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The Forefront of Space Science

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But the most important discovery was still to come. We had coordinated our observations with those of an X-ray satellite operated by NASA. And when we combined the optical and X-ray data, we found a fascinating connection between the two. When the X-ray light from the black hole increases, the visible light instead decreases (Figure 3). But once the X-rays have peaked, the visible light suddenly shoots up to show a sharp spike. The visible light is delayed with respect to X-ray by a precise, and very short time of only 1/6th of a second. What gives rise to these intriguing patterns of light? One idea involves "jets" of hot plasmas that are shot away from the black hole. This plasma travels at speeds close to that of light, and and may carry energy equivalent to 1000 Suns, some of which can be emitted as the fast optical flashes that we observe. The optical and X-ray patterns can now tell us in detail about the behaviour of highly energetic plasma in black hole environment. Imagine understanding all this detail without ever directly seeing the black hole itself! This is the power of the scientific method.

Figure 3
Figure 3. Graph showing the change in visible light and X-rays from GX 339-4, probed at a high time resolution faster than 1 second. Time progresses from left to right. The X-ray flare in blue shows a simple, broad peak, and smooth decline ranging over several seconds. But surprisingly, the visible light emission (red curve) instead decreases slightly, before displaying a fast spike just after the X-rays have peaked (delayed by 0.15 seconds). The visible light spike lasts just a small fraction of a second, much smaller than X-rays. This sequence of events was seen repeatedly over several hours of observation, and the graph shows the average behaviour from these observations.

Super massive black holes: a case fit for Sherlock Holmes

Although these black holes are certainly violent, they pale in comparison to so-called "super massive black holes", which are the real monsters of the Universe. Super massive black holes are found at the centers of all large galaxies, and each one can weigh anywhere from a million to 10 billion Suns, as their name suggests. Our own Galaxy also hosts one with a mass of about 3 million Suns. In many galaxies, these are surrounded by matter which glows hot and bright before falling to its doom into the hole. Remarkably, there is an intimate connection between these black holes, and the galaxies in which they lie, in that the black hole mass grows in close proportion to the stars in the galaxy. Why is this remarkable? Because it means that somehow the black hole, with a size somewhat smaller than our Solar system, knows about the galaxy on physical scales more than 1 million times larger! How this occurs is one of the hot topics of current black hole research.

One of the main challenges in this work is to understand how the accreting matter actually flows in from the large scales of the galaxy towards the center. The matter may flow in as a stream, or it may be instead that entire stars which stray too close are shred apart and swallowed whole by the black hole. In order to study this, we first need to cleanly separate out, or 'resolve', the black hole environment - in particular, we must be able to distinguish the black hole from the stars that immediately surround it. This is made difficult by the vast distances to other galaxies. Imagine trying to separate the two headlights of a car in London from Tokyo, at a distance of approximately 10000 km - that is the equivalent feat of what we are trying to do when resolving the environments of super massive black holes! The largest telescopes in the world are steadily bringing us closer to this goal.

One of the perks of being an astronomer is the freedom to travel to exotic places. I was fortunate to work in the South American country of Chile, at Europe's Very Large Telescope. With a huge primary mirror 8 m in size, this telescope is the southern hemisphere equivalent of Japan's Subaru telescope in Hawaii, which I am also now using to study black holes. It is located in the Atacama Desert, one of the driest places on Earth. Seeing thousands of stars under the crisp desert night was a truly moving experience.

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