that can detect higher energy photons that pass through the
upper atmosphere.
2 Observations of Radio and X-ray Pulsars
2.8 Development of X-ray Astronomy
2.8.1 Opening X-ray Astronomical Observations
During more than 50 years after X-rays discovered by Wilhelm Röntgen, X-rays were
widely applied in industrial manufacture, medical diagnosis and scientific research.
However, nobody knew anything about the X-rays from space. The X-rays can only be
able to be detected in outer space as they cannot pass through the Earth’s atmosphere.
And thus, progress on cosmic X-ray research is very slow. American astronomers
first detected the X-ray radiations from the Sun by sounding rockets until 1949. By
utilizing three Geiger counters carried with the sounding rocket, an Italian-American
astrophysicist Riccardo Giacconi (1931–2018) was leading a research team to detect
lunar fluorescence X-rays which resulted from the Sun in 1962, but they unexpectedly
found a strong X-ray source from Scorpius (Sco X-1), and cosmic X-ray diffusion
background. And thus, the prelude for human beings to investigate X-ray astronomy
was opened.
The cosmic X-ray radiations mainly come from four aspects: (1) the electron
transitions of atoms in the rarefied gas whose temperature is more than an order of 10
5
K, like the coronae and accretion disks; (2) the cyclotron radiations of non-relativistic
electrons in the extremely strong magnetic fields, like the accretion columns of
neutron stars; (3) the synchrotron radiations of relativistic electrons in the extremely
strong magnetic fields, like the Crab nebula; (4) the blackbody radiations in the dense
gas whose temperature gets to an order of 10
6 K, like the neutron stars’ surface and
accretion disks.
For detecting the high-energy radiations from celestial bodies, there are unique
advantage and important significance by using X-rays. On the one hand, there are
the extensive ground-state neutron atoms in interstellar space, whose energies are
more than 13.6 eV (ultraviolet frequency band); except Lyman series, these neutron
atoms have continuous absorption effects, which gradually weaken with increase in
frequency; the interstellar gas widely absorbs the ultraviolet rays whose energies
are higher than 13.6 eV, but are very transparent for the X-rays whose energies
are higher than 0.1 keV; thereby, X-rays can pass through interstellar space and
bring information on the Milky Way and extra-galactic objects. On the other hand,
for cosmic high-energy radiations, the number of photons gradually decreases with
radiation energy increasing; therefore, a large amount of information from cosmic
objects mainly concentrates on observable low-energy photons, which are the X-ray
frequency bands in electromagnetic spectrum.
In earlier cosmic X-ray detecting activities, besides using the sounding rockets,
hot air balloons are also utilized to carry large-scale detection equipment into the
upper atmosphere in order to detect the X-rays. Although the flight altitude of the
balloons is usually lower than that of the sounding rockets, the resident space time
of the balloons is so long that can detect higher energy photons that pass through the
upper atmosphere.