The Dance Of Two Black Holes

Astronomers have measured the rotational speed of a black hole by observing an outburst of light that occurred close to the centenary celebration of Einstein’s General Theory of Relativity.

AsianScientist (Mar. 21, 2016) – By analyzing bright outbursts from one of the most massive black holes in the universe, an international team of astronomers have accurately measured its rotational rate. This finding—a timely one, as it coincides with the 100th anniversary of Einstein’s General Theory of Relativity—provides indirect evidence for the existence of a massive spinning black hole binary system emitting gravitational waves. Their work was published in The Astrophysical Journal Letters.

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-magnetic radiation due to matter falling into their massive black holes.

Interestingly, careful analyses of these observations show that OJ287 has produced 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 Professor Mauri Valtonen of the University of Turku, Finland and collaborators, including theoretical astrophysicist A. Gopakumar from the Tata Institute of Fundamental Research in India, to theorize that OJ287 harbors two black holes. Their model involves a massive black hole with the comparatively smaller black hole revolving around it.

The small black hole passes through the larger black hole’s accretion disk—a disk of interstellar material formed by matter falling into black holes—during its orbit, which causes the disk material to heat up to very high temperatures. This causes peaks in the brightness, and the double peaks arise due to the ellipticity of the orbit, as shown in the figure below.

An illustration of the binary black hole system in OJ287. Credit: Gary Poyner
An illustration of the binary black hole system in OJ287. Credit: Gary Poyner

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. The research team first measured the precession rate, or the change in orientation of the rotational axis, of the smaller hole’s orbit. Their analysis revealed the rotation rate of the massive black hole, along with accurate estimates for the masses of both black holes.

The General Relativistic model for OJ287 also predicted that the next outburst could occur around 25 November 2015, which marks the 100th anniversary of Einstein’s General Theory of Relativity.

An observational campaign was launched to catch this predicted outburst. The predicted optical flare began around November 18, 2015 and reached its maximum brightness on December 4, 2015. The timing of this bright outburst allowed the research team 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.

This is encouraging news for the Pulsar Timing Array efforts that will directly detect gravitational waves from such systems in the near future.


The article can be found at: Valtonen et al. (2016) Primary Black Hole Spin in OJ 287 as Determined by the General Relativity Centenary Flare.

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Source: Tata Institute of Fundamental Research; Photo: Shutterstock.
Disclaimer: This article does not necessarily reflect the views of AsianScientist or its staff.

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