Black Hole's Lunch Reveals Its Mass
An artist's concept of a black hole, surrounded by an accretion disk. The gas in this disk is heated to millions of degrees Celsius and emits X-ray radiation.
Credit: Nasa/Honeywell Max-O digital group/Dana Berry.

A group of Cambridge astronomers have detected what appears to be the rapid spinning of hot gas as it falls into a supermassive black hole. These first of their kind observations help pinpoint the mass of the hungry maw that is devouring material from an inward-spiraling disk.

The hefty black hole lies at the center of the galaxy NGC 3516, which is about 100 million light years away in the direction of the Big Dipper. Material pulled in by the hole's immense gravitational field forms a disk that is heated to high temperatures, causing it to glow over a broad range of frequencies.

The XMM-Newton satellite observed a sudden brightening of the disk in X-rays. Andrew Fabian from Cambridge University and his colleagues have interpreted this as an X-ray flare occurring close to the black hole. An X-ray flare is somewhat like a solar flare, Fabian said, but its energy is 10 trillion times stronger. The light from the flare excited nearby iron atoms in the accretion disk, causing them to emit a bundle of photons at a specific frequency.

"It is as if a bit of the disk is being painted," Fabian said.

The light from the iron atoms changed over time, implying that the "painted" gas was orbiting a very massive object. Calculations involving Einstein's general relativity showed that this object, the black hole, weighs between 10 and 50 million solar masses.

This result, which will be published in an upcoming issue of the Monthly Notices of the Royal Astronomical Society, agrees with other estimates for the mass of the black hole.

Four Quick Years

The XMM observation lasted about a day, recording four revolutions of the excited gas. The implied radius is similar to the Earth's distance to the sun, which prompted Fabian to imagine what it might be like riding along side the swirling gas. With a "year" lasting only seven hours, an intrepid disk surfer would be traveling at about a tenth of the speed of light.

The sky around Fabian's observer would be overwhelmed by the black hole, whose width is between 40 and 200 times that of our sun. How exactly all this would look is difficult to fathom, as the immense gravity would bend light in strange ways.

"It would be a spectacular sight," Fabian said.

But the wonderment would be over all too soon. Fabian suspects that the highlighted gas fell into the unforgiving black hole just a few days after the measurements. The exact timing of the demise depends on the viscosity in the disk and whether the black hole itself is spinning or not.

Pattern Recognition

Flares are a common occurrence around these supermassive black holes. In fact, the tell-tale observations of NGC 3516 were made three years ago, but it took this recent analysis to uncover the flare's secret. The lead author, Kazushi Iwasawa, produced a map of the X-ray light that revealed a saw tooth pattern in the data.

"The eye is very good at picking out patterns," Fabian remarked.

The researchers perceived a variation in the excited iron's frequency, which they attributed to the rotation of the gas as it switches between moving towards us to moving away from us. The frequency shift, called a Doppler shift, is similar to the pitch changes in a "bullroarer" - a traditional instrument that is swung by a string around the player's head.

Besides the Doppler shift, there is also an overall shift down in frequency, as photons lose some of their energy climbing out of the gravitational well around the black hole.

Possible Fluke

Although the saw tooth pattern matches theoretical expectations for gas spinning around a black hole, it is difficult to make definitive statements with only four periods of revolution.

"I think we are lucky. We are at the very limits of detectability," Fabian said.

The researchers ran simulations to confirm that the periodic variation they were seeing wasn't just a random fluke in the noise. Fabian said that he and his colleagues are optimistic that the effect is real, but some astronomers have reservations.

"The reactions range from 'Wow' to 'I'm not really sure about this,'" Fabian said.