One must admit that spectra, the curves that plot the number of photons and their energy, appear to be rather uninspiring to the layman -- There's nothing worse than a graph! But like ones body temperature curve, they mean a lot.

For astronomers, spectra are like fingerprints from the stars and galaxies. The information they hold may be incomplete, like a tattered newspaper, but it tells a story. While images from a telescope are attractive, spectra can reveal the innermost secrets of certain cosmic monsters.

Many of the brighter X-ray emissions in the Lockman Hole were previously identified with the ROSAT satellite. For these, XMM-Newton has now provided very detailed spectra, as is shown in a picture montage by Guenther Hasinger from the Astrophysics Institute of Potsdam (AIP).

(courtesy Guenther Hasinger/AIP -- click to enlarge)

The main image [above] plots the soft X-ray photons, already observed by ROSAT, in red. Green corresponds to intermediate X-ray photons, also detectable by NASAs Chandra X-ray Observatory. Blue is used to show the hardest X-ray photons, only detectable by XMM-Newton.

The individual spectra [right edge of this page] "roll out" this color information into graphs, plotted in red, where the X-ray emission (the number of arriving photons) is plotted against the energy at which it is emitted. The green lines refer to model spectra fitted to these graphs.

Many of the sources are associated with black holes, monstrous gravitational wells into which matter is being sucked and in which even light disappears. Stellar-mass black holes arise after the death of a massive star that has used all its fuel. But there exist also supermassive black holes, which are present in the center of almost every galaxy.

"The origin of these supermassive ones remains a complete mystery," said Guenther Hasinger. "Forming these from many single black holes would probably take too much time. They might therefore be part of the original collapse of gas into a galaxy, in other words the seed objects of galaxy creation."

Next page: detectives of the invisible universe

~

Detectives of the invisible

The spectra of the X-ray emission from the sources can reveal a lot about the properties of the active nucleus. If low-energy X-rays are sparse compared with those at higher energies, it indicates that there is much absorbing gas between us and the nucleus -- or possible black hole.

The absorbing material may be in a ring or doughnut shape, surrounding the X-ray source. In some extreme cases, when we are looking at this accretion disk edge-on, the nucleus may be hidden from our view, and only the highest energy X-rays can escape.

Alternatively when there is little absorption, and the low-energy X-rays are strong, the spectra display practically straight lines (known as "power-law spectra"). They indicate that we are getting a close-to-face-on view, looking right down into the nucleus and associated black hole.

Other sources show bumps or wiggles in their spectra. This can be attributed to the emission of iron atoms very close to the maelstrom of the black hole. From the shape of a bump, one can infer the geometry of the emitting region -- for instance, our distance from the hole and the angle at which we are observing the central accretion disk.

Clearly identified emission lines in the spectra are the fingerprints of different elements that are swirling around very close to the event horizon, where matter finally disappears. From the displacement of these lines from their normal position in a spectrum one can measure the velocities, close to the speed of light, at which these atoms are moving. This, in turn, indicates how fast the black hole itself is rotating. An iron line also tells us how close the accretion disk is reaching to the very edge of the black hole.

A new black hole?

"All but one of these nine sources had already been identified by ROSAT. Whether they are unobscured sources with straight power-law profiles, or objects whose X-rays are partially absorbed, the XMM-Newton spectra confirm the models -- the way we had imagined we would see these sources in greater detail," explained Guenther Hasinger.
"But the bright source -- practically in the center of the image -- is one of XMM-Newtons new discoveries!

"We have given it the number 24021. Its nature is still unclear; it has a very different spectrum, practically no X-rays below the 2 keV energy level and a power-law profile above 2 keV. We havent determined its redshift (how far away it is), and to know more we will need to observe this source with the new generation of 8- to 10-meter (315- to 395-inch) telescopes."

Those who say spectra are dull must be lacking in imagination. The spectral fingerprints like those shown here are revealing how black holes form and grow. If the Lockman Hole observation is an example, XMM-Newtons spectrometric mission promises to be extremely rewarding.