| Subject: Re: black hloes |
Author:
Blobrana
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Date Posted: 00:21:13 10/06/04 Wed
In reply to:
Teddy
's message, "Chandra listens to black hloes" on 12:27:27 09/10/03 Wed
A group of Cambridge astronomers have detected what appears to be the rapid spinning of hot gas falling into a super massive black hole. This first of their kind observations help pinpoint it’s mass as it devours material from an inward-spiralling disk.
The black hole lies at the centre of the galaxy NGC 3516, which is about 100 million light years away in the constellation Ursa Major. The blackhole’s immense gravitational field forms an accretion 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 accretion disk in X-rays. This has been interpreted this as an X-ray flare occurring close to the black hole. An X-ray flare is somewhat like a solar flare, but the 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,”
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 agrees with other estimates for the mass of the black hole.
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. With a “year” lasting only seven hours the gas would be travelling at about a tenth of the speed of light.
This phenomenon would be short lived though, with the highlighted gas falling into the 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.
Flares are a common occurrence around these super massive black holes. In fact, the telltale observations of NGC 3516 were made three years ago, but it took this recent analysis to revealed a saw tooth pattern in the data.
“The eye is very good at picking out patterns,”
The researchers perceived a variation in the excited iron’s frequency. This frequency shift they attributed to the Doppler effect of the rotating gas as it switches between moving towards us to moving away from us.
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.
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.
The researchers ran simulations to confirm that the periodic variation they were seeing wasn’t just a random fluke in the noise. But it seems that the effect is real, though some astronomers have reservations.
“The reactions range from ‘Wow’ to ‘I’m not really sure about this,’”
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