A recent observational campaign involving more than two dozen optical telescopes and NASA's space based SWIFT X-ray telescope allowed a team of astronomers to measure very accurately the rotational rate of one of the most massive black holes in the universe. The rotational rate of this massive black hole is one third of the maximum spin rate allowed in General Relativity. This 18 billion solar mass heavy black hole powers a quasar called OJ287 which lies about 3.5 billion light years away from Earth. Quasi-stellar radio sources or `quasars' for short, are the very bright centers of distant galaxies which emit huge amounts of electro-mag

netic radiation due to the infall of matter into their massive black holes.

This quasar lies very close to the apparent path of the Sun's motion on the celestial sphere as seen from Earth, where most searches for asteroids and comets are conducted. Therefore, its optical photometric measurements already cover more than 100 years. A careful analysis of these observations show that OJ 287 has produced quasi-periodic optical outbursts at intervals of approximately 12 years dating back to around 1891. Additionally, a close inspection of newer data sets reveals the presence of double-peaks in these outbursts.

These deductions prompted Prof. Mauri Valtonen of University of Turku, Finland and his collaborators to develop a model that requires the quasar OJ287 to harbour two unequal mass black holes. Their model involves a massive black hole with an accretion disk (a disk of interstellar material formed by matter falling into objects like black holes) while the comparatively smaller black hole revolves around it. The quasar OJ287 is visible due to the slow accretion of matter, present in the accretion disk, onto the largest black hole. Additionally, the small black hole passes through the accretion disk during its orbit which causes the disk material to heat up to very high temperatures. This heated material flows out from both sides of the accretion disk and radiates strongly for weeks. This causes peaks in the brightness, and the double peaks arise due to the ellipticity of the orbit, as shown in the figure.

The binary black hole model for OJ287 implies that the smaller black hole's orbit should rotate, and this changes where and when the smaller hole impacts the accretion disk. This effect arises from Einstein's General Theory of Relativity and its precessional rate depends mainly on the two black hole masses and the rotation rate of the more massive black hole. In 2010, Valtonen and collaborators used eight well timed bright outbursts of OJ287 to accurately measure the precession rate of the smaller hole's orbit. This analysis revealed for the first time the rotation rate of the massive black hole along with accurate estimates for the masses of the two black holes. This was possible since the smaller black hole's orbit precess at an incredible 39 degrees per individual orbit. The General Relativistic model for OJ287 also predicted that the next outburst could occur around the time of GR Centenary, 25 November 2015, which marks the 100th anniversary of Einstein's General Theory of Relativity.

An observational campaign was therefore launched to catch this predicted outburst. The predicted optical flare began around November 18, 2015 and reached its maximum brightness on December 4, 2015. It is the timing of this bright outburst that allowed Valtonen and his co-workers to directly measure the rotation rate of the more massive black hole to be one third of the maximum spin rate allowed in General Relativity. In other words, its Kerr parameter is accurately measured to be 0.31 and its maximum allowed value in General Relativity is one. In comparison, the Kerr parameter of the final black hole associated with the first ever direct detection of gravitational waves is only estimated to be below 0.7.

The observations leading to accurate spin measurement have been made due to the collaboration of a number of optical telescopes in Japan, South Korea, India, Turkey, Greece, Finland, Poland, Germany, UK, Spain, USA and Mexico. The effort, led by Staszek Zola of Poland, involved close to 100 astronomers from these countries. Interestingly, a number of key participants were amateur astronomers who operate their own telescopes. Valtonen's team that developed and contributed to the spinning binary black hole model include theoretical astrophysicist A. Gopakumar from TIFR, India, and Italian X-Ray astronomer Stefano Ciprini who obtained and analyzed the X-ray data.

The occurrence of the predicted optical outburst of OJ287 also allowed the team to confirm the loss of orbital energy to gravitational waves within two percent of General Relativity's prediction. This provides the first indirect evidence for the existence of a massive spinning black hole binary emitting gravitational waves. This is encouraging news for the Pulsar Timing Array efforts that will directly detect gravitational waves from such systems in the near future. Therefore, the present optical outburst of OJ287 makes a fitting contribution to the centenary celebrations of General Relativity and adds to the excitement of the first direct observation of a transient gravitational wave signal by LIGO.

What does a black hole sound like?

Actually, since sound waves don't propagate in the near-vacuum of outer space, we can't hear black holes. But if we could, they might sound somewhat similar to the static from a badly tuned TV set. 

That's the word from the authors of a study published Friday in the journal Science Advances. In addition to giving us a sense of what black holes might "sound" like, the study sheds new light on the behavior of accretion disks. Those are the disk-shaped collections of matter, such as gas and dust, that surround black holes.

Accretion disks are often used as tools to study black holes since, unlike black holes themselves, they give off light.

"Since black holes cannot be observed directly it is only because of the existence of these disks that we can infer what a black hole might 'sound' like," Dr. Simone Scaringi, lead author of the study and a Humboldt research fellow at the Max-Planck-Institute for Extraterrestrial Physics in Germany, said in an email. "It is important to realize, however, that because space is almost a vacuum, there is no real sound. What we did is observe brightness variations over time for accreting black holes (and other systems too). I then converted these light variations into sound variations."

In other words, Scaringi and his colleagues took observations of the shifting light patterns from accretion disks (taken by NASA's Kepler space telescope, ground-based telescopes, and the European Space Agency's XMM-Newton satellite) and converted them into sound waves.

For instance, if the light intensity from an accretion disk fluctuated 10 times a second, it was converted to a sound wave of 10 cycles per second, or 10 Hertz, Space.com reported.

Scaringi then had to "translate" the sounds into a range that humans can hear.

"[T]he variations we see in accreting systems are very low or very high in frequency, depending on the size of the system, and would fall outside of the human hearing range," Scaringi said. "Because of this I had to shift the 'sounds' of accreting systems into the human audible range for us to listen to." 

The researchers said that they were surprised to find similar brightness variations not only in accretion disks surrounding black holes but also in disks surrounding other celestial objects, including white dwarf stars and young stellar objects.