VSOP brightness temperature in NRAO 530 exceeds the inverse Compton limit Original Research Article

We present here space-based VLBI observations with VSOP and a southern hemisphere ground array of the gamma-ray blazar NRAO 530 at 1.6 and 5 GHz. The 5 GHz show a single component with a brightness temperature of ∼ 3 × 1012 K, significantly in excess of the inverse Compton limit and of the equipartition brightness temperature limit (Readhead 1994). This is strong evidence for relativistic motion in a jet and implies model-dependent Doppler boosting factors in the range 6 to 60. We show that a simple homogeneous sphere probably does not model the emission region accurately. We favor instead an inhomogeneous jet model which requires a Doppler boosting factor of 15.