From: njzy@vax5.cit.cornell.edu (T. Joseph Lazio, Cornell University)
Newsgroups: sci.astro
Subject: Re: Compton Catastrophe
Date: 21 Jan 93 10:06:10 EST
Distribution: sci
Organization: Cornell University

In article <psima01.727536934@convex>,
psima01@convex.zdv.uni-tuebingen.de (Michael Maisack) writes: 
> 
> can soemone please explain to me what the "Compton catastrophe"
> is, how it comes about and what the limits are?

 The "Compton catastrophe" is the term given to the thermostat-like
 action produced by inverse-Compton scattering in a magnetized, 
 relativistic plasma.

 Qualitatively, it works in the following manner.  A relativistic 
 plasma, e.g. a gas of electrons all moving near the speed of 
 light, will produce synchrotron radiation if the plasma is 
 threaded by a magnetic field.  Synchrotron radiation is a 
 fundamental radiation process due to magnetic fields:  In the 
 presence of a magnetic field, a charged particle, e.g. electron, 
 is confined to spiral around the magnetic field lines.  As a result, 
 it emits photons.

 If synchrotron radiation was the only important radiation process, 
 the story would end here.  The only energy loss process would 
 be synchrotron emission.  However, the photons can interact on the 
 electrons producing an additional energy loss due to inverse 
 Compton (IC) scattering.  IC scattering is when an electron 
 scatters off a photon, increasing the energy of the photon and 
 lowering the energy of the electron.

 The thermostat mechanism arises because these two processes compete.
 In order to produce synchrotron radiation, i.e. photons, one requires
 relativistic electrons.  However, IC scattering acts to reduce the 
 number of available relativistic electrons.  In fact, the ratio 
 between the IC luminosity, L_IC, and the synchrotron radiation, L_s, 
 has the simple form,

         L_IC         U_rad
         ----    =    -----
         L_s          U_B

 where U_rad is the energy density in the radiation field and U_B 
 is the energy density in the magnetic field.

 The so-called "Compton catastrophe" arises when U_rad/U_B > 1. 
 In this case, IC scattering is reducing the number of relativistic
 electrons faster than the relativistic electrons can produce 
 synchrotron radiation, thereby shutting down the entire process.

 Astronomically, this process is thought to be important in the 
 cores of Active Galactic Nuclei (AGN).  In particular, the number 
 of AGN with so-called brightness temperatures > 1.0E12 K is very 
 small.  This limit of 1.0E12 K is taken to be reflection of the 
 "Compton catastrophe" at work.

 This is largely from memory; corrections and/or additions welcome.
 If desired, I can dig up my radio astrophysics notes and be a bit
 more quantitative.

-- 
 T. Joseph Lazio                | Why relativity? and Why  
 514 Space Sciences             | turbulence?  I really believe   
 Ithaca, NY  14853-6801         | [God] will have an answer for the
 (607) 255-6420                 | first [question].
 lazio@astrosun.tn.cornell.edu  |   -- W. Heisenberg, on his death bed
ICBM:                           |
 42 deg. 20' 08" N              |              STOP RAPE
 76 deg. 28' 48" W              |
Newsgroups: sci.astro