2. Finding black holes: X-ray astronomy

Two images of Orion. The image of the left shows the constellation of Orion taken in optical (i.e. visible) light, as you might see it from the ground. The three stars in a row near the middle mark Orion's belt; the red star above and to the left of them is Betelgeuse, a red giant; and the very bright star towards the very left is Sirius A, part of a binary system. (There are really two stars here Sirius A and Sirius B. They are locked in orbit around each other. It is Sirius A that we can see with the naked eye. With a suitably large telescope it is possible to see both stars.)
The image on the right shows the same patch of sky viewed with an X-ray telescope (in this case ROSAT). The sky appears very different. Most of the stars that appear bright in the optical image are not bright in the X-ray image and, vice versa, there are many strong sources of X-rays that are not bright in the optical image.
An interesting side note is that Sirius (the bright star near the bottom left) is clear in the X-ray image too, only this time it is Sirius B that we see. Sirius B is a white dwarf - the cooling corpse of a dead star - which is so hot it shines much more brightly in X-rays than it does in optical light.

2.1 What are X-rays?

X-rays are a form of light. They are in fact more energetic than the visible light our eyes are sensitive to. X-radiation is part of the electromagnetic spectrum, just like visible light, radio waves, microwaves, etc. Here's a schematic of the whole spectrum... (image courtesy of NASA)

The spectrum is plotted with lower frequency (=lower energy) radiation on the right, with radio waves being the lowest frequency, least energetic form of radiation. On the left are higher frequency (=higher energy) forms of radiation. In this plot the optical (visible) light that our eyes are sensitive to falls somewhere near the middle.

X-ray radiation is much higher energy radiation than optical light, and so is near the far left of the above plot. An X-ray photon typically carries 1000 times as much energy as a photon of optical light. This also means that X-rays tend to be produced at extremely high temperatures (millions of degrees C). The thermometer shows the types of temperatures that will emit different types of radiation.

2.2 The discovery of X-rays

X-rays were discovered in 1895 by Wilhelm Conrad Roentgen (left), a German physics professor working in a laboratory in the Physical Institute of the University of Wurzburg. For this discovery he won the first Nobel prize in physics (1901).
The other figure on the left shows an X-ray image of the hand of Albert von Kolliker, made at the conclusion a lecture given by Roentgen to the Wurzburg Physical-Medical Society on 23 January 1896. The X-rays are able to pass through the flesh of the hand, but get absorbed by the bone (and the metal ring!). This interesting property of X-rays (that they are better at penetrating skin/muscle than bone) meant they were immediately put into use by the medical community and radiology (use of X-rays from medical diagnosis) quickly became a standard diagnostic tool in medicine.
This is somewhat different from how X-rays are used in astronomy. In order to X-ray a broken finger bone you would place your hand between a strong source of X-radiation and a photographic plate. The X-rays passing through your hand would be absorbed preferentially by the bones, but pass through the fleshy parts. This would leave dark patches on the negative where the bones are. When using X-rays like this the subject (in this case your hand) being X-rayed is placed between an artificial source of X-rays and a detector of radiation (e.g. photographic plate). See the illustration below (image courtesy of NASA).

X-ray astronomy works differently. Many astronomical objects are their own sources of X-rays and X-ray astronomy is primarily concerned with the study of these sources. Instead of using an artificial source of X-rays to illuminate a subject, X-ray astronomy is concerned with how X-rays are produced in the cosmos. There are various ways of producing X-rays in the laboratory, X-ray astronomy (or high-energy astrophysics) is all about trying to understand and use the physics responsible for producing X-rays from stars, galaxies, etc. These objects are not (usually anyway) "X-rayed" in the same sense as your hand is when you break a finger. It is how heavenly bodies generate X-rays that is of interest to X-ray astronomers.

2.3 X-rays from the Sun

One very important point about X-rays is that they tend to get absorbed by matter. A few millimeters of bone is effective at absorbing X-rays. A few meters of air will also absorb most X-rays. This means X-rays are naturally absorbed by the Earth's atmosphere, making it impossible for cosmic X-rays to reach the surface of our planet. This is news good for us, because too much X-radiation can cause cell mutations. But it makes observing cosmic X-rays rather tricky. In order to detect any X-rays from space you need to get your X-ray detector above the atmosphere. The image on the left shows how effective the different types of electromagnetic radiation (including X-rays) are at penetrating through the Earth's atmosphere. X-rays are among the worst and can only penetrate down to a few 100 km altitude. To get to this height you would need to carry your X-ray detector on a high-altitude ballon at the very least.

During the 1940's the pioneers of X-ray astronomy used X-ray detectors carried on high-altitude balloons and rockets (originally captured German V-2 rockets!) which could take them above most of the atmosphere and so out of the way of the absorption. These experiments showed that our Sun is a source X-rays.
The figure to the left shows a modern X-ray image of the Sun (from the Yohkoh satellite). The surface of the Sun does not emit much in the way of X-rays, its temperature is only around 5800K (this is why we see it as a brilliant source of optical light). However, above the surface of the Sun is what is known as the solar corona. In the corona tenuous gas is heated to millions of degrees by intense and rapidly changing magnetic fields. This super-heated gas is hot enough to emit X-rays, which is what the picture reveals. For more information on the sun as an X-ray source see this site: http://www.sunblock99.org.uk/

2.4 The birth of X-ray Astronomy: X-rays from beyond the Sun

For a while it was thought that observing anything other than the Sun in X-rays would be hopeless. Everything would be too faint (even the next nearest stars are too far away) to detect. This is view was found to be seriously wrong in 1962, more or less by accident.
American Science and Engineering (ASE, a private company based in Cambridge, Massachusetts) was paid by the US Air Force to develop X-ray detectors for flight above the atmosphere, with the purpose of detecting X-rays from nuclear weapon tests. During this programme the team led by Riccardo Giacconi designed an experiment that was (officially) intended to search for X-ray emission from the surface of the moon. The moon itself is not a source of X-rays but it was expected to produce emission as a result of irradiation by the Sun and the solar wind.
The ASE experiment payload (right) was launched from White Sands, New Mexico, at 1 minute before midnight on 18 June 1962 using a USAF Areobee 150 rocket. This rocket was above 80 km for a total of 5 min and 50 sec, and reached a maximum altitude of 225 km. When the data were analysed it was found that the main source of X-rays was not coming from the direction of the moon at all but instead from the direction of the constellation of Scorpius. Unfortunately the primative detector could not pinpoint the location of the source any better than that. The mysterious source of cosmic X-rays became known as Sco X-1 (the 1st X-ray source in the constellation of Scorpius). Within a few years it became clear that Sco X-1 was not alone and there were many more bright X-ray sources out there. The race was on to find out what they were. X-ray astronomy had begun.

2.5 The Story of Cygnus X-1

The story of Sco X-1 is interesting but it turned out not a be a black hole (it's a neutron star X-ray binary) so I won't go into it here. The first X-ray source in Cygnus (henceforth Cyg X-1) was discovered by the Giacconi group shortly after their discovery of Sco X-1, and also by a group at the Naval Research Laboratory. Over the years the location of this X-ray source became more accurately determined. The X-ray source was found to lie very close to the position of a 9th magnitude blue star called HD 226868. This was confirmed when a radio flare was detected, at the same time as a jump in the X-ray brightness, and found to be from the same direction. HD 226868 is the optical counterpart to the X-ray source Cyg X-1.

Optical astronomers then turned their ground-based telescopes to HD 226868 and found it to be an "O-type supergiant" (a huge star, probably 18 times more massive than the Sun). But it seemed unlikely that this star, albeit a big one, is such a powerful source of X-rays. By carefully monitoring the emission from the star, they could use the Doppler effect to measure the star's velocity. This is shown to the right and showed the star to "wobble" periodically, once every 5.6 days. In order words HD 226868 appears to be rocking backwards and forwards ever 5.6 days. What this means is that HD 226868 is not alone - it has a much darker (to optical telescopes at least!) companion star and the two are orbiting their common centre of mass every 5.6 days. This is a very similar technique to that used by the "planet finders" to discover planets in orbit around nearby stars.

As better data were obtained it became possible to estimate the mass of the "invisible" companion, today the mass is thought to be around 10 times that of the Sun. So Cyg X-1 is a binary system containing one O-type supergiant star (HD 226868) and one much darker but very massive companion star. Could this dark star be the source of the X-rays from Cyg X-1? If you're following me you should be able to guess the answer (YES!)

The high mass of the dark companion star, its lack of optical emission and the coincidence of the bright X-ray source mean that it is almost certainly a black hole. The system probably looks something like this image by Rob Hynes (see left). It shows the giant star HD 226868 (which we see as a 9th magnitude star in the optical image) and its tiny companion star Cyg X-1 (which is the source of the X-rays).

But why is Cyg X-1 a bright X-ray source if it's a black hole? Isn't a black hole supposed to be black? (No X-rays, no light. Nothing.)

A black hole in isolation will be black. But this one is most certainly not in isolation. It's one half of a binary system with the giant star HD 226868. Stars as massive as HD 226868 produce powerful "winds." The powerful star blows off material from its sruface. This wind is many orders of magnitude more powerful than the solar wind in our solar system. The space around HD 226868, in which Cyg X-1 lives, is rich in the matterial being blown off by the giant star. This is what Cyg X-1 "feeds" on. The wind material is attracted by the black hole's gravity. It is pulled into the immediate vicinity of the black hole. However, it probably won't fall immediately through the event horizon. The reason for this is angular momentum.

When an ice skater draws his arms in, he spins faster. When he spreads them out he spins slower - that's angular momentum. It's what keeps things spinning. The same thing happens with matter falling into a black hole. Most of the gas that is being pulled in by the black hole's gravity will start revolving, ever so slightly, around the black hole. Just as ice skaters spin faster when pull their arms in, the gas spins faster as it gets closer to the black hole. The rotating matter is thought to form a disc of gas spiraling around the hole. In a nutshell: A fraction of the matter lost from the giant star (through its wind) is pulled towards the black hole (accreted) where angular momentum will make it form a rotating disc around the black hole.

But why does this make X-rays? As the accreting matter spirals closer to the black hole it gets faster and faster and heats up. The velocity of the gas disc is highest the closer it gets to the black hole. Close to the event horizon the speeds can reach a significant fraction of the speed of light. This stuff is really moving! As a result it can reach temperatures of well over a million degrees. At this temperature it emits enormous amounts of X-rays. That's why Cyg X-1 is such a bright X-ray source. (Well, that's one reason anyway. Cyg X-1 also has a corona, similar to the Sun's but much more powerful, and this also produces strong X-ray emission.) Cyg X-1 produces X-rays with something like 10,000 times the power that our Sun produces. This is true gravity power!

The plot below shows a schematic of an accretion disc around a black hole like Cyg X-1. The units of distance are the radius of the black hole. This is 3 km per solar mass (see the previous page) so for Cyg X-1, with a mass of 10 times the Sun, the radius of the black hole is about 30 km. Not very big, hey? It's the closest part of the disc, within say 100 radii (3000 km), where the matter is moving fastest and the X-ray emission emerges.

Even back in 1974 enough was known about Cyg X-1 for it to be considered a very likely black hole - a black hole candidate. So much so that two of the leading theoretical relativity experts, Kip Thorne and Stephen Hawking, made a bet with each other about whether Cyg X-1 really was a black hole. They both thought it probably was but decided to have some fun anyway. Thorne wagered it was a black hole, Hawking that it wasn't. A copy of the bet is shown to the right. In 1990 Hawking conceded the bet.

2.6 Modern X-ray Astronomy

X-ray astronomy has come a long way since the days of Giacconi's first rocket experiments. We now know of about 100,000 X-ray cosmic sources, including many more black hole binaries like Cyg X-1. In the past few years the two most powerful X-ray telescopes to date have been launched.

NASA's Chandra X-ray Observatory

ESA's XMM-Newton X-ray Observatory

Maintained by Simon Vaughan (sav2 at star. le. ac. uk)
Last updated: 17/9/2003
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