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The centre of our Galaxy harbours a 4 million solar mass black hole that isunusually quiet: its present X-ray luminosity is more than 10 orders ofmagnitude less than its Eddington luminosity. The observation of ironfluorescence and hard X-ray emission from some of the massive molecular cloudssurrounding the Galactic Centre has been interpreted as an echo of a pastflare. Alternatively, low-energy cosmic rays propagating inside the cloudsmight account for the observed emission, through inverse bremsstrahlung of lowenergy ions or bremsstrahlung emission of low energy electrons. Here we reportthe observation of a clear decay of the hard X-ray emission from the molecularcloud Sgr B2 during the past 7 years thanks to more than 20 Ms of INTEGRALexposure. The measured decay time is compatible with the light crossing time ofthe molecular cloud core . Such a short timescale rules out inversebremsstrahlung by cosmic-ray ions as the origin of the X ray emission. We alsoobtained 2-100 keV broadband X-ray spectra by combining INTEGRAL and XMM-Newtondata and compared them with detailed models of X-ray emission due toirradiation of molecular gas by (i) low-energy cosmic-ray electrons and (ii)hard X-rays. Both models can reproduce the data equally well, but the timevariability constraints and the huge cosmic ray electron luminosity required toexplain the observed hard X-ray emission strongly favor the scenario in whichthe diffuse emission of Sgr B2 is scattered and reprocessed radiation emittedin the past by Sgr A*. Using recent parallax measurements that place Sgr B2 infront of Sgr A*, we find that the period of intense activity of Sgr A* endedbetween 75 and 155 years ago.

Fading hard X-ray emission from the Galactic Centre molecular cloud Sgr B2