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For more than a decade, astronomers have studied several mysterious sources of X-rays near the centre of the Milky Way. The X-rays are a puzzle because they come from clouds that are not hot enough to produce them. That has left astrophysicists scratching their heads over the origin of these emissions.

One idea is that the emissions are echoes of X-rays from another, more powerful source that must have erupted earlier. And indeed various lines of evidence seem to support this. For example, a few years ago, scientists discovered that the X-rays were flickering in exactly the way you’d expect if the irregularly shaped clouds were being illuminated by another source.

But what could generate such a flare? The answer depends on the size of the original. If relatively small, then the culprit could be a binary system made up of an ordinary star and a white dwarf or neutron star, which are known to emit X-rays at intensities of up to 10^35 ergs.

But if the intensity of the original flare were much larger than this, there can be only one culprit: the supermassive black hole at the centre of the galaxy: Sagittarius A* or Sgr A* (pronounced Sagittarius A-star) .

Today, Masayoshi Nobukawa and buddies at Kyoto University in Japan provide the first unambiguous evidence that Sgr A* is the guilty party. They say they’ve measured the intensity of the X-rays from various different clouds over several years using the Suzaku orbiting X-ray telescope.

What they’ve found is interesting. The X-rays from these clouds are all dropping in exactly the same way albeit shifted in time by a few years. That can only be possible if the clouds at different distances were illuminated by the same X-ray flare as it fizzed out.

Nobukawa and co also calculate the brightness of this original flare at more than 10^39 ergs, more than any ordinary star or binary system could produce.

Their conclusion is that Sgr A* must be responsible. This flare would have bathed us in X-rays some time ago but the echo from the molecular clouds is reaching us only now.

This echo also allows astronomers to work out when the supermassive black hole erupted. All they have to do is measure the clouds’ distances in light years from the galactic centre.

It turns out that these molecular clouds are only a few hundred light years from Sgr A*, which means that the supermassive black hole must have erupted during the Renaissance period (roughly from the 14th-17th centuries).

That’s a neat bit of astronomy, which raises an interesting question. Astronomers can see today that Sgr A* emits X-rays with an intensity of about 10^34 ergs and constantly emits small flares. It would be only natural to think that if this object has emitted big flares in the recent past, it will repeat the performance in the near future.

But even if that happens, astronomers might still have to wait for some echoes to study it in detail. Our view of Sgr A* is so badly obscured that most of any light coming our way is lost.

Ref: arxiv.org/abs/1109.1950: New Evidence For High Activity Of The Super-Massive Black Hole In Our Galaxy


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