18 billions of suns support Einstein

A central black hole, with a mass equal to 18 billion times that of the Sun, is orbited by a smaller one, and the interaction of the system with its surroundings produces brightness changes that allow astronomers to study the evolution of the orbit.

This evolution is dominated by one of the most intriguing predictions of Einstein's theory of General Relativity: the emission of gravitational waves. This outstanding confirmation of Einstein's centennary theory has been recently published in the journal Nature, and Calar Alto staff, telescopes and instruments have contributed to the discovery…

Astronomers believe that very massive black holes lurk at the centres of most galaxies but often, as in the case of our own Galaxy, they remain silent and are difficult to detect. But in other cases the black holes are surrounded by disks of material that falls onto them (accretion disks). The infalling material is heated and emits huge quantities of radiation: the active nucleus of a galaxy can appear, then, as a quasar.

One of these objects is OJ 287, the centre of a galaxy placed at 3.5 billion light-years in the constellation of Cancer. But this object exhibits the peculiarity of sending every twelve years quasi-periodic pulses of energy, superposed to its normal activity. The study recently published in Nature confirms one of the suspected interpretations for this behaviour: this quasar is powered by a binary black hole. A very massive black hole is surrounded by an accretion disk: the classical quasar display. But a second, much lighter black hole orbits around the very massive black hole and blasts into the accretion disk twice per orbit: this generates the quasi-periodic pulses.

The research group led by Dr. Mauri Valtonen of University of Turku, Finland, has carefully analysed the behaviour of this system. Timing the brightness changes over many years, they have been able to plot the orbit of the small black hole, and this provides a precise way to measure the big hole's mass: 18 billion solar masses. Also, they have followed the evolution of the orbit and have checked that its size and orientation change accordingly to the predictions of Einstein's theory of General Relativity.

This theory displays all its power when dealing with extreme gravitational fields, and there are not so many situations allowing to test the theories of gravitation in such strong field situation. The study clearly shows an example of gravitational waves at work, one of the most exotic predictions of Einstein's theory. Effectively, the binary black hole orbit shrinks and evolves in a way that can only be explained if it is losing huge quantities of energy in form of gravitational radiation.

The observations leading to this discovery have been done thanks to the joint collaboration of a number of observatories at Japan, China, Turkey, Greece, Finland, Poland, Great Britain and Spain. More than 25 astronomers from 10 countries took part in the campaign. Two points deserve a special mention: first, that all the telescopes involved belonged to the category that nowadays is called of “small” apertures (only two of them were close to 2.5 m in diameter), and second, that a number of key participants were amateur astronomers who operate their own telescopes.

Calar Alto observatory participated in the observational campaigns with its 2.2 m telescope, equipped with instrument CAFOS, to perform photometric and polarimetric observations. The polarimetric data were crucial to confirm the conclusions of the study, as stated in the paper published in Nature, and only two of the participant observatories provided this kind of data.

The model of the binary black hole developed by Valtonen and collaborators predicts a new outburst of OJ 287 quasar in 2016. No doubt many telescopes will be looking to Cancer around the predicted dates of that year, and Calar Alto instruments will count among them.