Einstein's Cross under the gravitational microlens

The Spanish interuniversity research group has obtained measurements for the innermost region of a disc of matter in orbital motion around a supermassive black hole tucked inside the quasar known as Einstein's Cross (Q2237-0305). It constitutes the most precise set of measurements achieved to date for such a small and distant object, and was made possible thanks to years of monitoring as part of the OGLE and GLITP gravitational microlensing projects, which have had their lenses trained on this quasar for 12 and 9 years, respectively.

Typically, astronomers can only detect bright objects that emit a lot of light or large objects that block background light. Microlensing can be used to detect objects that either emit little light or are too far away to be measured the usual way. It measures how much the light emitted bends around other objects lying directly in the path between it and us, with the foreground object acting as a lens and magnifying our view of very distant objects, like quasars and regions within them.

At the edge of a black hole

Quasars are very small, very distant objects that emit vast amounts of light. Their energy comes from mass falling onto the accretion disc, a disc of matter spinning extremely fast around a massive central body; in the case of quasars, around a supermassive black hole. Q2237-0305's accretion disc is comparable to the size of our solar system, but it is so far away that it has only been possible to measure its structure via microlensing.

By studying the variation in brightness of the four different images of the disc provided by OGLE and GLITP, researchers have been able to obtain precise measurements of the structure of its innermost region, right at the edge of the black hole (also known as the event horizon).

José Antonio Muñoz, lecturer at the Department of Astronomy and Astrophysics at the Universitat de València, who took part in this research alongside colleagues at the universities of Granada, Cadiz and the Canary Islands Astrophysics Institute, explains: "In recent years we have shown how microlensing allows us to analyse the structure of quasar accretion discs. Now we have obtained precise measurements of the innermost region of one, likely its innermost stable orbit closest to the black hole at its centre."

His colleague at the University of Granada, Jorge Jiménez Vicente, adds that "the big breakthrough in this work is that we have been able to detect a structure on the innermost edge of such a small disc, so far away. It is like being able to detect a one euro coin located over 100,000 kilometres away."

Only one in every 500 quasars can be measured in this way. The information obtained will be of enormous use for researchers to gain a deeper understanding of quasars, key to the formation and evolution of galaxies.

Jiménez Vicente points to a future, when large-scale monitoring programs -like the 8.4 metre Large Synoptic Survey Telescope planned for northern Chile by 2022- are up-and-running, where "the detection of high magnification microlensing events like this one will be possible for thousands of quasars." This "will open up a unique window onto the innermost edges of supermassive black holes at the heart of quasars," concludes the Muñoz of the UV.