1895 X-Rays Are Discovered

Wilhelm Röntgen

Wilhelm Conrad Röntgen was a German physicist, who in November 1895, produced and detected electro-magnetic radiation in a wavelength range known as x-rays or Röntgen rays. In 1901 this achievement earned him the first Nobel Prize in Physics.

Röntgen called the radiation x-radiation to denote its unknown nature. This mysterious radiation had the ability to pass through many materials that absorb visible light. X-rays also have the ability to knock electrons loose from atoms. Over the years these exceptional properties have made x-rays useful in many fields, such as medicine, research into the nature of the atom, and the study of black holes and other outer space objects.

Röntgen discovered its medical use when he made a picture of his wife's hand on a photographic plate using x-rays. The photograph of his wife's hand was the first photograph of a human body part using x-rays. Within 5 years of the initial discovery, many medical professionals were using x-rays for diagnostic purposes.

Eventually, x-rays were found to be another form of light (electro-magnetic radiation). Light is the by-product of the constant vibration of all matter. Light can take on many forms. Radio waves, microwaves, infrared, visible, ultraviolet, x-ray and gamma radiation are all different forms of light.. We tend to use the term light for only visible light, but in science light (electro-magnetic radiation) has a much broader meaning.

The chair you are sitting on may look and feel motionless. But if you could see down to the atomic level you would see atoms vibrating hundreds of trillions of times a second and bumping into each other, while electrons zip around at speeds of about a million miles per hour.

In honor of his accomplishments, in 2004 the International Union of Pure and Applied Chemistry (IUPAC) named element 111, roentgenium after him. Roentgenium is a radioactive element with multiple unstable isotopes.  Top

Medical (Bremsstrahlung) X-Rays

Brehmsstrahlung Foot Image

To generate x-rays, we must have three things. We need to have a source of electrons, a means of accelerating the electrons to high speed, and a target material to receive the impact of the electrons.

X-rays for medical diagnostic procedures are produced by accelerating electrons with a high voltage inside an x-ray tube and projecting them to collide with a metal target. X-rays are produced when an electron is suddenly "decelerated" upon collision with an atom nucleus of the metal target. These x-rays are called "bremsstrahlung" x-rays. See the electron/atom diagram to the left.

If the electrons have sufficient energy, they can knock an electron out of the inner shell of the target metal atoms. Then electrons from higher states drop down to fill the vacancy, emitting x-rays with "precise" energies that can be used for very special medical purposes (like cancer treatment).

This precise energy emission is called "Atomic Emission". The energy of such an emission x-ray is determined by the exact chemical structure of the element's atom. So in the cosmic arena, emission line x-rays can be used as indicators that some particular element (hydrogen, helium, oxygen, etc.) is present in a distant star or other remote celestial object.

X-ray Tube

See the x-ray tube illustration to the left. Current flows in from the left hand side and heats up the filament cathode (pink). There is a very high voltage between the filament cathode and the anode. Electrons (red) are accelerated to high speeds and collide with the metal anode. Bremsstrahlung x-rays are produced and exit the tube at the bottom.

The resultant image is generally presented in a "negative" format. The x-ray image shows various parts of the body in different shades of black and white. This is because different tissues absorb different amounts of radiation. Calcium in bones absorbs the most x-rays, so bones look white. Fat and other soft tissues absorb less, and look gray. Air absorbs the least, so lungs look black.

When you have an x-ray taken, you may have to wear a lead apron to protect certain parts of your body. The amount of radiation you get from an x-ray is small. For example, a chest x-ray emits a radiation dose similar to the amount of radiation you are naturally exposed to from the environment over a 10 day period.  Top

Compton Scattering

Compton Scattering

Compton Scattering occurs when an incoming x-ray photon scatters off an electron that is initially at rest. The electron gains energy and the scattered outgoing x-ray photon has a frequency less than that of the incoming x-ray.

How can Compton Scattered X-rays be used to destroy a cancer tumor?

When a patient is exposed to million electron-volt x-rays, most of the x-rays pass right through the patient, but a small fraction undergo Compton Scattering and leave some of their energy behind in the form of high powered electrons.  These high powered electrons kill tissue and can be used to destroy a cancer tumor.  By approaching a tumor from many different angles through the patient's body, the treatment can minimize the injury to healthy tissue around the tumor while giving the tumor itself a fatal dose of radiation.

X-rays are produced when low energy photons of the cosmic microwave background (CMB) radiation are boosted to x-ray energies by colliding with high energy electrons in a jet of particles speeding away from the vicinity of a black hole. This process is Compton Scattering.   Top

Inverse Compton Scattering

Inverse Compton Scattering

Inverse Compton Scattering involves the scattering of low energy photons to high x-ray energies by ultra-relativistic electrons so that the low-energy photons gain and the electrons lose energy. The process is called inverse because the electrons lose energy rather than the photons, the opposite of the standard Compton Scattering effect.

Inverse Compton scattering is very important in astro-physics.

The accretion disc surrounding a black hole is thought to produce a rather low energy thermal x-ray spectrum. The lower energy x-rays produced from this spectrum are scattered to higher energies by relativistic high speed electrons emanating from the nearby black hole corona - i.e. Inverse Compton scattering.

The Inverse Compton effect is also observed when photons from the cosmic microwave background (CMB) move through the hot gas surrounding a galaxy cluster. The CMB low energy x-rays are scattered to higher energies by the electrons in the gas, resulting in the Sunyaev-Zel'dovich effect (SZ effect). Observations of the SZ effect (Inverse Compton scattering) has provided a redshift independent means of detecting dense clusters of galaxies.

Pulsars have very strong magnetic fields, billions and even trillions as times as powerful as the earth’s. This incredible magnetism can accelerate electrons to speeds to very nearly that of light. When such an electron slams into a photon of light, the photon can pick up the energy of the electron by Inverse Compton scattering. An ordinary photon of light can be energized tremendously, becoming a super-high-energy gamma ray. Gamma rays begin at the upper end energy of x-rays. Both are the same kind of electro-magnetic radiation. Gamma rays arbitrarily start at 100,000 electron volts (100 keV) whereas x-rays arbitrarily end at 100 keV.  Top

Synchrotron Radiation

Synchrotron Raduation

A charged particle moving in a magnetic field radiates energy. At non-relativistic velocities, this results in cyclotron radiation while at relativistic (close to the speed of light) velocities it results in synchrotron radiation. Synchrotron radiation is electro-magnetic radiation that is emitted by electrons that are spiralling along a field line that is being accelerated by a super strong magnetic field.

Synchrotron radiation was seen for the first time at General Electric in the United States in 1947 in a new type of particle accelerator, a synchrotron - hence the name. It was first considered a nuisance because it caused the accelerated particles to lose energy, but it was then recognized in the 1960s as light with exceptional properties that overcame some of the shortcomings of x-ray tubes.

Synchrotron radiation from cosmic sources has a distinctive spectrum, i.e. the distribution of photons versus energy. The synchrotron radiation falls off less rapidly than does the spectrum of radiation from a hot gas. The radiation produced in this fashion has a characteristic polarization and the frequencies generated can range over the entire electromagnetic spectrum. When synchrotron radiation is observed in supernova remnants, cosmic jets, and other celestial sources, it reveals information about the high-energy electrons and magnetic fields that are present.

Supermassive black holes produce synchrotron radiation because of the jets formed by the acceleration of ions through their "tubular" zones of extremely long magnetic fields. A few jets have appeared to be travelling at several times the speed of light. This phenomenon is a result of the jet travelling very near the speed of light at a very "small angle" directly towards the earth. (It is like looking straight down the end of a pencil at a tiny angle.) An illusion is created because at every point of the path, the high-velocity jets are emitting light. It is in fact a very "small angle" illusion - the recent newer light appears to be old light, but this is due to the very small angles involved and the two get mixed up. The newer light gets interpreted (in error) as older light. Net-net, the speed of light in a vacuum does remain a constant.  Top

"Charge Exchange" Radiation

Synchrotron Raduation

Charge Exchange is an atomic collision process in which a neutral atom (see example: hydrogen) collides with a charged ion (helium) and louses its electron. An x-ray photon is emitted as the captured electron drops to a lower energy state. This process is called "charge exchange" because an electron is exchanged between a neutral atom and an ion.

Charge exchange is especially important for comets, where ions in the solar wind collide with neutral atoms in the comet's atmospheres.

Charge exchange results in x-ray emissions at energies characteristic of the ions involved. When the ions originate in the solar wind this process is known as solar wind charge exchange (SWCX). The ions that result in the emission of x-rays include highly-charged oxygen, carbon and neon, which are all present in low percentages in the solar wind. SWCX emission can be seen throughout the Solar System, for example in planetary exospheres such as the Earth, Mars and Jupiter. The emitted x-rays can be used to identify particle changes in the solar wind.  Top

Black Hole X-Ray Modeling

Black Hole Cygnus X-1

Cygnus X-1 (Cyg X-1) is a dual (binary) set of celestial objects - a black hole that orbits a supergiant star 8,000 light years away. See a real Cyg X-1 photo to the left, the black hole is the smaller object. (The name X-1 means it was the first x-ray source found in the constellation Cygnus.)

To the left below is a plot of how bright the black hole Cyg X-1 is in x-rays. The vertical axis is brightness and the horizontal axis is the energy of the x-rays. The round black points labeled "X-ray data" represent how bright Cyg X-1 measured at each frequency of x-ray. It is the brightest around 2 keV and has emissions all the way up to 100 keV. The solid line line, a curve with a hump-like shape, is the result of mathematical modeling.

Mathematical models are based on the size, composition and temperature of the accretion disk plus the mass of the black hole. Depending on how hot the disk is, astronomers can estimate how bright its accretion disk will be at different frequencies. When the components of the model are summed up, they make up a curve that can be compared to the original data.

Black Hole X-Ray Model

The model result of Cyg X-1 is the solid black line and lies approximately on top of the real data - an exceptionally good fit. The final model is made up of three parts:

The light the accretion disk itself emits is shown by the long-dash line (labeled in blue). Note that its top energy level is only about 4.2 keV, quite low. This is because the accretion disk emits very "soft" x-rays.

There is also a corona, and much like the corona of our sun, it is a region of hot gas that is much less dense than the accretion disk, but emits very powerful "hard" x-rays. The light the corona emits is shown with the short-dash line (labeled in red). Note that the corona has very strong x-rays all the way up to and beyond 100 keV. 100 keV is the beginning range of gamma rays. (See the Gamma Ray Burst page.)

The third component is the light the accretion disk reflects from the corona, its emission is shown with the dotted line (labeled in green). Why is the green reflection line so complicated ? This complex dotted line is a result of all the different kinds of materials that are rotating in the accretion disk.  

At the end of the day, a good model has to agree with observations. The model that best matched the data for Cyg X-1 required a black hole 12 times the mass of the sun. This is one way astronomers can determine the mass of a black hole in a binary system. This is also a way to tell if a binary has a black hole or a neutron star in it. Neutron stars cannot be more massive than 3 times the mass of the sun. So if you find an object that is more than 10 times the mass of the sun, it definitely has to be a black hole.  Top

The Cosmic "X-Ray" Background

Cosmic X-Ray Background

Most of us are familiar with the Cosmic Microwave Background (CMB), which is a remnant of the Big Bang. The CMB is a low-energy diffuse glow of microwave radiation that completely surrounds everything in the universe.

But there is another background that exists, known as the "Cosmic X-ray Background" (CXB). Just as the CMB is a diffuse microwave glow, the CXB is a diffuse x-ray glow. See the German ROSAT x-ray image of the CXB to the left. It is a false-color image, where the red, green and blue colors represent low, medium and high x-ray energies. But, unlike the CMB, the x-ray background is not a remnant of the big bang.

Local Bubble

Decades of mapping the regional sky in soft x-rays had revealed a local glow, known as "The Local Bubble". The Local Bubble was discovered in the 1970s and 1980s. Recent measurements also made it clear that our sun and solar system reside in a region of space where normal interstellar gas is unusually sparse. The Local Bubble is some-what peanut-shaped, about 300 light years long, and filled with almost nothing. See the NASA artist's conception to the left.

Gas inside the bubble is very thin and very hot - roughly a million degrees. This is a a sharp departure from ordinary interstellar material. Evidence suggests that our solar system is moving through a region of space that may have been blasted clear by several supernova explosions during the past 20 million years.

However within the last decade, some scientists have been challenging the supernova interpretation, suggesting that much, if not all of the soft x-ray background is instead a result of "charge exchange" with the solar wind. Charge exchange happens when the electrically-charged ions of the solar wind come in contact with electrically neutral gas atoms. Atoms in the neutral gas and ions in the solar wind exchange electrons resulting in an x-ray emission that looks a lot like the x-ray glow from old supernovas. (Charge exchange has been fairly recently observed with comets. See the Charge Exchange Radiation section above.) So the question raised was - is the x-ray glow that fills the sky a sign of normal "charge exchange" in the solar system or evidence of humongous supernova explosions in the past?

To find out an international team of researchers led by NASA developed an x-ray detector that could distinguish between the two possibilities.  The device was named DXL, for Diffuse X-ray emission from the Local galaxy. On December 12, 2012, DXL launched atop a NASA rocket, reaching a peak altitude of 160 miles, and spent a grand total of five minutes above the earth's atmosphere.  That was all the time it needed to measure the amount of "charge exchange" from the solar wind.

The results clearly indicate that about 40 percent of the soft x-ray background originates from within the solar system.  The 60 percent balance comes from the Local Bubble of hot gases, the relic from ancient supernova explosions outside of our solar system, and possibly other interstellar sources. NASA is planning the next flight of DXL which will include additional instruments to better characterize our solar system's x-rays. The launch is currently planned for December 2015.