Observing a quasar accretion disc using gravitational lensing

The accretion discs around supermassive black holes are too distant and too small to observe using conventional methods. However, by combining gravitational macrolensing by an intermediate galaxy, and gravitational microlensing by stars within that galaxy, astronomers are able to probe the structure of these discs. This video explains the method used.

The video begins by showing a simulation of the view from Hubble, with the lens galaxy and bright lensed images of the quasar. It then rotates round to a view from the side, showing the relative positions of the objects and the path of the light beams as they are lensed by the galaxy. This is gravitational macrolensing. Note that the distances are shortened.

The video then zooms into three different points: The accretion disc at the heart of the quasar, the point at which the quasar’s light passes through the lens galaxy, and the view from Earth’s orbit showing Hubble’s location. The beams of light coming from the accretion disc are bluer when they come from the centre of the disc, and redder when they come from the edges. This is because the centre of the disc, the part closest to the black hole, is much hotter than the edge.

As a star passes through the path of the light, beams of light from different parts of the disc get deflected towards the telescope at different times: note how the cross-hairs scanning across the disc of the quasar matches the colour that is deflected towards the telescope at any given time. This is gravitational microlensing.

This method can be used to observe the colour profile of the disc with unprecedented precision, comparable to being able to distinguish individual grains of sand on the surface of the Moon.