Mystery of twin quasar brightness revealed

The quasar Q0957+561, some 9 billion light years away in the Ursa Major constellation, was discovered in 1979 by an Anglo-American team. Since then scientists have managed to explain why two images are observed of the same object -separated by a little less than two thousandths of a degree in the sky-, but up until now had not been able to explain the origins of the “rapid” (lasting several months) variations in their brightness.

“Just as the force of the Earth's gravity determines the movement of a rocket or the moon, the concentration of a vast mass (such as a cluster of galaxies situated between Q0957+561 and the Earth) is capable of substantially deviating rays of light from the far away quasar and producing two images of it, A and B”, so SINC was told by Luis J. Goicoechea, from the Gravitational Lenses Group at the University of Cantabria, Spain, and one of the authors of the study published in Astronomy & Astrophysics. This phenomenon is known as “gravitational lensing” and is similar to mirages seen in a desert or double vision that short-sighted people have.

However, the researcher explained that the matter that concerned astronomers, after the discovery of Q0957+561 30 years ago, was knowing whether fluctuations in the brightness of its two images were caused by intrinsic variations in the brightness of the quasar or, on the other hand, produced by the gravity of planets or stars found in a galaxy’s halo situated in the centre of the cluster which acts as a gravitational lens. The temporary scales of the fluctuations (various months) suggested to scientists the possible existence of planets forming this galaxy’s dark matter.

“It seems the attractive hypothesis of a large population of dark objects with planetary mass in the galaxy’s halo has disappeared, since the variability found in our experiment is of intrinsic origin to the quasar”, commented Goicoechea. The researchers have confirmed that fluctuations in the brightness detected in image A were repeated 14 months later in Image B. If they had been caused by the gravity of planets or stars, the fluctuations in image A would not have been exactly replicated in image B.

To measure the brightness of the two images of the twin quasar, the astronomers used the robotic telescope ‘Liverpool’, located in La Palma (Canary Islands). The study focused on the analysis of the blue and red colours in the light, with the results showing that over the months the maximum width of the brightness varied around 10% in relation to the average. Those fluctuations were also observed in image B approximately 14 months after being detected in A.

“This delay between components has been determined with a relative error of 0.5%, i.e. with great precision”, Goicoechea pointed out. Scientists have also observed that the quasar does not simultaneously emit all the optic radiation at the same time, emitting firstly the blue light then the red. In image A they measured a delay of around four days between a very prominent variation of the blue brightness and the corresponding fluctuation of the red brightness.

Presence of a supermassive black hole

The researchers believe that most of these variations are due to a disk of hot gas situated in the heart of Q0957+561. This quasar is one of the most studied, and astronomers believe that inside it there is a supermassive black hole, with a mass equivalent to more than one billion suns, around which the disc of gas would be situated. “This structure emits the optic radiation observed and feeds off the supermassive black hole”, Goicoechea indicated.

The astronomer explained it as follows: “Perpendicularly to the plane of the disc there is a jet, which was detected in radio waves and ultraviolet rays, and at a certain height there are high energy fluctuations, which firstly reach the most internal and hottest area of the disc (responsible for blue radiation), and then a little later, the most external and coldest part (where red radiation is generated)”. The regions of the disc are therefore radiated, and a part of that radiation is reflected while the other is reprocessed and re-emitted with less energy. This phenomenon is known as “reverberation” and several very recent studies indicate that it could be produced in a large number of quasars.