Explaining the spectrum of Sagittarius A* with a model of an accreting black hole

Abstract

THE radio source at the centre of our Galaxy1, Sagittarius A* (Sgr A*), seems to be a low-luminosity version of active galactic nuclei—a massive black hole that is accreting gas from the surrounding region1,2. This idea is supported by observations of the gas and stars within 1 pc of Sgr A*, which appear to move under the influence of a large central mass1,3,4. A recent determination of the upper limit5,6 to the hard X-ray emission from the Galactic Centre has posed a problem for this picture, however, as the mass accretion rate implied by applying a standard accretion model to the X-ray data is far below that estimated from the observations of gas flows. Here we present a new model of accretion onto Sgr A*, in which most of the energy released is carried along with the gas and lost into the black hole of mass ∼7 x 105 solar masses, rather than appearing as radiation. The model fits the observed spectrum of Sgr A* from radio to hard X-ray wavelengths, and reconciles the low observed luminosity with a high mass-accretion rate.