We use 5 years of Fermi-LAT data towards the Galactic-centre giant molecular cloud complex, Sagittarius B, to test questions of how well-mixed the Galactic component of cosmic rays are and what the level of the cosmic-ray sea in different parts of the Galaxy is. We use dust-opacity maps from the Planck satellite to obtain independent methods for background subtraction and an estimate for the mass of the region. We then present high-quality spectra of γ-ray emission from 0.3 to 30GeV and obtain an estimate of the cosmic-ray spectrum from the region. We obtain an estimate of the mass of the region of 1.5±0.2x107M☉ using the Planck data, which agrees well with molecular-line-derived estimates for the same region. We find that the γ-ray flux from this region is fitted well with a cosmic-ray spectrum that is the same as is observed locally, with evidence of a small over-density at intermediate (1-10GeV) energies. We conclude that the γ-ray and cosmic-ray spectrum in the region can be well-fitted using a local cosmic-ray spectrum.
Galaxy: center - gamma rays: ISM - cosmic rays - ISM: clouds - HII regions - ISM: individual objects: Sgr B