# Compton Catastrophe

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If you keep scattering the same electrons, as in Synchrotron Self-Compton, there is a danger, if things are dense enough, of a runaway amplification of radiation energy density, or a Compton Cooling Catastrophe. However, we've never seen anything with a brightness temperature of $10^{12}K$. What sets this inverse Compton limit at this temperature? Comparing, for a single electron, the luminosity of inverse Compton scattering to synchrotron scattering: $${L_{IC}\over L_{sync}}={{4\over3}\beta^2\gamma^2\sigma_TcU_{ph}\over {4\over3}\beta^2\gamma^2\sigma_TcU_B}={U_{ph}\over U_B} \begin{cases} >1&catastrophe\\ <1 &no\ catastrophe\end{cases}$$ Now we're going to make an approximation that we are on the Rayleigh-Jeans side of the blackbody curve, so that: \begin{aligned}U_{ph}=U_{ph,sync}&\propto\nu_mI_\nu(\nu_m)\\ &\propto\nu_m{2kT_B\over\lambda_m^2}\\ &\propto\nu_m^3T_B\\ \end{aligned} where $\nu_m$ is the frequency of peak of synchrotron emission. Now $U_B\propto B^2$ is pretty obvious: $$\nu_m\sim\gamma_m^2\nu_{cyc}\propto\gamma_m^2B$$ where this $\gamma_m$ is not $\gamax$. Making the approximation that we are in the optically thick synchrotron spectrum, so that $\gamma m_ec^2\sim kT$, then we get $\nu_m\sim T_B^2B$. 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, $${U_{ph}\over U_B}=C{\nu_m^3T_B\over\nu_m^2}T_B^4 =\left({\nu_m\over10^9Hz}\right)\left({T_B\over10^{12}K}\right)^5=1$$ 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.