Dissecting a black hole with natural magnifying glasses
BY DR EMILY BALDWIN
ASTRONOMY NOW

Posted: 16 December, 2008

Combining the natural magnifying power of galactic lenses with ESO’s Very Large Telescope, astronomers have scrutinised a supermassive black hole 10 billion light years away.

By using a technique known as gravitational lensing – the effect of a foreground galaxy or star magnifying a distant object by bending light with its gravitational field – astronomers can hone in on an object that would otherwise be too distant to observe in much detail. In this case, a team of astronomers were able to study the inner parts of a black hole’s accretion disc with a level of detail a thousand times better than that of the best telescopes in the world, providing the first observational confirmation of the prevalent theoretical models of such discs.

The Einstein Cross and the galaxy that caused this cosmic mirage. The cross-shaped configuration consists of four images of a single very distant source. The multiple images are a result of gravitational lensing by a foreground galaxy. The inset shows a close up of the Einstein Cross. The central blob is the nucleus of the lensing galaxy, surrounded by the four mirage images of the distant quasar. Image: ESO/F. Courbin et al.

The team studied the ‘Einstein Cross’, a cross-shaped configuration consisting of four images of a single very distant light source, in this case a quasar approximately ten billion light-years away, fortuitously magnified by a foreground lensing galaxy which is ten times closer. "The combination of this natural magnification with the use of a big telescope provides us with the sharpest details ever obtained," says team leader Frederic Courbin. The quasar is known as QSO 2237+0305.

This galactic magnification effect is known as macrolensing, but in addition to macrolensing by a galaxy, stars in the lensing galaxy act as secondary lenses to produce an additional magnification. Since this secondary magnification is based on the same principle as macrolensing but on a smaller scale, it is known as microlensing. Given that the stars are moving in the lensing galaxy, the microlensing magnification also changes with time, and from Earth, the brightness of the quasar images flickers around a mean value.

“With lensing and microlensing, surface brightness is conserved, so if you change the apparent size of an object you also change its brightness, and lensing does exactly that,” Courbin explains. “Now, the area actually magnified by the stars is limited and depends on the mass of the stars. By chance, in the Einstein Cross, the size of the area magnified by the stars is comparable to the apparent size of the quasar accretion disc on the plane of the sky.”

This animation demonstrates the natural technique of macro- and micro-lensing. Image: ESO.

The microlensing affects various emission regions of the disc in different ways, with smaller regions being more magnified. Because differently sized regions have different colours (or temperatures), the net effect of the microlensing is to produce colour variations in the quasar images, in addition to the brightness variations. By observing these variations in detail for several years, astronomers can measure how matter and energy are distributed about the supermassive black hole that lurks inside the quasar. Therefore, the team observed the Einstein Cross three times a month over a period of three years using ESO's Very Large Telescope (VLT), monitoring all the brightness and colour changes of the four images.

“The disc itself has a temperature profile, that is, its colour is not the same as you go from the central quasar to the outer regions,” Courbin tells Astronomy Now. “As a consequence, when the stars are moving in the lensing galaxy they magnify different parts of the accretion disc, such that the microlensing makes the quasar images change colour with time. This change of colour allows us to reconstruct the colour profile or temperature profile of the disc, with quite high accuracy. Microlensing gives us some sort of 1D “scan” across the quasar accretion disc that gives the relative sizes of the regions of different colours in the disc.”

Measuring the way the temperature is distributed around the central black hole is a unique achievement, and various theories exist for the formation and fuelling of quasars, each of which predicts a different profile. Two leading theories suggest that either the temperature profile of the disc is driven by X-ray emission from matter falling onto the central black hole or that accretion discs result from the angular momentum transfered from the central black hole to the disc due to the spin of the black hole. “Our observations marginally favour the latter model,” reports Courbin. “However, what is nice with our observations is that they are model-independent.” Indeed, this is the first time that accurate and direct measurements of the size of a quasar accretion disc has been made in this way.

"Thanks to this unique dataset, we could show that the most energetic radiation is emitted in the central light day away from the supermassive black hole and, more importantly, that the energy decreases with distance to the black hole almost exactly in the way predicted by theory," says Alexander Eigenbrod, who completed the analysis of the data.

The study demonstrates that by coupling the natural effects of gravitational lensing with the world’s most sensitive telescopes, astronomers can probe regions on scales as small as a millionth of an arcsecond, corresponding to the size of a one euro coin seen at a distance of five million kilometres, that is, about 13 times the distance to the Moon. "This is 1,000 times better than can be achieved using normal techniques with any existing telescope," adds Courbin.

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