28 July 2011

Chandra-The Other Space Telescope

Posted by Dan Satterfield

Combined image from the Chandra X-ray telescope and the Very Large Telescope (VLT) showing hot gas falling into a massive black hole.

The public know all about the Hubble Space Telescope but most have just heard the name Chandra a few times if at all. The images from Chandra have led to as many if not more amazing discoveries and this post is about just one of them. A friend of mine (and neighbor) works on the Chandra project, so I’m always keen to hear about new images and research.

The Hubble telescope sees mainly in visible light that your eye can detect but Chandra sees electromagnetic radiation from the much, much shorter wavelengths, in the x-ray portion of the spectrum. Light is light, and if we evolved with X-ray eyes, the Chandra would get the same attention of the Hubble and the HST would be the exotic telescope seeing in that unfamiliar red, green and blue light! (We evolved eyes that see in the visible range, because that is the wavelength the sun produces most of its light)

Raw Chandra X-ray image of NGC 3115 where the massive black hole is located..

Chandra recently made a spectacular image of hot gases falling into an incredibly massive black hole. This is the first time that such a thing has been directly seen and measured. When I say massive, black hole, I do mean massive! It is 2 billion times the mass of the sun and the biggest of these monsters so close to the Earth. It’s only  32 million light years away which is about 13 times further away than our nearest large spiral galaxy Andromeda.

There is a big mystery in this image that you cannot see but that researchers can measure. The physics predicts how much X-ray light this gas being sucked onto the black hole should give off as it gets heated up, and the amount seen does not match up. This has been seen before and is a well-known mystery.

Here is a description of the issue from a NASA press release:

By imaging the hot gas at different distances from this supermassive black hole, astronomers have observed a critical threshold where the motion of gas first becomes dominated by the black hole’s gravity and falls inward. This distance from the black hole is known as the “Bondi radius.”

As gas flows toward a black hole, it becomes squeezed, making it hotter and brighter, a signature now confirmed by the X-ray observations. The researchers found the rise in gas temperature begins about 700 light years from the black hole, giving the location of the Bondi radius. This suggests the black hole in the center of NGC 3115 has a mass about two billion times that of the sun, making it the closest black hole of that size to Earth.

The Chandra data also show the gas close to the black hole in the center of the galaxy is denser than gas further out, as predicted. Using the observed properties of the gas and theoretical assumptions, the team then estimated that each year gas weighing about 2 percent the mass of the sun is being pulled across the Bondi radius toward the black hole.

Making certain assumptions about how much of the gas’s energy changes into radiation, astronomers would expect to find a source that is more than a million times brighter in X-rays than what is seen in NGC 3115.

“A leading mystery in astrophysics is how the area around massive black holes can stay so dim, when there’s so much fuel available to light up,” said co-author Jimmy Irwin, also of the UA in Tuscaloosa. “This black hole is a poster child for this problem.”

There are at least two possible explanations for this discrepancy. The first is that much less material actually falls onto the black hole than flows inside the Bondi radius. Another possibility is that the conversion of energy into radiation is much less efficient than is assumed.