How Günther Hasinger and his colleagues discovered black holes
A diffuse light fills the entire sky: the so-called cosmic X-ray background. Neither the naked eye nor optical telescopes can see it, because the Earth’s atmosphere blocks these waves of energy. Satellites equipped with an X-ray eye, however, can.
In the mid-1990s, Günther Hasinger, along with Maarten Schmidt, Joachim Trümper and later Nobelist Riccardo Giacconi demonstrated that this glowing background is actually fed by numerous discrete sources – similar to the way Galileo used his telescope in the 17th century to discern that the Milky Way is composed of myriad individual stars.
With the help of X-ray satellites, the scientists were able to look through the Lockman Hole in the constellation Ursa Major. Since, in this direction, there is very little in the way of absorbent materials such as dust and hydrogen clouds, the window of observation opens into the deep recesses of space, to far-off extragalactic objects. Behind the individual lights of the X-ray background are hundreds of millions of black holes.
Thanks to their irresistible pull, these gravity traps ensnare interstellar material, or even entire stars. In today’s universe, almost all galaxies have massive black holes at their core – there is even one at the center of the Milky Way (MAXPLANCKRESEARCH 1/2003, p. 56 ff). When material falls into the chasm of a black hole, it races around the cosmic maelstrom at nearly the speed of light, heating up so much that, before disappearing, it releases highly energized radiation as a sort of last call for help. If they are well fed at the center of active galaxies, the black holes, which themselves are invisible, are among the most brilliant objects in space.
Astronomical detective work
The chemical elements in the clouds surrounding black holes radiate X-rays with characteristic wavelengths, providing a fingerprint by means of which they can be identified. Iron atoms are especially well suited for astronomical detective work, since this metal is the most prevalent in the cosmos, and it emits large amounts of radiation when heated, leaving a distinctive mark (line) in the spectrum. Much like the police snag speeders using radar traps, astronomers establish the high speeds at which iron atoms revolve around a black hole by way of shifts in the wavelengths of the light. Due to the great mass of black holes, this relativistic Doppler effect is coupled with the gravitational red shift –and both phenomena are in line with the theory of relativity.
The special theory of relativity postulates that clocks run more slowly the faster they move through space. According to the general theory of relativity, the same slowdown also happens to clocks in the proximity of large masses. Applied to electromagnetic radiation, this means that the wavelength of the light emitted by iron atoms is altered at the longer, red end of the spectrum. This results in a wider, asymmetrical line – a sort of smeared fingerprint.
Günther Hasinger and his team discovered such an abnormal spectral fingerprint in the X-ray background. From the strength of the signals, the astronomers ascertained, for example, the number of iron atoms within the material. “We were surprised that the abundance of iron in the diet of these young black holes is approximately three times that of our own solar system, which was created much later,” says Hasinger. The centers of galaxies in the early universe apparently had an extremely efficient means of producing iron – possibly because active galaxies contain many massive stars that spawn chemical elements, including iron, relatively quickly. The broadness of the line indicates that the iron atoms come very close to the black hole, and thus that the majority of black holes in space are probably spinning very rapidly. As a result, the surrounding space is being stirred, too, like mixing a batter. As Hasinger explains, “That is why material flying around a black hole in the same direction can come very close to these monsters of mass without falling in. And it is here that the higher speeds and greater gravitational red shift can be measured.”
These discoveries have been made with the help of the European X-ray satellite XMM-Newton. It is also thanks to Günther Hasinger that the orbiting observatory can see so precisely through the vastness of space: Unlike light waves, X-rays are not easily bundled. Only specially formed, extremely polished, low-dispersion mirrors can be used. This is the field that Hasinger chose for his thesis, in which he examines the technological problems faced in the polishing of X-ray surfaces (“Hasinger’s Triangle”), and contributes to the later appearance of the mirror from the ROSAT satellite in the GUINNESS BOOK OF WORLD RECORDS as the smoothest surface in the world. Scaled to the size of Germany’s Lake Constance, the highest “mountain” on the mirror would be a ripple on the surface of the lake caused by a small pebble being thrown in.
Supernovae, neutron stars, black holes: material that is subjected to pressure, mass that warps the space time continuum, temperatures that reach into the hundreds of millions of degrees and objects that move at nearly the speed of light – these are the things that astronomers hunt for, launch satellites to find, spend careers explaining. They are the stuff of Günther Hasinger’s astronomical work.