Reconciling the spectrum of Sagittarius A* with a two-temperature plasma model

The radio source Sagittarius A* is thought to be powered by gas accreting onto a supermassive black hole at the centre of our Galaxy1,2. Using the high infrared accretion rates3, however, standard accretion models4 are unable to explain the observed low luminosity and spectral energy distribution5,6,7,8, which has led to the consideration of a new model: advection-dominated accretion flows9,10,11,12. In an advection-dominated flow, most of the accretion energy is stored as thermal energy in the gaswhich is then lost as the gas falls into the black hole. This model requires the protons to have a much higher temperature than the electrons, and the gas therefore has a two-temperature structure10,13,14. Although this model explains the low total luminosity15,16,17,18 and much of the spectral energy distribution (from millimetre wavelengths to hard X-rays), it has been difficult to reconcile with low-frequency radio observations. Here we show that a neglected emission process associated with the protons naturally explains the radio observations without any ‘fine tuning’ of the model parameters. This result simultaneously supports the two-temperature model of the gas and suggests that an advection-dominated accretion flow onto a black hole of 2.5 × 106 solar masses provides an accurate description of Sagittarius A*.