EXPLAINING THE SPECTRUM OF SAGITTARIUS-A-ASTERISK WITH A MODEL OF AN ACCRETING BLACK-HOLE

THE radio source at the centre of our Galaxy(1), Sagittarius A (Sgr A ), seems to be a low-luminosity version of active galactic nuclei-a m assive black hole that is accreting gas from the surrounding region(1, 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 c entral mass(1,3,4). A recent determination of the upper limit(5,6) to the hard X-ray emission from the Galactic Centre has posed a problem f or this picture, however, as the mass accretion rate implied by applyi ng a standard accretion model to the X-ray data is far below that esti mated 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 carr ied along with the gas and lost into the black hole of mass similar to 7 x 10(5) solar masses, rather than appearing as radiation. The model fits the observed spectrum of Sgr A from radio to hard X-ray wavelen gths, and reconciles the low observed luminosity with a high mass-accr etion rate.