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Gyro-orbit size, brightness temperature limit and implausibility of coherent emission by bunching in synchrotron radio sources
We show that an upper limit on the maximum brightness temperature for a
self-absorbed incoherent synchrotron radio source is obtained from the size of
its gyro orbits, which in turn must lie well within the confines of the total
source extent. These temperature limits are obtained without recourse to
inverse Compton effects or the condition of equipartition of energy between
magnetic fields and relativistic particles. For radio variables, the intra-day
variability (IDV) implies brightness temperatures K in the
co-moving rest frame of the source. This, if interpreted purely due to an
incoherent synchrotron emission, would imply gyro radii cm, the size
of the universe, while from the causality arguments the inferred maximum size
of the source in such a case is cm. Such high
brightness temperatures are sometimes modeled in the literature as some
coherent emission process where bunches of non-thermal particles are somehow
formed that radiate in phase. We show that, unlike in case of curvature
radiation models proposed in pulsars, in the synchrotron radiation mechanism
the oppositely charged particles would contribute together to the coherent
phenomenon without the need to form separate bunches of the opposite charges.
At the same time we show that bunches would disperse over dimensions larger
than a wavelength in time shorter than the gyro orbital period
( sec). Therefore a coherent emission by bunches
cannot be a plausible explanation of the high brightness temperatures inferred
in extragalactic radio sources showing variability over a few hours or longer.Comment: 8 page
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