Is the Galactic Centre gamma-ray source 1E1740.7 - 2942 accreting from a molecular cloud?

FOR twenty years there have been detections of an intermittent source of 511-keV electron-positron annihilation radiation at the centre of our Galaxy^1-3. Until recently, none of the balloon or satellite experiments lucky enough to catch the source in its 'on' state had spatial resolution better than several degrees, but on 13-14 October 1990 the French SIGMA experiment aboard the Soviet GRANAT spacecraft observed a day-long burst of narrow-band γ rays, probably annihilation radiation, from within 1.5 arcmin of the known X-ray source IE 1740.7-2942, about 50 arcmin from Sgr A West, which contains the dynamical centre of the Milky Way^4-6. Here we report millimetre-wavelength observations showing that this variable X-ray source lies on a line of sight to the dense 10⁵M_solar molecular cloud G-0.86-0.08, near the Galactic Centre. We suggest that the γ-ray source, probably a compact object, possibly a black hole, is accreting dense gas directly from the molecular cloud.