Probing massive black hole with X-ray observations

Chris Reynolds (Astronomy Dept., Univ. of Maryland, College Park)

 

Massive black holes and `active' galaxies

 In the center of most galaxies resides one of Nature's most extreme creations. Through processes that are still largely unknown, huge amounts of matter have collapsed inwards to form a massive black hole which nowadays can be seen in the cores of many galaxies (including our own Milky Way, and the Andromeda galaxy which is shown to the left). These black holes typically have between a million and a billion times the mass of our own Sun.

What is a black hole? The modern theory of black holes started in 1916 when Karl Schwarzschild solved the equations of General Relativity which had been formulated by Einstein the previous year. Although it took people some time to figure out what the mathematical equations were actually telling them, Schwarzschild had actually found the equations describing an object which is so dense and massive, with such a strong gravitational attraction towards it, that nothing (not even light) can escape its pull. These objects became known as black holes . They are best thought of as a region of space where gravity overwhelms all other forces. Absolutely nothing (including the most powerful rocket or even light rays) can escape the attraction of a black hole once it ventures too close. The point of no return, or the distance within which everything must fall into the black hole, is called the event horizon .

 Astronomers have been finding these massive black holes in almost all galaxies close enough for us to be able to study in detail. In some fraction of galaxies, large amounts of gas are falling into the black hole. Any small rotation of the gas is amplified as it falls towards the black hole, and the gas eventually forms a disk which orbits around the black hole (see the figure to the right of this text). Such a disk is called an accretion disk .

As the gas slowly spirals through the accretion disk towards the black hole, it releases a large amount of energy. A more familar analogue is the energy that is released and converted to hydro-electricity when water is made to fall through some height inside a hydro-electric powerplant. The accretion disk acts like a kind of natural hydro-power plant, except that it produces large amounts of infra-red, optical, ultraviolet and X-ray light instead of electricity. Since such galaxies are seen by astronomers to house a powerful source of radiation at their center, they are called active galaxies.
 

 X-rays from active galaxies and the Iron emission line

As mentioned above, a large fraction of the energy released by the gas as it falls onto the black hole is converted into X-rays. It is thought that the X-rays come from material that is very close to the black hole (i.e. at distances of just a few times the event horizon size). Observations with X-ray telescopes allow astronomers to test and measure the conditions in this very interesting region of space.

 In particular, Nature has provided us with one way in which we can get detailed information about conditions close to the black hole. Iron atoms in the accretion disk become `excited' by absorbing X-rays. The response to such excitation is to emit X-ray of their own, but with a very particular frequency (or, no make a musical analogy, a very particular tone). X-ray emission at this special frequency is called iron line emission . Ever since the launch of the joint US/Japanese X-ray astronomy satellite ASCA in 1993, we have been able to observe measure these X-ray frequencies (or tones) with some precision. The observed frequency of these X-rays is strongly influenced by the orbital motion of the gas around the black hole (in the same way that the tone of a car's engine depends upon how fast it is moving towards/away from the listener). Also, as predicted by Einstein's theory, the gravity of the black hole lowers the frequency of the X-rays that are emitted nearby. Astronomers can clearly detect both of these effects when observing the iron emission line [see figure to the left - this shows the iron emission line from the active galaxy MGC-6-30-15 as measured by the ASCA observatory. This work was published in Nature by Tanaka et al. in 1995].
 

 Measuring the spin of black holes

 One of the most important questions facing those astronomers working on black holes is whether they rotate rapidly. A rapidly rotating black hole can act as a `fly-wheel' in the sense that energy is stored in the black hole rotation. In principle, this energy can be tapped by a variety of processes and may be responsible for some spectacular astronomical phenomena. For example, the image to the right is a picture of the galaxy Cygnus-A taken with a radio telescope, the Very Large Array in Socorro NM. This image shows two faint and narrow jets of gas that emerge from the center of this galaxy and pump huge amounts of energy into two giant bubbles. These jets, which are travelling at 95% the speed of light for hundreds of thousands of light-years, may well be created from a spinning black hole.

To understand these processes, astronomers must attempt to measure the rotation of black holes in different types of galaxies. Since black holes are perfectly featureless `smooth' objects, measuring their rotation is a very difficult task. They must search for the subtle effect that a rotating black hole has on its environment and, in particular, the accretion disk. So far, there are hints and suggestions that some massive black holes are indeed rapidly spinning. Unfortunately, X-ray telescopes are not yet sensitive enough to provide definitive proof that some massive black holes are rapidly spinning.

What will be possible with future X-ray telescopes?

Both NASA and the European Space Agency (ESA) are planning to build very sensitive X-ray telescopes within the next decade. With this increased sensitivity, astronomers will be able to examine many new aspects of these massive black holes. One exciting area of research that will open up is the study of X-ray flares. The X-ray emission described above is not steady and unchanging. Instead, it is emitted in violent and dramatic flares. These flares are probably due to eruptions from the surface of the accretion disk that are somewhat similar to eruptions on the surface of the Sun that produce solar flares.

When one of these X-ray emitting eruptions occurs, an X-ray echo sweeps out across the accretion disk. Astronomers should be able to see this X-ray echo by looking for subtle changes in the shape of the iron emission line as the echo illuminates different parts of the disk. The panel of images to the right (to be read from left to right) shows a calculation of what this echo would look like if we could observe it, together with the predicted shape of the iron emission line. These have been calculated by Dr. Christopher Reynolds (University of Colorado) and Dr, Andrew Young (University of Maryland). We also show simulated data from NASA's planned Constellation X-ray mission . Our calculations show that we can follow the changes in the iron line shape with enough detail to infer the natire of the X-ray echo. Making such measurements will allow us to measure the mass and rotation rate of the black hole, as well as the properties of the nearby matter as it starts it final plunge towards the black hole.

More information

For more information about the topics covered on this web site, please contact Chris Reynolds .