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This false colour picture shows the region near the centre of the Milky Way as seen by Integral in gamma rays. The position of the super-massive black hole Sgr A* and of the molecular cloud Sgr B2 are indicated. The projected distance between Sgr A* and Sgr B2 is approximately 350 light years. Only now is Sgr B2 being exposed to the gamma rays emitted by Sgr A* 350 years ago, during one of its 'high' states. This powerful radiation is absorbed and then re-emitted by the molecular hydrogen gas in Sgr B2, which becomes fluorescent in the gamma rays. The other bright objects on the right-hand side of the picture are known gamma ray emitters.

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This animation shows what astronomers believe has happened in the past near the centre of the Milky Way. A strong burst of gamma rays, emitted by Sgr A* about 350 years ago and lasting several years, has travelled through space and has now reached Sgr B2, a cloud of molecular hydrogen gas. The powerful gamma ray energy lightens up the gas, which absorbs and then re-emits the radiation it receives. Studying the radiation reflected by Sgr B2 has allowed astronomers to reconstruct for the first time in detail the hectic past of Sgr A*.