It is tough to image something that devours any electromagnetic radiation that gets too close to it, light included. But, since black holes warp space so severely, once mass or energy gets too close, it can no longer be observed by someone in our universe. Because of this, astronomers who study black holes are forced to use less direct methods for determining the nature of these beasts.
Thankfully for those interested in black holes, matter leaves behind a trail of X-rays as it enters its death throes prior to being accreted into a black hole. As matter swirls around a black hole, it gets accelerated to speeds near that of light, and becomes so hot it begins to emit X-rays. Since various elements emit unique spectral lines, astronomers can look at the X-ray spectra coming from a supermassive black hole and get a sense of what is falling into it.
In the current edition of Nature, a publication describes how scientists using the ESA's XMM-Newton X-ray telescope observed the X-ray spectrum that is being emitted from the supermassive black hole at the center of the galaxy 1H0707-495. This black hole was thought to be obscured by the dust and gas clouds near it, but thanks to these new observations, astronomers are getting their closest look ever at a supermassive black hole.
The view comes courtesy of a large concentration of iron atoms in the accretion disk, which produces characteristic lines in the spectrum. The spectral lines, known as the iron L and K lines, are distorted as they leave the accretion disk. The type and amount of distortion in the L and K lines allows astronomers to make inferences about the inner region around the black holes, including the speed that the ions are orbiting the black hole, the escape energy needed, and the spin of the black hole. These all distort the iron lines in a predictable manner.
Using 48 hours worth of observation time and an in-depth statistical analysis, the team found that an echo time of 30 seconds was seen between when the light is first observed and the time its reflection off the accretion disk is seen. Using this measurement, the astronomers were able to determine the size of the reflection region, which allows a derivation of the black hole's mass. The black hole at the center of 1H0707-495 weighs in at a hefty 3 to 5 million solar masses.
Other distortions in the spectral lines reveal that the supermassive black hole is consuming mass at a rate of about two Earths per hour. This is near the theoretical limit for the rate at which black holes of this size can accrete matter. Using this technique, the team saw closer to a black hole than anyone has to date, probing to within two times the radius of the black hole itself.
Nature, 2009. DOI: 10.1038/nature08007
Listing image by ESA (Image by C. Carreau)