PKS 0405-385: The smallest radio quasar?

We have observed profound variability in the radio flux density of the quasar PKS 0405-385 on timescales of less than an hour; this is unprecedented among extragalactic sources. If intrinsic to the source, these variations would imply a brightness temperature TB ∼ 1021 K, some 9 orders of magnitude larger than the inverse Compton limit for a static synchrotron source, and still a million times greater than can be accommodated with bulk relativistic motion at a Lorentz factor γ ∼ 10. The variability is intermittent with episodes lasting a few weeks to months. Our data can be explained most sensibly as interstellar scintillation of a source component that is less than 5 μas in size - a source size which implies a brightness temperature TB >5 × 1014 K, still far above the inverse Compton limit. Simply interpreted as a steady, relativistically beamed synchrotron source, this would imply a bulk Lorentz factor γ ∼ 103. © 1997. The American Astronomical Society. All rights reserved.