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To discuss synchrotron self-absorption, we need to discuss what the spectrum of an optically thick medium of relativistic electrons looks like. If we take a bunch of spiraling around magnetic field lines with , then the energy of the photons emitted by these electrons is . This emitted photon can be absorbed be a neighboring relatvistic electron in the self-absorption process.
If this gas were optically thin, we’d just see a sharply peaked spectrum around with going as and going as . That is, when we are in the optically thin regime, more electrons in the synchrotron lead to more emission. Now let’s add more electrons to this gas. For a while in the optically thick regime, the more electrons we add, the more emission we see.
At some point, though, self-absorption starts making a difference, and we get less emission per electron added.
Let’s examine the peak amplitude of emission (that is, at ). To do this, imagine we have an optically thick ball of blackbody (perfectly absorbing and emitting) particles with a temperature carefully chosen so that . Then:
We are assuming we are in the Rayleigh-Jeans tail. Note that we are not arguing that we are operating in thermal equilibrium. Instead, we know that blackbodies are perfect absorbers and perfect emitters. By taking a blackbody as our model, we are defining the maximum intensity, we can observe. The actual observed intensity will be below , but this gives us a good upper limit.
If , then:
Since , we have that . Thus, for an optically thick gas:
It is important to remember that each has a unique corresponding . Also note that optical thickness depends inversely on frequency, so if we plot vs. , we get a power law for high frequencies ( for low optical thickness), and a dependence for low frequencies ( for high optical thickness, under self-absorption). This implies that there will be some turnover point at where is maximal. This turnover point can give us information about the strength of the B-field and the density of electroncs in the region.
Synchrotron Self-Compton (SSC)
Electrons undergoing synchrotron radiation create a photon bath which other electrons will then interact with via Inverse Compton Scattering. For original Synchrotron Radiation, that , between some minimum and maximum frequency cut-off, goes as , and that the number of photons per is , where . These frequency cut-offs were set by and . After this radiation is processed by SSC, approximately every photon is upscattered to a new energy . We are assuming that the relationship between an incoming photon frequency and its final frequency are related via a delta function. Thus:
Keep in mind that is normalized to so the integral comes out to 1 (it just accounts for the “shape” of the energy distribution function). is what contains the actual # density of ’s. It is the fraction scattered, and is generally . .
For a fixed , we find that .
In deriving the interaction of synchrotron radiation with synchrotron electrons, we derived the following formula for flux:
Now the integral over is along “slant paths” through the rectangle (, ). Some of these slant paths will not stretch all the way to or because of the boundaries imposed by and . That is to say, photons emitted by an electron at can be scattered off another electron at or vice versa. So we need to be a little more precise about the bounds on the integral:
But since , the integrand is simply , giving us:
Recall that was normalized so that , so saying that is just some multiple of , it must be that
Note for , looks like (using the above relationship):
Spectra for synchrotron radiation, with and without self-interactions. The model used to generate this figure assumes a rectangular slab as the emitting/absorbing region and that the initially emitted photons only interact with other electrons once before leaving the radiating slab. The radiating region is 1 pc thick, has an electron number density of 1 cm
, and is immersed in a uniform magnetic field of 100
G. The electrons obey a power law distribution in energy, with a minimum Lorentz factor of 10, a maximum Lorentz factor of 10,000, and a power-law index of -2.5. Note that
has a slope of 5/2 for low frequencies, and a slope of -3/4 after the turnover, as expected from the above discussion about self-absorption. Also note the bounds on the range of frequencies for the SSC spectrum—the lower bound is
times greater than the minimum frequency of the original synchrotron spectrum, and the upper bound is
times greater than the upper bound of the original spectrum.
If you keep scattering the same electrons via Synchrotron Self-Compton and things are dense enough, there is a danger of a runaway amplification of radiation energy density, or a “Compton Cooling Catastrophe”. However, we’ve never seen anything with a brightness temperature of . What sets the “inverse Compton limit” at this temperature? Comparing, for a single electron, the luminosity of inverse Compton scattering to synchrotron scattering:
Now we’re going to make an approximation that we are on the Rayleigh-Jeans side of the blackbody curve, so that:
where is the frequency of peak of synchrotron emission.
Now (see Energy Density):
where this is not . Making the approximation that we are in the optically thick synchrotron spectrum, so that , then we get . We can say that the kinetic temperature is the brightness temperature because we are talking about the average kinetic energy of the electrons generating the synchrotron radiation with a particular brightness temperature (i.e. another frequency of synchrotron radiation will have another brightness temperature, and another set of electrons moving with a different amount of kinetic energy). Thus,
A way of think about this is that, in order to avoid having infinite energy in this gas of electrons, there has to be a limit on the brightness temperature (which is determined by the density of electrons). This is a self-regulating process–if the brightness temperature goes too high, an infinite energy demand is set up, knocking it back down.