Brighter than a trillion suns: an intense X-rated drama

You may be unaware of the celestial wonder known as OJ 287 but, as you will see, it is one of the most outlandish objects in the cosmos. Astronomers have known of periodic eruptions from OJ 287 since 1888 and in recent decades a mind-boggling explanation has emerged. It seems that the outbursts arise deep in the heart of a distant galaxy where two supermassive black holes are locked in a deadly embrace.

What is a black hole?

A black hole forms when a huge quantity of matter collapses under its own gravity to form an object whose gravitational attraction is so intense that nothing can escape, not even light. This fate awaits the most massive stars at the end of their lives.

Such stellar mass black holes may be a whopping five, ten, or even a hundred times the mass of the Sun. The first stellar mass black hole to be identified is known as Cygnus X-1. A black hole’s size is characterized by its event horizon. This is the sphere of no return: once inside all roads lead inexorably inwards. The radius of the event horizon of a 10 solar mass black hole is just 30 kilometres.

Astronomers believe that at the centre of every galaxy there lurks a black hole on another scale entirely. These are the supermassive black holes whose mass may be millions or even billions of times that of the Sun. We do not, as yet, fully understand how they grow to be so enormous in the time available since the Big Bang.

Brighter than a trillion stars

Over time galaxies collide and merge, and this may bring their central supermassive black holes into close proximity. Indeed, OJ 287 is the most well-studied example of such a system where two colossal black holes dance around each other performing a celestial tango de la muerte. Astronomers estimate that the primary black hole is a staggering 18 billion solar masses, while its much smaller companion is a mere 150 million solar masses. This gives the primary’s event horizon a radius of over 50 billion kilometres. To put this into context, the distance between the Sun and the outermost planet Neptune is 4.5 billion kilometres. So, the primary black hole is a vast bottomless pit that would dwarf the entire solar system.

Surrounding this chasm is the black hole’s accretion disc—an incredibly hot swirling disc of plasma with a temperature of billions of degrees—so hot that it emits X-rays and gamma rays. As the secondary dances around its gigantic partner, it periodically crashes through this seething whirlpool of fire releasing a blast of radiation that is picked up by telescopes here on Earth, and this is how we know of this amazing system.

These two-week-long flares are brighter than the combined light of an entire giant galaxy of a trillion stars. The radiation blast is produced mainly by hot plasma from the accretion disc spiralling into the secondary black hole. The OJ 287 system is 5 billion light years distant, so the light in these flares has been travelling our way since before the Earth formed. It is only because the flares are so bright that we can see them from such an incredible distance.

The clash of the cosmic titans

There are two flares every 12 years, the most recent in February 2022, as the secondary black hole plunges and re-emerges through the primary’s accretion disc. Like a cosmic duel in Lucifer’s inner sanctum, the two writhing supermassive black holes twist, twirl, and cavort around each other. Researchers led by Finnish astrophysicist Mauri Valtonen of Turku University and his colleague Achamveedu Gopakumar from the Tata Institute of Fundamental Research in Mumbai, India have used the precise timing of the flares to build a detailed picture of the orbit of the black holes based on our best theory of gravity—Einstein’s theory of general relativity. This enables them to predict when future flares will occur. The extreme nature of OJ 287 challenges our understanding of the fundamental laws of nature, offering tests for general relativity that have not been possible before. A wide range of astronomical instruments will be ready and waiting when the next blast is due to arrive. In the years ahead, we are sure to learn much more about this amazing system that illustrates just how weird the universe can be.

References: Mauri J Valtonen et al, ‘Refining the OJ 287 2022 impact flare arrival epoch’, Monthly Notices of the Royal Astronomical Society, Volume 521, Issue 4, June 2023, Pages 6143–6155, https://doi.org/10.1093/mnras/stad922

_{Feature image: Black Hole and a Disk of Glowing Plasma by Daniel Megias via iStock.}