The inverse Compton catastrophe and high brightness temperature radio sources

Context. The inverse Compton catastrophe is the dramatic rise in the luminosity of inverse-Compton scattered photons predicted to occur when the synchrotron brightness temperature exceeds a threshold value, usually estimated to be 1012 K. However, this effect appears to be in contradiction with observation because: (i) the threshold is substantially exceeded by several intra-day variable radio sources, but the inverse Compton emission is not observed, (ii) powerful, extra-galactic radio sources of known angular size do not appear to congregate close to the predicted maximum brightness temperature. Aims. We re-examine the parameter space available to synchrotron sources using a non-standard electron distribution, in order to see whether the revised threshold temperature is consistent with the data. Methods. We apply the theory of synchrotron radiation to a population of monoenergetic electrons. The electron distribution and the population of each generation of scattered photons are computed using spatially averaged equations. The results are formulated in terms of the electron Lorentz factors that characterise sources at the threshold temperature and sources in which the particle and magnetic field energy density are in equipartition. Results. We confirm our previous finding that intrinsic brightness temperatures TB ∼ 1014 K can occur without catastrophic cooling. We show that substantially higher temperatures cannot be achieved either in transitory solutions or in solutions that balance losses with a powerful acceleration mechanism. Depending on the observing frequency, we find strong cooling can set in at a range of threshold temperatures and the imposition of the additional constraint of equipartition between particle and magnetic field energy is not warranted by the data. Conclusions. Postulating a monoenergetic electron distribution, which approximates one that is truncated below a certain Lorentz factor (γmin), alleviates several theoretical difficulties associated with the inverse Compton catastrophe, including anomalously high brightness temperatures and the apparent lack of clustering of powerful sources at 1012 K.