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X-ray emissions, detectors, and telescopes: A brief history of X-ray astronomy

First published on 7th October 2012. Last updated on 25th January 2018 by Dr Helen Klus

June 18th 2012 marks half a century since the first X-ray source was discovered outside of the Solar System[1a]. This began a race to map the X-ray sky, leading to the discovery of the most extreme objects in the universe.

1. X-rays

German physicist Wilhelm Röntgen first discovered X-rays in December 1895[2]. Röntgen was firing beams of electrons across a vacuum tube, and noticed that they made the inside of the tube fluorescent. He experimented with these rays - naming them 'X' for their unknown nature - and found that they do not have a charge, and can penetrate all kinds of matter.

Röntgen demonstrated this a week later with an X-ray image of his wife Anna's hand. X-rays penetrated the skin to be detected by a photographic plate. Within three weeks, British physician John Hall-Edwards began using X-ray detectors under clinical conditions, and, a month later, to aid surgery[3].

X-ray photograph of Anna Bertha Röntgen's hand.

Wilhelm Röntgen's image of Anna Bertha Röntgen's hand. Image credit: Wilhelm Röntgen/Public domain.

X-rays were shown to be highly energetic 'particles' of light, known as photons[4]. X-ray photons are produced in X-ray machines when electrons are fired at metal targets. This creates X-rays through a number of processes:

1.1 Bremsstrahlung

Bremsstrahlung (German for 'braking radiation') occurs when electrons are slowed down and deflected by atoms in the target metal. The electron loses energy, emitting it in the form of an X-ray photon[5].

Diagram of Bremsstrahlung, where an electron is slowed by another particle, and releases a photon.

Bremsstrahlung. Image credit: NASA's Imagine the Universe/Public domain.

1.2 Atomic emission

In atomic emission, when an electron hits an atom in the target metal, it knocks off one of the electrons in the outer shell of the atom. It's replaced by an electron from another shell, closer to the atom's nucleus, but the atom is not stable in this state, and so the electron soon 'falls' back to the shell it came from. This state requires less energy, and so the excess energy is released as an X-ray photon. This process creates emission lines in the light's spectrum[6].

Diagram of atomic emission.

Atomic emission. Image credit: CXC/S. Lee/CC-NC-A.

1.3 Fluorescence

The X-ray fluorescence that Röntgen witnessed occurs when a high-energy particle, or photon, knocks away an electron from the innermost shell of an atom. This makes the atom unstable, and so an electron from an outer shell will 'fall' down to replace it, releasing energy as an X-ray photon.

Diagram of fluorescence

Fluorescence. Image credit: NASA/CXC/M.Weiss/CC-NC-A.

X-rays can also be produced in a number of other ways, including charge-exchange, inverse Compton scattering, and synchrotron or cyclotron emission.

1.4 Charge-exchange

Charge-exchange occurs when a positive ion, an atom that has lost an electron, collides with a neutral atom and captures one of its electrons. The captured electron moves into the most stable orbit and releases energy as an X-ray photon.

Diagram of charge exchange.

Charge exchange. Image credit: NASA/CXC/M.Weiss/CC-NC-A.

1.5 Inverse Compton scattering

Inverse Compton scattering occurs when a low-energy photon collides with an extremely fast electron, and energy is transferred to the photon, turning it into an X-ray. If a high-energy photon loses energy, this is known as Compton scattering[7].

Diagram of Compton scattering, where a photon changes energy after colliding with a charged particle.

Compton scattering. Image credit: NASA's Imagine the Universe/Public domain.

1.6 Cyclotron and synchrotron emission

Cyclotron[8] and synchrotron[9] emission occurs when electrons are accelerated by a magnetic field, which causes them to emit photons. In cyclotron emission, the electron moves relatively slowly, whereas in synchrotron emission, the electron moves at close to the speed of light.

Diagram of synchrotron radiation, where an electron moves in a spiral while in a magnetic field. This makes the electron release photons.

Synchrotron radiation. Image credit: NASA's Imagine the Universe/Public domain.

2. X-ray telescopes

X-rays can’t travel through the atmosphere, and so it was not possible to know if there were X-rays in space until detectors could be sent above the clouds. American geophysicist Edward Hulburt devised a method to do this with a rocket in 1929[10a].

In 1949, American scientist Herbert Friedman led a team that used captured German rockets in order to directly detect X-rays from the Sun[10b]. The British Skylark rocket program produced high quality X-ray images[11], but until 1962, it was thought that most stars would only emit faint X-ray radiation.

In 1962, Riccardo Giacconi, Herb Gursky, Bruno Rossi, and Frank Paolini detected X-rays from the Sun and Moon using an Aerobee 150 rocket, while working for American Science and Engineering, Inc. and the Massachusetts Institute of Technology[1b]. They found that X-rays are emitted all over the sky, but that most X-rays were being emitted from somewhere, designated Sco X-1, in the constellation of Scorpius. Sco X-1 was later identified as a neutron star in a binary system with a low mass companion[12].

More detections were made the longer the detector was in space, and so balloons were used while Giacconi and others attempted to launch an X-ray telescope into orbit. This resulted in NASA's Uhuru satellite.

X-ray telescopes focus X-rays at a detector that records their position and energy. X-rays will only reflect from surfaces covered in heavy elements like gold, and only at a very shallow angle. This means that many parabolic and hyperbolic mirrors are needed in order to focus[13]. Detectors include photographic plates, proportional counters, X-ray CCDs, micro-channel plates, and calorimeters[14].

Diagram of an X-ray telescope, showing that X-rays are reflected from a parabolic, and then hyperbolic surface, before reaching a focal point.

Image credit: NASA/CXC/D.Berry/CC-NC-A.

Diagram of an X-ray telescope, showing the path of X-rays through an X-ray telescope.

Image credit: NASA/CXC/S. Lee/CC-NC-A.

Proportional counters, X-ray CCDs, and micro-channel plates measure the electric charge caused by X-rays exchanging energy with electrons. This produces a current, and the strength of the current is related to how much energy the X-ray originally had. Calorimeters directly measure the energy released as heat when an X-ray is absorbed by an atom. Proportional counters were used in the 1962 discovery and on Uhuru, which was launched in 1970.

Uhuru detected over three hundred X-ray sources, including X-ray binaries, supernova remnants, active galaxies, and clusters of galaxies[15]. Giacconi went on to work on the Einstein Observatory, which detected X-ray jets from active galaxies when it launched in 1978[16].

Many X-ray telescopes were launched into space from the early 1980s to the 2000s. These included EXOSAT (the European X-ray Observatory Satellite, 1983-1986), Ginga (1987-1991), and ROSAT (the Röntgen Satellite, 1990-1999).

These were followed by RXTE (the Rossi X-ray Timing Explorer, 1995-2012), BeppoSAX (1996-2003), Chandra (1999-present), and XMM-Newton (the X-ray Multi-Mirror Mission - Newton, 1999-present), which were all launched in the 1990s. The most recent X-ray telescopes to be launched include Swift XRT (the Swift X-ray telescope, 2004-present), Suzaku (2005-present), AGILE (the Astrorivelatore Gamma ad Immagini ultra LEggero, 2007-present), and NuSTAR (the Nuclear Spectroscopic Telescope Array, 2012-present). AGILE uses calorimeters as detectors.

3. X-ray sources

3.1 The Solar System

We have known that the Sun emits X-rays since 1949[10c]. Stars like the Sun produce X-rays in their outer atmosphere when flares interact with magnetic fields. Massive stars can emit more X-rays than the Sun because they have a stronger stellar wind. X-rays have since been observed from comets, and from most of the planets, and many of the moons in the Solar System, often due to their magnetic fields[17].

3.2 The X-ray background

In 1962, X-ray observations showed that small amounts of X-rays are being emitted from all over the sky[1c]. This implied that there were thousands of unresolved sources in all directions. It took decades before most of these sources were identified and we still haven't accounted for all of them.

The Chandra satellite made the deepest ever X-ray observations between 1999 and 2002. Over 500 sources were detected, these included X-ray binaries, magnetars, supernovae, clusters of galaxies, and active galactic nuclei[18].

Photograph of the deepest ever X-ray observations.

The deepest ever X-ray observations, the two large red shapes are clusters of galaxies. Image credit: NASA/CXC/PSU/D.M.Alexander et al/CC-NC-A.

3.3 X-ray binaries

The X-ray observations made in 1962 also showed an X-ray source in the constellation of Scorpius, known as Sco X-1[1d]. Many similar sources were found in the 1960s and, by 1967, these were identified as X-ray binaries[19]. X-ray binaries are binary star systems composed of a compact object - a white dwarf, neutron star, or black hole - and a companion star, which could be a main sequence star. A main sequence star is a star that is still undergoing nuclear fusion. X-rays are produced when matter is transferred from the companion star to the compact object.

There are generally two types of X-ray binaries: high mass X-ray binaries (HMXB) and low mass X-ray binaries (LMXB). In HMXB, a compact object gains matter from a high mass companion star, and in LMXB they gain matter from a low mass companion[20].

The companion star in HMXB is usually either a supergiant star, or an OBe star. OBe stars are massive blue stars that are surrounded by a disc of gas that can extend to about 1 AU (where 1 AU is the distance between the Earth and the Sun).

Supergiants generally transfer matter constantly, via a stellar wind. OBe stars, on the other hand, tend to only transfer matter periodically, when the two stars are closest together. The companion star in a LMXB is usually about the same mass as the Sun, and must be very close to the denser star in order for matter to be exchanged.

X-ray binaries that contain a white dwarf produce X-rays when matter is transferred between them because nuclear fusion begins again on their surface. These types of binaries are known as cataclysmic variables (CVs) or, if matter is constantly fusing, super soft X-ray sources (SSXS or SSS). If enough matter is accumulated, then the white dwarf will explode in a type Ia supernova[21].

X-ray binaries that contain a neutron star produce X-rays when matter falls onto the surface, and its gravitational potential energy is converted to heat. This causes matter to become more energetic, and X-rays are produced by a number of mechanisms, such as synchrotron radiation and inverse Compton scattering[22].

X-ray binaries containing a black hole produce X-rays because the matter that falls into orbit around it travels at different speeds, creating friction. This heats up the material, and X-rays are emitted close to the black hole, where it is hottest. Jets of matter are also sometimes emitted along the black hole's rotation axis[23].

3.4 Magnetars

Neutron stars have strong magnetic fields because they contain charged particles, like electrons and protons, and spin extremely quickly, sometimes thousands of times a second. Some neutron stars have a magnetic field so high that quantum field theories are needed to describe their behaviour[24][25].

Isolated neutron stars can emit X-rays if their magnetic field is so high that its lines twist, causing stress that can break the crust on the neutron star's surface. Neutrons stars that emit X-rays in this way are known as magnetars.

Some magnetars emit large bursts of gamma and X-rays at irregular intervals. These are known as soft gamma repeaters (SGRs). SGRs were first discovered in 1979 by a variety of detectors[26], and the theory behind them was explained in the 1990s[27].

3.5 Supernovae

Type Ia supernovae occur when white dwarfs begin fusion again, and all other types of supernova occur when a massive star ceases nuclear fusion and a neutron star or black hole is formed. X-rays are produced during these supernovae when matter falls onto the inner core of the star at about 1/5th of the speed of light[28]. Supernova remnants were first detected as X-ray sources in the early 1970s[29].

Photograph of a type Ia supernova remnant.

The remnant of a type Ia supernova, the Tycho Supernova Remnant. The background is in optical light. Higher energy X-rays are shown in blue, and lower energy X-rays are shown in red. These come from expanding debris, shaped by tangled magnetic field lines. Image credit: X-ray: NASA/CXC/Rutgers/K.Eriksen et al.; Optical: DSS/Public domain.

Photograph of a type IIb supernova remnant.

The remnant of a type IIb supernova, Cassiopeia A. Higher energy X-rays are shown in blue, lower energy X-rays are shown in green, optical light is shown in yellow, and infrared light is shown in red. Image credit: NASA/JPL-Caltech/O. Krause (Steward Observatory)/Public domain.

3.6 Clusters of galaxies

Clusters of galaxies could be considered the largest and most massive objects in the universe. They contain hundreds to thousands of galaxies, held together by mutual gravitation, and are millions of light-years wide. Clusters emit X-rays because they have such a strong gravitational force that any matter between galaxies falls towards the centre of the cluster very quickly, causing particles to collide[30].

In 1966, X-rays were detected from the galaxy M87 in the Virgo Cluster, and this led to the realisation that there's more matter in the space between galaxies than in the galaxies themselves[31]. Only around 2% of the mass of a cluster comes from stars, about 11% comes from the matter between galaxies, and the rest is dark matter, matter that does not interact with any kind of light[32].

Evidence of dark energy can also be found by studying the X-rays produced from galaxy clusters[34]. Dark energy is the name scientists have given to the unknown energy that is causing the expansion of the universe to accelerate.

Theories of dark energy make predictions about the amount of clusters that exist over time, and this can be verified by counting the number of clusters found as we look deeper into space. This is because light from distant objects takes so long to get here that the deeper we look into space, the further we look back in time. Another method for demonstrating the effects of dark energy involves looking at the X-rays produced by supernovae in different time-periods.

3.7 Active galactic nuclei

Supermassive black holes are black holes that are millions of times as massive as the Sun, and are found at the centre of most galaxies, including the Milky Way[35]. They are thought to have formed from the merger of many smaller black holes.

If a large amount of gas orbits a supermassive black hole, then it can emit X-rays due to friction. Supermassive black holes like this are known as active galactic nuclei (AGN). AGN were first detected in the 1970s[36].

Jets of matter are sometimes emitted along the rotation axis of a supermassive black hole. These can extend for thousands of light-years. As these jets run out of energy, they flare out, creating radio lobes. These mostly emit lower energy radio waves[37] but can sometimes emit light of other wavelengths[38][39][40][41].

Photograph of an AGN in X-ray, radio, and optical wavelengths.

Composite image showing AGN Centaurus A, in X-ray, radio, and optical light. Image credit: X-ray: NASA/CXC/CfA/R.Kraft et al; Radio: NSF/VLA/Univ.Hertfordshire/M.Hardcastle; Optical: ESO/WFI/M.Rejkuba et al/CC-NC-A.

4. The Future of X-ray astronomy

There are currently a number of X-ray observatories in orbit, including Chandra, XMM-Newton, and Suzaku. Chandra was launched by NASA in 1999, and was expected to remain operational for at least 5 years. Its life expectancy has since been extended, and it should work until at least 2014.

XMM-Newton was launched by the European Space Agency (ESA), also in 1999. It was originally intended to last 2 years but, like Chandra, has exceeded its expected lifespan, and could continue working until 2018.

Suzaku was launched by the Japan Aerospace Exploration Agency (JAXA) in 2005, its X-ray spectrometer broke down within two weeks, but the other X-ray telescopes on board are still functional and its spectrometer is due to be replaced in 2014.

It's hoped that we'll be able to continue sending better telescopes into space as technologies improve, but the short-term future of X-ray astronomy is uncertain. These missions are expensive, and so they have to compete for funding with other projects, which could also lead to exciting and important observations, such as telescopes that can image exoplanets[42].

NASA, ESA, and JAXA were intending to launch a new X-ray observatory, IXO (the International X-ray Observatory), in 2021, but NASA pulled out in 2011. The ESA are still involved with a remodelled version of the project, renamed ATHENA (the Advanced Telescope for High ENergy Astrophysics), although its launch date is currently not known. NASA has no current plans to launch a new X-ray observatory, although it will consider new mission proposals in 2015.

Martin Elvis, who worked on the Einstein observatory and first showed that AGN are strong X-ray sources, argues that the future of space-based telescopes may be in the hands of private companies[43a]. While space agencies are an expense that governments are finding hard to justify, orbital telescopes cost little in comparison to the turnover made by some businesses. Elvis states that "in 2007 the entire global space industry…amounted to just two-thirds of Walmart's turnover". He goes on to argue that there's money to be made from making space exploration safer, cheaper, and easier.

Elvis points out that space contains "truly vast resources, with trillions of dollars in street value, and capable of solving today's oil-based energy crisis"[43b]. These include helium-3, which is found on the Moon, and can be used to create energy from nuclear fusion that does not produce pollution or radioactive waste.

Asteroids are also valuable as they contain iron, water, and methane, all of which are needed if people are going to explore space, but are difficult and expensive to launch into orbit. Perhaps most importantly, from a business perspective, asteroids also contain rare metals such as silver, gold, and platinum. Elvis states that a small asteroid may contain "about $30 billion, and provide nearly two year's production of platinum at current levels"[43c].

The future of X-ray astronomy is uncertain, but it may turn out to be very exciting. Our understanding of the universe is currently dependent on funding, and there would be dramatic advancements if private companies were able to make space exploration cost effective.

UPDATE: As of 2017, Chandra, XMM-Newton, Swift, AGILE, and NuSTAR are still operational. Suzaku ceased functioning in September 2015, and ATHENA is due to launch in 2028.

5. References

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The Star Garden is a science news and science education website run by Dr Helen Klus.

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