Einstein’s Cross Helping Astronomers Measure The Spin of Supermassive Black Holes

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Einstein’s Cross helping astronomers measure the spin of supermassive black holes. Researchers have used a new method to determine the spin of five accretion discs zipping at more than 70 percent of the speed of light.

The rotation of gigantic dust and gas discs swirling into supermassive black holes billions of light-years away may sound like a hard thing to evaluate, but astronomers have discovered a way to do it now.

Researchers have used a new method to determine the spin of five accretion discs-finding that one, in a quasar called the Einstein Cross, is zipping at more than 70 percent of the speed of light.

All this comes down to X-ray wavelength observations coupled with a cosmic effect called gravitational lensing.

See, in the Universe, there is some stuff that has so much mass that they generate a strong gravitational field around them. For example, massive galaxies and galaxy clusters.

The gravitational force is so powerful that when the light behind that field moves through it that it bends the path of light. This is what gravity lensing is: where bent light creates various object images, enabling us to see the information that would otherwise have been too distant to be clearly understood.

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There are quite a few lensed objects out there that have given us insight into the evolution of the Universe, including lensed quasars.

These are among the Universe’s brightest objects: highly luminous galactic cores driven by feeding actively supermassive black holes. As the material’s accretion disc swirls around the black hole, its friction produces such intense radiation that we can see it even from billions of light-years away.

Einstein's Cross Helping Astronomers Measure The Spin of Supermassive Black Holes
Einstein’s Cross Helping Astronomers Measure The Spin of Supermassive Black Holes. Chandra observations of lensed quasars. (NASA/CXC/Univ. of Oklahoma/X. Dai et al.)

We can see even more detail when viewed through a gravitational lens. In this study, the research team added a final key element to help evaluate their rotation. Microlensing to calculate the rotation rate of five of these lensed quasars (varying from 8.8 billion to 10.9 billion light years away).

This is just like lensing on a galaxy scale, but lower, using the lensing impact produced in the lensing galaxy by individual stars rather than galaxies or galaxy clusters. The method generates extra magnification-which, in turn, implies that the observed X-ray emission must be produced by a smaller region.

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We understand that with its rotation, a spinning black hole drags around space-time: a phenomenon called frame-dragging. This means the accretion disk’s inner edge can orbit closer to the black hole than a black hole that doesn’t spin. The quicker the spin of the black hole, the closer the orbit.

X-rays are produced when a fast spinning accretion disk generates a corona of high temperature above the disk, near the black hole. These X-rays reflect the internal edge of the accretion disc and are distorted by the gravitational forces of the black hole.

A smaller area of X-ray emission implies a very tight orbit, which in turn means that the black hole must be spinning pretty fast. This is what the team used to calculate the spin rate of black holes, based on observations using NASA’s Chandra X-ray Observatory.

The Einstein Cross black hole was spinning the fastest, as close as we saw at the highest possible speed. The X-ray emission, which emerged from a region only 2.5 times the size of the event horizon of the black hole, stated a spin rate of 70 percent of the speed of light.

This means that the event horizon is spinning at the speed of light.

The other four black holes were not so dramatic. The detected X-ray emission came from regions four to five times the size of their event horizons, indicating that they were spinning at about half the rate of the Einstein Cross black hole.

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Furthermore, all five showed elevated distortion levels, which also means close proximity to the black hole-and high spin rates.

These black holes are quite large, weighing between 160 and 500 million times that of the Sun. The supermassive black hole of the Milky Way galaxy is only about 4 million solar masses, and comparatively quiet.

Scientists believe that the rotations of the quasar black hole became so fast as they continuously accreted things along the same spin orientation for long time-billions of years. They just kept getting faster because there was nothing to slow them down.

“Unfortunately,” the researchers write in their paper, “the spin measurement technique presented in this paper can be used only to analyze the small sample of targets whose X-ray spectrum can be measured using the current generation X-ray telescope with sufficient signal-to-noise ratios.”

The research has been published in The Astrophysical Journal.

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