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Paper: Equipartition, Minimum Energy Density and Maximum Brightness Temperature of a Synchrotron Source
Volume: 407, The Low-Frequency Radio Universe
Page: 180
Authors: Singal, A.K.
Abstract: We show that an upper limit of ~ 1012 K on the peak brightness temperature for an incoherent synchrotron radio source, commonly referred to in the literature as an inverse Compton limit, may not really be due to inverse Compton effects. We show that a somewhat tighter limit Teq ~ 1011 is actually obtained for conditions of equipartition of energy between radiating particles and magnetic fields, which happens to be a configuration of minimum energy for a self-absorbed synchrotron radio source. An order of magnitude change in brightness temperature from Teq in either direction would require departures from equipartition of about eight orders of magnitude, implying a change in total energy of the system up to ~ 104 times the equipartition value. Constraints of such extreme energy variations means that brightness temperatures may not depart much from Teq. This is supported by the fact that peak brightness temperatures much lower than ~ 1011 K are also not seen in VLBI observations, even though there is no such theoretical bound on the lower side like the inverse Compton limit on the upper side. Absence of significant departures from equipartition means Tm ~ 1012 K or higher may hardly ever occur, making the inverse Compton limit arguments rather redundant.
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