A Brief History of X-ray Astronomy

7th October 2012

Photograph of a supernova remnant.

SN 1006, the remnant of a type Ia supernova. Low-energy X-rays are red, and high-energy X-rays are blue. Image credit: NASA/CXC/Rutgers/J.Hughes et al/CC-NC-A.

This year marks 50 years 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 two processes: Bremsstrahlung and atomic emission.

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 a charged particle is slowed by another particle, and emits a photon.

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

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 inverse Compton scattering.

Inverse Compton scattering. Image credit: CXC/S. Lee/CC-NC-A.

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.

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

2. X-ray telescopes

X-rays are unable to penetrate the water in the atmosphere, and so no one knew if there were X-rays in space until it was possible to send detectors high above the surface of the Earth. In 1929, American geophysicist Edward Hulburt devised an experiment to send an X-ray detector into the upper atmosphere in a rocket, before letting it parachute back down[10a]. This method could be used to detect ultraviolet radiation and X-rays.

After World War II, captured German rockets were made available to American scientists, such as Herbert Friedman, who led a team that used this method to directly detect X-rays from the Sun in 1949[10b]. The British Skylark rocket program produced high quality X-ray images shortly after the launch of Sputnik[11]. The Sun does not emit many X-rays and, until 1962, it was thought that most stars would only emit faint X-ray radiation.

Riccardo Giacconi, Herb Gursky, Bruno Rossi, and Frank Paolini used an Aerobee 150 rocket to detect X-rays from the Sun and Moon, while working for American Science and Engineering, Inc. and the Massachusetts Institute of Technology in 1962[1b]. To their surprise, their results showed that X-rays are emitted in small amounts from all over the sky, and more X-rays were being emitted from a source in the constellation of Scorpius than had ever been observed elsewhere. This source was designated Sco X-1, and was later identified as a neutron star that is in a binary system with a low mass companion[12].

In the years that followed, the X-ray sky was mapped more thoroughly, with X-ray detectors raised into the upper atmosphere in balloons. The longer the detector remained above the atmosphere, the better the results would be, and so Giacconi and others worked on a project to launch an X-ray telescope into orbit, culminating in NASA's Uhuru satellite.

X-ray telescopes focus X-rays at a detector, where the X-rays are stopped, and their position and energy are recorded. X-rays travel through most things, and will only reflect from a mirrored surface if it's made of a heavy element like gold, and only if they hit it at a very shallow angle. In order to focus, many parabolic and hyperbolic mirrors are needed, and so they are made of thin materials like gold foil[13]. The first detectors were the photographic plates used by Röntgen. Other detectors include 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 of the first X-ray source outside of the Solar System, and on Uhuru, the first orbiting X-ray astronomy satellite, which was launched by NASA in 1970.

Uhuru detected 339 X-ray sources, including X-ray binaries, supernova remnants, active galaxies, and clusters of galaxies[15]. Giacconi later worked on the first fully imaging X-ray telescope, the Einstein Observatory, which was launched in 1978, and detected X-ray jets from active galaxies[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), which observed cataclysmic variables, Ginga (1987-1991), which observed black holes, and ROSAT (the Röntgen Satellite, 1990-1999), which detected over 150,000 X-ray sources, including comets.

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), were all launched in the 1990s, with Chandra using CCDs and micro-channel plates as detectors. 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.

Artist’s impression of an X-ray binary, showing gas flowing from a yellow star, and then circulating a black hole, creating a disc.

Artist's impression of an X-ray binary containing a black hole. Image credit: ESA, NASA, and Felix Mirabel (French Atomic Energy Commission and Institute for Astronomy and Space Physics/Conicet of Argentina)/CC-A.

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].

Some of the best evidence for dark matter comes from the Bullet Cluster[33]. This was formed from a collision between two galaxy clusters. Individual galaxies are so far apart that most did not collide when this happened, but the gas between the galaxies did. This meant that the galaxies and the gas were separated. The gas emits X-rays, which are shown in pink in the image below. The stars within galaxies are visible in optical light, which are shown in white and orange.

Dark matter was also separated from the gas during the collision. This is because it was not slowed by interactions with the light around it, and so moved ahead of the gas as each cluster passed through the other. In the image below, the dark matter is mostly in the blue coloured region. Dark matter does not interact with light, but it does interact with gravity and so scientists can determine where it is using a method known as gravitational lensing.

Photograph of the Bullet Cluster

The Bullet Cluster. Image credit: NASA, ESA, CXC, M. Bradac (University of California, Santa Barbara), and S. Allen (Stanford University)/Public domain.

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].

Photograph of a jet emanating from active galaxy M87.

Active galaxy M87, optical image taken by the Hubble telescope in 1998. Image credit: NASA and The Hubble Heritage Team (STScI/AURA)/Public domain.

Jets can sometimes emit light of other wavelengths, and the first optical jets were observed by American astronomer Heber Doust Curtis in 1918, coming from the galaxy M87[38]. X-ray observations by the Einstein observatory later showed that it contains a supermassive black hole[39].

AGN can be identified in many different ways depending on the angle they are viewed from. They appear most powerful when viewed along a region close to the jet. AGN that are viewed from this angle are known as blazars and quasars. When viewed at a 90° angle to this, AGN appear less luminous, and are known as radio galaxies[40][41].

Artist’s impression of an AGN. A jet emanates from the black hole. The system is known as a blazar when viewed at a 0 degree angle to the jet, as a radio galaxy when viewed at a 90 degrees, and a quasar of Seyfert 1 galaxy when viewed at an angle in between.

Illustration of an AGN. Image credit: Aurore Simonnet, Sonoma State University/NASA/Public domain.

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 2016, 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

  1. (a, b, c, d) Giacconi, R., Gursky, H., Paolini, F. R., and Rossi, B. B., 1962, 'Evidence for X rays from sources outside the solar system', Physical Review Letters, 9, pp.439-443.

  2. Röntgen, W. C., 1896, 'On a new kind of rays', Science, pp.227-231.

  3. Blue Plaque Places, 'John Hall-Edwards blue plaque in Birmingham', last accessed 15-02-16.

  4. NASA, 'X-rays', last accessed 15-02-16.

  5. Sommerfeld, A., 1909, 'Über die Verteilung der Intensität bei der Emission von Röntgenstrahlen' (About the distribution of intensity in the emission of X-rays), Physikalische Zeitschrift, 10, pp.969-976.

  6. Bohr, N., 1913, 'On the constitution of atoms and molecules', The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 26, pp.1-25.

  7. Compton, A. H., 1923, 'A quantum theory of the scattering of X-rays by light elements', Physical review, 21, pp.483-502.

  8. Lawrence, E. O. and Livingston, M. S., 1932, 'The production of high speed light ions without the use of high voltages', Physical Review, 40, pp.19-37.

  9. Elder, F. R., Gurewitsch, A. M., Langmuir, R. V., and Pollock, H. C., 1947, 'Radiation from electrons in a synchrotron', Physical Review, 71, pp.829.

  10. (a, b, c) Naval Research Laboratory, Solar and Heliospheric Physics Branch, 'Spacelab-2 Mission', last accessed 15-02-16.

  11. Pounds, K. A., 1986, 'British X-ray astronomy', Quarterly Journal of the Royal Astronomical Society, 27, pp.435-444.

  12. Steeghs, D. and Casares, J., 2002, 'The mass donor of Scorpius X-1 revealed', The Astrophysical Journal, 568, pp.273.

  13. NASA, 'X-ray Telescopes', last accessed 15-02-16.

  14. NASA, 'X-ray Detectors', last accessed 15-02-16.

  15. NASA, 'The Uhuru Satellite', last accessed 15-02-16.

  16. Giacconi, R., et al, 1979, ' The Einstein/HEAO 2/x-ray observatory', The Astrophysical Journal, 230, pp.540-550.

  17. Bhardwaj, A., et al, 2002, 'Soft X-ray emissions from planets, moons, and comets', arXiv preprint astro-ph/0209107.

  18. Tananbaum, H., et al, 2014, 'Highlights and discoveries from the Chandra X-ray Observatory', Reports on Progress in Physics, 77, pp.066902.

  19. Shklovsky, I. S., 1967, 'On the Nature of the Source of X-Ray Emission of SCO XR-1', The Astrophysical Journal, 148, pp.1.

  20. Reig, P., 2011, 'Be/X-ray binaries', Astrophysics and Space Science, 332, pp.1-29.

  21. NASA, 'Cataclysmic Variables', last accessed 15-02-16.

  22. Klus, H., 2015, 'Breaking the quantum limit: the magnetic field of neutron stars in extra-galactic Be X-ray binaries', PhD thesis.

  23. Institute of Astronomy X-Ray Group, 'Black Holes and X-ray binaries', University of Cambridge, last accessed 15-02-16.

  24. Duncan, R. C., 2000, 'Physics in ultra-strong magnetic fields', Gamma-ray Bursts, 5th Huntsville Symposium, 526, pp.830-841.

  25. Kouveliotou, C., Duncan, R. C. and Thompson, C., 2003, 'Magnetars', Scientific American, 288, pp.34-41.

  26. Mazets, E. P., Golenetskij, S. V., and Guryan, Y. A., 1979, 'Soft gamma-ray bursts from the source B1900+ 14', Soviet Astronomy Letters, 5, pp.343.

  27. Kouveliotou, C., 1998, 'An X-ray pulsar with a superstrong magnetic field in the soft gamma-ray repeater SGR1806− 20', Nature, 393, pp.235-237.

  28. Burrows, A., 2000, 'Supernova explosions in the Universe', Nature, 403, pp.727-733.

  29. Gorenstein, P., Kellogg, E. M., and Gursky, H., 1970, 'X-ray characteristics of three supernova remnants', The Astrophysical Journal, 160, pp.199.

  30. Institute of Astronomy X-Ray Group, 'Clusters of Galaxies', University of Cambridge, last accessed 15-02-16.

  31. Byram, E. T., Chubb, T. A., and Friedman H., 1966, 'Cosmic X-ray Sources, Galactic and Extragalactic', Science, 152, pp.66.

  32. Balaguera-Antolínez, A., 2010, 'The REFLEX II galaxy cluster catalogue', PhD thesis.

  33. NASA, 'A Clash of Clusters', last accessed 15-02-16.

  34. NASA, 'Chandra Discovery Sheds Light on Dark Energy', last accessed 15-02-16.

  35. Schödel, R., et al, 2002, 'A star in a 15.2-year orbit around the supermassive black hole at the centre of the Milky Way', Nature, 419, pp.694-696.

  36. Lynden-Bell, D., 1969, 'Galactic Nuclei as Collapsed Old Quasars', Nature, 223, pp.690-694.

  37. Hardcastle, M. J., 2005, 'Jets, hotspots and lobes: what X-ray observations tell us about extragalactic radio sources', Philosophical Transactions of the Royal Society of London A: Mathematical, Physical and Engineering Sciences, 363, pp.2711-2727.

  38. Curtis, H. D., 1918, 'Descriptions of 762 nebulae and clusters photographed with the Crossley reflector', Publications of Lick Observatory, 13, pp.9-42.

  39. Lea, S. M., Mushotzky, R. and Holt, S. S., 1982, 'Einstein Observatory solid state spectrometer observations of M87 and the Virgo cluster', The Astrophysical Journal, 262, pp.24-32.

  40. NASA, 'Blazars and Active Galaxies', last accessed 15-02-16.

  41. NASA, 'Japanese and NASA Satellites Unveil New Type of Active Galaxy', last accessed 15-02-16.

  42. NASA, 'NASA's Discovery Program: Missions', last accessed 15-02-16.

  43. (a, b, c) Elvis, M., 2012, 'After Apollo', Harvard International Review, last accessed 15-02-16.

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