Basics of Electromagnetic Radiation

classical radiative process

In classical electromagnetic theory, radiation is emitted by any charged particle that is inaccelerated motion:

provided that the acceleration is measured in the frame in which the particle is instantaneously at rest.

electron scattering

This example shows the tracks of 70 electrons as they impact a thin foil of gold. The incoming beam is represented by the heavy red line. The colors of the tracks indicates the energies of the particles, with red being higher and blue lower values.

electron scattering

cyclotron radiation

cyclotron radiation

The composite model spectrum shown in blue consists of a suitably scaled M-star spectrum (red), cyclotron model spectra (green) and the 20!000!K white dwarf model. At a field strength of 110!MG one can access the very low harmonic numbers usually hidden in the infrared or far infrared.

A cataclysmic variable star hosting a strongly magnetic white dwarf primary as accretor.

pulsar spin down

crab pulsar

Observation of pulsed gamma-rays above 25 GeV

thermal bremsstrahlung

Bremsstrahlung: from German bremsen "to brake" and Strahlung "radiation", i.e. "braking radiation" or "deceleration radiation"), is electromagnetic radiation produced by the deceleration of a charged particle

For a plasma in thermal equilibrium:

such that

thermal bremsstrahlung

thermal bremsstrahlung

coma cluster

Figure 5: Coma in optical and Xray.

The temperature is close to that expected from galaxy velocity dispersion, kT

mp!2 which means that the origin of the thermal energy of the gas is tied up with

the background gravitational potential of the cluster, as is the velocity dispersion

of galaxies. In other words, 3 2 kT mp".

One can easily determine the temperature of the gas from the shape of the

spectrum. The observed intensity then (assuming a physical lengthscale) then

gives the particle density. The typical density in rich clusters is 10 3 cm 3. This

also indicates a total gas mass. It turns out that the total gas mass is comparable

to that of member galaxies in poor clusters, but it is several times that of galaxy

masses for rich clusters. Typically one hasMgas 5Mgals.

Homework on ram pressure stripping : see Shu

There are some emission lines in addition to the continuum and one can infer

the presence of highly ionized heavy elements like iron. The transitions take place

in the inner shells (e.g., n=1,2 (K,L)). The temperatures derived from degree of

ionization is consistent with that derived from continuum shape : suggests that

gas is collisionally ionized. The typical abundance is of order 1 3 solar, and it

is hardly dependent on the total cluster mass. For very poor groups, there is still

some uncertainty in the measurement of the metallicity. This indicates that ram

pressure is not important for the enrichment of ICM, otherwise there would be a

dependence on cluster temperature (Renzini 1997).

There is a small metallicity gradient, with metallicity decreasing outwards, as

was found from high resolution spectroscopy with ASCA.

The origin of this metallicity is still uncertain, although the general idea is

that galactic winds from member galaxies has enriched it. There is a good corre-

5

Fig. 4.— Coma at optical and x-ray energies.