An article published in “The Astrophysical Journal Letters” describes a research on the quasar Q2237+0305 nicknamed Einstein Cross or Einstein’s Cross. Through the technique of gravitational microlensing a team of Spanish astrophysicists carried out the most accurate measures of the innermost region belonging to the disc of materials spinning around the supermassive black hole that feeds this quasar.

The quasar Q2237+0305 is located about 8 billion light years from Earth but between us and it, about 400 million light years away, there’s a galaxy. It distorts the light from the quasar with its very strong gravity, so much that the light reaches us from four different directions. For this reason, the Einstein Cross is also called G2237+0305 (with the G at the beginning instead of the Q).

The four images we see of the quasar Q2237+0305 form a cross while the reference to Einstein is due to the fact that the distortion, called gravitational lensing, is an effect predicted by the theory of general relativity formulated by the great scientist. The nickname Einstein Cross is used for the quasar Q2237+0305 but indicates a more general phenomenon because others of that kind are known such as the supernova SN Refsdal.

Quasars emit enormous amount of light, coming from the disc of materials that spins around a supermassive black hole. Its immense gravity heats up those gases and dust so much that they emit electromagnetic radiation. Their flow allows us to see the quasar billions of light years away but it and the disc of material surrounding it have such a small apparent size that the amount of light that reaches us is really reduced.

In these cases, one speaks of gravitational microlensing because the effect of gravitational lening concerns objects of which a limited amount of light reaches us. The disc of the quasar Q2237+0305 has a size comparable to that of our solar system but it’s so far away that it’s possible to try to make some measurements only using the technique of gravitational microlensing.

Thanks to two gravitational microlensing projects, OGLE (Optical Gravitational Lensing Experiment) and GLITP (Gravitational Lens International Time Project), the astrophysicists who conducted this research had available data on the brightness variation of the four images of the disc of the quasar Q2237+0305.

The exam of all these data, collected for over a decade, resulted in precise measurements of the inner disc of material around the supermassive black hole that powers the quasar Q2237+0305. That area could be the last stable orbit before the event horizon of the black hole, beyond which gravity becomes so strong that even light can no longer escape it.

This result is unique because it came thanks to the effect of gravitational lensing that in this case multiplied the image of the quasar Q2237+0305. New more sophisticated telescopes are being built so in the next decade better large-scale observations of quasars will be possible to study more deeply these phenomena and their influence on the formation and evolution of the galaxies that host them.