Variable Hard X-Ray Emission from the Sgr B2 Molecular Cloud

The 4 million solar masses black hole at the Galactic centre (GC) is remarkably quiet today. The molecular cloud Sgr B2, among others, emits strong Fe Kα photons as well as hard X-rays up to 100 keV. This has been interpreted as the result of the illumination of the cloud by a past high luminosity period of Sgr A^*. This interpretation is still controversial since irradiation by subrelativistic cosmic rays might also account for the observed spectra. We present here the results of 7 years of INTEGRAL monitoring of Sgr B2 and show that its hard X-ray (20-60 keV) flux has decreased by nearly 40% during that period. This cannot be accounted for by cosmic ray interpretations and support the idea that Sgr A^* was more active in the past and has been quiescent since about 100 years.