For the first time, the European Space Agency (ESA)’s orbiting X-ray observatory, XMM-Newton, has proved the existence of a “gravitational vortex” around a black hole.
As per ESA, the discovery solves a mystery that has eluded astronomers for more than 30 years, and will help scientists understand more about the behavior of matter very close to black holes.
The findings, aided by NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) mission could also open the door to future investigations of Albert Einstein’s general relativity.
When matter falls into a black hole, it heats up as it plunges to its doom. And before it passes into the black hole, it gets lost from view forever, reaching millions of degrees, and at which temperature it shines X-rays into space.
Back in the 1980s, pioneering astronomers using early X-ray telescopes discovered that the X-rays coming from stellar-mass black holes in our galaxy flicker. The changes follow a set pattern. When the flickering begins, the dimming and re-brightening can take 10 seconds to complete. The phenomenon was dubbed the Quasi Periodic Oscillation (QPO).
“It was immediately recognized to be something fascinating because it is coming from something very close to a black hole,” said Adam Ingram, University of Amsterdam, the Netherlands, who began working to understand QPOs for his doctoral thesis in 2009.
Later in the1990s, astronomers though that the QPOs were associated with a gravitational effect predicted by Einstein’s general relativity: that a spinning object will create a kind of gravitational vortex.
In 2004, NASA launched Gravity Probe B to measure this so-called Lense-Thirring effect around Earth. After painstaking analysis, scientists confirmed that the spacecraft would turn through a complete precession cycle once every 33 million years.
However, around a black hole, the effect would be much more noticeable because of the stronger gravitational field, and hence astronomers began to suspect a link to the periods of the QPOs.
Together with colleagues, Ingram published a paper in 2009 suggesting that the QPO is driven by the Lense-Thirring precession of this hot flow. This is because the smaller the inner flow becomes, the closer to the black hole it would approach and so the faster its Lense-Thirring precession cycle would be. The question was: how to prove it?
Seeing this wobbling is where XMM-Newton came in. Ingram and colleagues from Amsterdam, Cambridge Durham, Southampton and Tokyo applied for a long-duration observation that would allow them to watch the QPO repeatedly. They chose black hole H 1743-322, which was exhibiting a four-second QPO at the time. They watched it for 260,000 seconds with XMM-Newton. They also observed it for 70,000 seconds with NASA’s NuSTAR X-ray observatory.
After a rigorous analysis process of adding all the observational data together, they saw that the iron line was wobbling in accordance with the predictions of general relativity. “We are directly measuring the motion of matter in a strong gravitational field near to a black hole,” says Ingram.
This is the first time that the Lense-Thirring effect has been measured in a strong gravitational field.
Larger X-ray telescopes in the future could help in the search because they are more powerful and could more efficiently collect X-rays. This would allow astronomers to investigate the QPO phenomenon in more detail.
The findings have been published in the Monthly Notices of the Royal Astronomical Society.
ESA’s XMM-Newton, which is the largest scientific satellite to have been built in Europe, was launched in December 1999. NuSTAR is a NASA Small Explorer (SMEX) mission launched on June 13, 2012.