The centre of our Galaxy harbours a 4 million solar mass black hole that is unusually quiet : its present X-ray luminosity is more than 10 orders of magnitude less than its Eddington luminosity .
The observation of iron fluorescence and hard X-ray emission from some of the massive molecular clouds surrounding the Galactic Centre has been interpreted as an echo of a past \mathrm { 10 ^ { 39 } erg s ^ { -1 } } flare .
Alternatively , low-energy cosmic rays propagating inside the clouds might account for the observed emission , through inverse bremsstrahlung of low energy ions or bremsstrahlung emission of low energy electrons .
Here we report the observation of a clear decay of the hard X-ray emission from the molecular cloud Sgr B2 during the past 7 years thanks to more than 20 Ms of INTEGRAL exposure .
This confirms the decay previously observed comparing the 6.4 keV line fluxes measured by various X-ray instruments , but without intercalibration effects .
The measured decay time is 8.2 \pm 1.7 years , compatible with the light crossing time of the molecular cloud core .
Such a short timescale rules out inverse bremsstrahlung by cosmic-ray ions as the origin of the X ray emission .
We also obtained 2-100 keV broadband X-ray spectra by combining INTEGRAL and XMM-Newton data and compared them with detailed models of X-ray emission due to irradiation 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 time variability constraints and the huge cosmic ray electron luminosity required to explain the observed hard X-ray emission strongly favor the scenario in which the diffuse emission of Sgr B2 is scattered and reprocessed radiation emitted in the past by Sgr A* .
The spectral index of the illuminating power-law source is found to be \Gamma \sim 2 and its luminosity \mathrm { 1.5 - 5 \times 10 ^ { 39 } erg s ^ { -1 } } , depending on the relative positions of Sgr B2 and Sgr A ^ { * } .
Using recent parallax measurements that place Sgr B2 in front of Sgr A ^ { * } , we find that the period of intense activity of Sgr A ^ { * } ended between 75 and 155 years ago .