The Bremsstrahlung, Synchrotron and Compton effects as emission processes in Astrophysics

The Bremsstrahlung, Synchrotron and Compton effects
as emission processes in Astrophysics

Jeff Stanger

An understanding of the fundamental radiation mechanisms observed in nature is essential in modern astrophysics. This understanding allows an astrophysicist to analyse spectra from distant astronomical sources and obtain information regarding the conditions present in the regions of the universe which the radiation has traversed.

Within many interesting regions of the universe several of these radiation mechanisms are of special importance. Through an exploration of examples we will see that the processes of bremsstrahlung emission, synchrotron emission and Compton scattering are commonly observed and therefore of great importance.

Bremsstrahlung Radiation

The bremsstrahlung or free - free emission mechanism is an example of a radiative process of considerable interest in modern astrophysics. This mechanism involves the emission of a photon by an electron due to an electrostatic interaction with another charged particle. In other words, when an electrons path is deviated by a charged atomic nucleus it can emit or absorb a photon (Figure 1).

This mechanism is observed in regions space containing ionised gas, such as gaseous nebulae, at radio frequencies and in the hot (8.8 x 10⁷K) intracluster gas of clusters of galaxies, at X-ray frequencies (Figure 2).

The thermal bremsstrahlung radiation observed in Figure 2 arises when free electrons that have a thermal distribution of energies (a spread of energies around a mean value relating to their temperature) and this produces a characteristic spectrum that can be readily identified.

This same thermal bremsstrahlung emission can be observed in the interstellar gas of the giant elliptical galaxy M87 in the Virgo cluster. This galaxy contains 10¹⁰ solar masses of hot gas (»10⁷K), which emits X-ray frequency thermal bremsstrahlung radiation. The analysis of this spectrum can indicate the variation of temperature (T_r) and mass density (r_r) as a function of radius (r) from the centre of the galaxy. These quantities can then be used to determine the total internal mass (M_r) as a function of radius for a galaxy, which includes both the luminous and dark matter in the galaxy. The results for M87 show that the total internal mass (M_r) increases linearly with radius (r) out to a distance of 300 kpc. The combination of visible wavelength observations and the internal mass data has determined that over 99% of the M87’s mass is dark matter.

Synchrotron Radiation

The synchrotron emission mechanism is another example of an important astrophysical process (Figure 3). Synchrotron radiation is observed in regions where relativistic electrons (those travelling close to the speed of light) spiral around magnetic field lines. This process results in strongly polarised radiation concentrated in the direction of the electrons motion (called “beaming”). Similar to bremsstrahlung, synchrotron has a characteristic shape of its spectra which is a power law spectrum. The shape of the spectrum produced is dependant on the energy distribution of the emitting electrons and is easily distinguishable from thermal blackbody radiation.

A knowledge of synchrotron radiation is essential in the study of a large group of astronomical objects called active galactic nuclei (AGN). A typical spectral profile for an AGN is shown in Figure 4.

AGN are thought to contain massive black holes and the associated structure in the heart of the galaxy NGC 4261 can be seen in Figure 5.

According to the unified theory for AGN the hot and dusty ion torus, shown to the right of Figure 5, emits unpolarised thermal radiation. This emission accounts for the ‘UV bump’ shown in Figure 4. This emission leads astronomers to believe that the thick ionised disk associated with this torus generates varying magnetic fields of up to 10⁴ Gauss (very large) across its surface. This induces a large electric field and accelerates particles to relativistic speeds away from the disk (contributing to bipolar jets). These particles spiral along the magnetic field lines and produce synchrotron radiation which is up to 60% linearly polarised.

This synchrotron radiation is thought to be a contributing source to the power law component of the spectrum in Figure 4 (illustrated as the dashed line labelled as characteristic synchrotron emission). The spectrum shown in Figure 4 does not exhibit the synchrotron emission at low frequencies due to a common effect called synchrotron self-absorption. This is where the plasma that the synchrotron emitting electrons are part of becomes opaque to their synchrotron emission. This results in the turnover frequency that can be seen in Figure 4 at infrared (IR) wavelengths.

Compton Scattering

A third example of an emission process of interest in astrophysics is Compton scattering (Figure 6). This scattering process takes place if the wavelength of radiation is much smaller than the size of the loosely bound particle it falls on. It is the process that is associated with the interstellar reddening, which is a reddening of radiation from a distant source that is proportional to the distance it travels. Longer wavelengths (i.e. red light) are not scattered as readily as shorter wavelengths (i.e. blue light) and therefore radiation passing through interstellar dust is reddened by Compton scattering. This leads to changes in the expected spectrum of astronomical objects such as broadening of spectral lines and needs to be taken into account when analysing the spectrum.

AGN (discussed previously) are very bright in X-ray radiation. They are much brighter than the contribution possible from synchrotron radiation. The soft (low energy) X-ray component of this part of the spectrum is thought to originate from thermal bremsstrahlung but this still leaves unexplained excess. This is thought to be explained by inverse Compton scattering. This process scatters photons to much higher energies through collisions with relativistic electrons as opposed to normal Compton scattering which leads to a loss in energy to the photon involved.

Inverse Compton scattering is also thought to account for gamma ray emissions in some quasars. It is also observed at the base of AGN jets (shown in the left image of Figure 5) and in the extended regions of radio galaxies. In these and many other cases the Compton scattering mechanism is considered more important than synchrotron radiation as an energy loss mechanism for electrons.

There is a vast body of research concerning the bremsstrahlung, synchrotron and Compton processes in astrophysics. This enables us to study the most exotic regions of the universe. An example is the overwhelmingly distant and strange black holes that power AGN. It is through our understanding of these emission mechanisms that we are able to probe their behaviour and hopefully understand them.

Image Credits

Figure 2

1. ROSAT Mission (http://www.xray.mpe.mpg.de/) and the

2. Max-Planck-Institut für extraterrestrische Physik (http://www.mpe.mpg.de/).

Accessed at http://heasarc.gsfc.nasa.gov/docs/rosat/gallery/clus_coma.html and used with permission

Figure 5

Walter Jaffe, Leiden Observatory; Holland Ford, STScI, NASA

http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html