Active galactic nuclei, which power quasars, are the most energetic objects we're aware of. Powered by the disk of matter swirling into the supermassive black holes at the center of galaxies, these objects pump out prodigious amounts of energy. But the accretion disks that power this output aren't especially large (roughly the size of our solar system), and they're buried in distant galaxies, making them impossible to image directly. But scientists have now figured how to use gravitational lensing to get a closer look.

Gravitational lensing of quasars is actually fairly old news. Massive objects like galaxies distort space in a way that can focus light from objects in the background. Most lensed objects are visible as light that passes near a massive object, providing a clear separation between the object being magnified and the foreground object doing the lensing. The authors of the new paper looked for something a bit different, namely the lensing of quasars caused as the light passes through an intervening galaxy.

For the most part, this causes problems. For one, the light of the galaxy will intermingle with the light of the quasar. To get around this, the authors looked for lensing objects that were red shifted enough that the galaxy's light had mostly dropped below that of the energies produced by the quasar. This required them to use the Hubble Space Telescope to image the lensed quasar in the UV area of the spectrum, where the quasar should still be visible, but not the lensing galaxy.

Even then, dust in the galaxy will frequently block out details of the quasar. And in a third of the lensed objects the paper looks at, this turned out to be a problem. In the remainder, the researchers searched for subtle changes in the color of the light being output by the quasar.

These are the product of what's called microlensing, when individual stars in the host galaxy pass between us and the quasar in the background, acting as a second lens and magnifying a small portion of the image. With this microlensing, the authors were able to actually detect the size of the accretion disk of the quasar (between four and 11 light days across) and get some information about the light being emitted from different regions of it. Since light emissions are proportional to temperature, this provides the clearest picture yet of the conditions inside the accretion disk.

Astrophysical Journal, 2011. DOI: not yet available.