X-Ray Astronomy — Although the more energetic X-rays (E > 30 keV) can penetrate the air at least for distances of a few meters (they would never have been detected and medical X-ray machines would not work if this was not the case) the Earth’s atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth’s surface.
X-rays in the 0.5 – 5 keV range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; ninety percent of the photons in a beam of 3 keV X-rays are absorbed by traveling through just 10 cm of air!
To observe X-rays from the sky, the X-ray detectors must be flown above most of the Earth’s atmosphere. There are three methods of doing so, however only satellites are used by scientists now.
Sounding rocket flights
A detector is placed in the nose cone section of a sounding rocket and launched above the atmosphere. This was first done at White Sands missile range in New Mexico with a V2 rocket in 1949. X-rays from the Sun were detected by the Navy’s experiment on board.
An Aerobee 150 rocket launched in June of 1962 detected the first X-rays from other celestial sources. The experiment package contained in this rocket is pictured at left. The largest drawback to rocket flights is their very short duration (just a few minutes above the atmosphere before the rocket falls back to Earth) and their limited field of view. A rocket launched from the United States will not be able to see sources in the southern sky; a rocket launched from Australia will not be able to see sources in the northern sky.
Balloon flights can carry instruments to altitudes of 35 kilometers above sea level, where they are above the bulk of the Earth’s atmosphere. Unlike a rocket where data are collected during a brief few minutes, balloons are able to stay aloft for much longer.
However, even at such altitudes, much of the X-ray spectrum is still absorbed. X-rays with energies less than 35 keV cannot reach balloons. One of the recent balloon-borne experiments was called the High Resolution Gamma-ray and Hard X-ray Spectrometer (HIREGS). It was launched from the Antarctic where steady winds carried the balloon on a circumpolar flight lasting for almost two months! A picture of the launch of HIREGS can be seen at right.
A detector is placed on a satellite which is taken up to an orbit well above the Earth’s atmosphere. Unlike balloons, instruments on satellites are able to observe the full range of the X-ray spectrum.
Unlike sounding rockets, they can collect data for as long as the instruments continue to operate. In one instance, the Vela 5B satellite, the X-ray detector remained functional for over ten years!
Satellites in use today include the XMM-Newton observatory, launched by ESA and the Chandra observatory, launched by NASA. Past observatories included ROSAT, the Einstein observatory, the ASCA observatory and BeppoSAX.
Sources of X-rays in the sky
Several types of objects emit X-rays. Firstly X-rays are emitted by black holes in active galactic nucleus, or AGN for short, galaxy clusters, supernova remnants, stars, binary stars containing a white dwarf (cataclysmic variable stars), neutron star or a black hole (X-ray binaries), the X-ray background, and some solar system bodies, the most notable being the Moon.
Black holes give off radiation because the matter falling into them gain gravitational energy which is released before the matter falls into the event horizon. The infalling matter has angular momentum, which means that the material cannot fall in directly, but spins around the hole. This material often forms an accretion disk.
Similar luminous accretion disks can also form around white dwarfs and neutron stars, but in these the infalling gas releases additional energy as it slams against the high-density surface with high speed. In case of a neutron star, the infall speed can be a sizeable fraction of the speed of light.
In some neutron star or white dwarf systems the magnetic field of the star is strong enough to prevent disc formation. The material in the disc gets very hot because of friction, and emits X-rays.
The material in the disc slowly loses its angular momentum and falls into the compact star. In neutron stars and white dwarfs, additional X-rays are generated when the material hits their surfaces. X-ray emission from black holes is variable, varying in luminosity in very short timescales. The variation in luminosity can provide information about the size of the black hole.
Clusters of galaxies are formed by the merger of smaller units of matter, such as galaxy groups or individual galaxies. The infalling material (which contains galaxies, gas and dark matter) gains kinetic energy as it falls into the cluster’s gravitational potential well.
The infalling gas collides with gas already in the cluster and is shock heated to between 107 and 108 K depending on the size of the cluster. This very hot gas emits X-rays by thermal bremsstrahlung emission, and line emission from metals (in astronomy, ‘metals’ often means all elements expect hydrogen and helium). The galaxies and dark matter are collisionless and quickly become virialised, orbiting in the cluster potential well.
The X-rays of the solar system bodies are produced by fluorescence. Scattered solar X-rays provide an additional component.