RadioAstron Observations of the Quasar 3C273: a Challenge to the Brightness Temperature Limit

TL;DR

RadioAstron observations of quasar 3C273 reveal brightness temperatures exceeding theoretical limits, challenging existing models of non-thermal emission near supermassive black holes and suggesting higher Doppler factors.

Contribution

First space VLBI measurements of 3C273 at unprecedented angular resolution, showing brightness temperatures above $10^{13}$ K, surpassing previous theoretical and observational limits.

Findings

Brightness temperature exceeds $10^{13}$ K.

Angular structure as small as 26 microarcseconds.

Challenges current models of jet emission and Doppler boosting.

Abstract

Inverse Compton cooling limits the brightness temperature of the radiating plasma to a maximum of $10^{11.5}$ K. Relativistic boosting can increase its observed value, but apparent brightness temperatures much in excess of $10^{13}$ K are inaccessible using ground-based very long baseline interferometry (VLBI) at any wavelength. We present observations of the quasar 3C273, made with the space VLBI mission RadioAstron on baselines up to 171,000 km, which directly reveal the presence of angular structure as small as 26 $\mu$as (2.7 light months) and brightness temperature in excess of $10^{13}$ K. These measurements challenge our understanding of the non-thermal continuum emission in the vicinity of supermassive black holes and require a much higher Doppler factor than what is determined from jet apparent kinematics.