BL LAC OBJECT PKS B1144−379: An extreme scintillator

Rapid variability in the radio flux density of the BL Lac object PKS B1144–379 has been observed at four frequencies, ranging from 1.5 to 15 GHz, with the Very Large Array and the University of Tasmania's Ceduna antenna. Intrinsic and line-of-sight effects were examined as possible causes of this variability, with interstellar scintillation best explaining the frequency dependence of the variability timescales and modulation indices. This scintillation is consistent with a compact source 20-40 μas or 0.15-0.3 pc in size. The inferred brightness temperature for PKS B1144–379 (assuming that the observed variations are due to scintillation) is 6.2 × 10¹² K at 4.9 GHz, with approximately 10% of the total flux in the scintillating component. We show that scintillation surveys aimed at identifying variability timescales of days to weeks are an effective way to identify the active galactic nuclei with the highest brightness temperatures.