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Astrophysics 102: Bremsstrahlung

December 3, 2010

If there ever was a language that from the outside seems designed by a language enthusiast, it has to be German. Forget Esperanto or Sindarin, the language that features the compound noun rindfleischetikettierungsuberwachungsaufgabenubertragungsgesetz (it’s actually a beef-labelling law) is one worthy of serious respect. Certainly amongst crossword-compilers and scrabble-board makers! I have a serious point here though. Many aspects of the English Language have been enriched by importing terms from German. One of the most famous is zugzwang, a chess term which means:

“…a situation where one player is put at a disadvantage because he has to make a move – the player would prefer to pass and make no move (but is compelled to do so). In game theory, it specifically means that it directly changes the outcome of the game from a win to a loss.”

Astronomy also has imported some German terms into its vernacular. One of the most famous is gegenschein (“counter glow”), which refers to:

“a rarely discernible faint glow known as the gegenschein (German for “counter glow”) [that] can be seen 180 degrees around from the Sun in an extremely dark sky. The gegenschein is sunlight back-scattered off small interplanetary dust particles. These dust particles are millimeter sized splinters from asteroids and orbit in the ecliptic plane of the planets.”

However, the most oft-used and referred formerly German term in astrophysics has to be bremsstrahlung (“braking radiation”). An impressive sounding word. But what is it? The English equivalent gives us a clue:

Bremsstrahlung is electromagnetic radiation produced by the acceleration of a charged particle when it is deflected by another charged particle.

It was originally discovered by Nikola Tesla during research he conducted between 1888 and 1897 (Image: The Vernadsky National Library of Ukraine).

Nikola Tesla

Although the definition of bremsstrahlung above could be interpreted to also include synchrotron radiation (which is caused by the acceleration of charged particles through a magnetic field), the standard definition refers usually only to the specific case of the acceleration of electrons by charged atomic nuclei (ions) via deflection from their Columb fields (Image: Nondestructive Testing)

Bremsstrahlung is also referred to as free-free emission because there is no particle capturing involved – the electrons are free before and after the interaction with the charged nuclei. If the electrons possess a thermal distribution of energies (i.e. a spread of energies around a mean value relating to their temperature) then the resulting radiation has a characteristic continuous spectrum. This is because the energy of the photon produced is related to the kinetic energy of the electron that is deflected, resulting in a spectrum that becomes more intense and shifts toward higher frequencies as the energy of the electrons involved increase (Image: Hyperphysics):

In an astrophysical context, bremsstrahlung is important because the photons emitted are usually observed as X-rays (although they can also be observed in the radio, and if energetic enough, as gamma-rays).

But where is bremsstrahlung produced? There are three main scenarios in which free-free emission is generated. The first is solar and stellar flares (Image: NASA):

A Solar Flare

I don’t intend to cover bremsstrahlung production in solar and stellar flares in this post (for more information please refer to this NASA web page). But I will talk in detail about the second and third scenarios in which bremsstrahlung is emitted.

Within galaxies, the regions between stars is filled with what is known as the Interstellar Medium (ISM). The vast majority of this interstellar medium is gas, with only a tiny amount (~1%) of dust. As a whole, it is extremely dilute, with an average density of only 1 atom per cm3! Usually found in a cold neutral state, around hot young stars in an HII region or the  exposed hot stellar core in a planetary nebula the gas is heated up and ionized (both the young stars and the stellar core emit intense ultraviolet radiation). The electrons produced as a result of the ionization then interact with the ions to produce bremsstrahlung (Image: DSS).

Galactic HII Region

As well as within galaxies, galaxy clusters are also a major source of bremsstrahlung. Between the galaxies in a cluster lies what is known as the Intracluster medium (ICM). This is an extremely tenuous gas consisting of ionized hydrogen and helium, although the mass of the ICM is typically approximately 15% of the mass of the parent galaxy cluster. At the centre of a galaxy cluster, the ICM is extremely hot – with a temperature of approximately 107 to 108 K, which means that the gas emits extremely strongly in X-rays via bremsstrahlung – typically this emission is much stronger than the optical equivalent. For example, below is a comparison of images of the Coma Cluster in X-rays and in optical (Image: Swinburne Astronomy Online):

The two bright points on the left (X-ray) image correspond to the two bright elliptical galaxies at the centre of the right (optical) image. As can be seen from the comparison, the emission from the ICM in X-rays is greater than the emission from individual galaxies in both optical and X-rays.

For a long time, it wasn’t known how and why this gas is still extremely hot –  since it is losing energy by emitting massive amounts of X-rays, it should cool down and condense, and formed more galaxies. But this is not happened. We now think this is because of the presence of massive black holes at the centre of active galaxies lying at the heart of galaxy clusters. These black holes swallow up any gas coming close to them emitting enormous amounts of energy in the process. This energy drives very narrow outflows of gas in the form of relativistic jets which reheats the intra-cluster gas. Thus the black holes act like thermostats, regulating the temperature of the gas surrounding them.

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