Riccardo Giacconi

X-Ray Astronomy

Category: Lectures

Date: 30 June 2004

Duration: 12 min

Quality: SD

Subtitles: EN

Riccardo Giacconi (2004) - X-Ray Astronomy

In 1959, after teaching at the University of Milano and spending a Fulbright Fellowship at the Universities of Indiana and Princeton, I joined American Science and Engineering, a private 30 person corporation in Cambridge, Massachusetts

X-ray astronomy had to wait for the development of space transportation, the reason being very simple, the x-rays that we’re looking at are absorbed in the atmosphere up to about 100 kilometres. So you have to put your detectors above 100 kilometres. This was done first by Herbert Friedman, who used V2 German rockets which had been captured after World War 2, and put detectors on it to go up and start looking for x-rays from the sun. This went on from ’48 to ’58 or so and a lot of information was obtained on the sun. But any attempt which was being made to discover sources from stars other than the sun or outside the solar system had failed. I fell into the field so to speak by chance. I had been hired by a corporation, a small corporation of 28 people at the time, and I was given the job to design a program of space research. This was in response, of course, to the fact that the United States was getting very nervous about the Soviet Union flying rockets and we had to catch up. And so there was a lot of opportunity for space research. And I started looking at different things. Bruno Rossi, who was the chairman of this corporation and also, of course, a professor at MIT and also chairman of the space science board at that time, had followed the discussions which were occurring at the space science board. Several people, Friedman, Leo Goldberg and others had discussed the fact that it would be nice to look at the sky in x-rays, because the x-rays would penetrate large spaces, interstellar spaces, and would be an index of violent processes occurring, high temperature processes occurring in the universe. I loved the geometry, as my mother had taught me. That geometry was great, god plays geometry. And I happen to come through a Pfluger encyclopaedia statement, that you could have total external reflection by x-rays impinging at raising incidence. And then of course, if you know geometry you construct in your mind a paraboloid. And so the first thing I did in x-ray astronomy was to design an instrument which was a telescope to actually collect and focus x-rays, so that you could have a tremendous improvement of signal-to-noise ratio, a factor of about hundred thousand or a million. What are you looking at? Well, the rocket goes up, this is already summing all of the rotation that occurs when you are above the region in which the x-rays are absorbed. So you're looking around the sky from 0 to 360 degrees during a spin and we had 2 detectors with different windows, this was the thicker window mica, this was the thinner window. Here is the magnetic field. Here is the moon, remember that the air force was interested in us looking for the moon so we had to have the moon up. And then the number of counts. Now what do we observe? We observe an enormous peak here in counts, which we didn’t expect at all. That is, we expected at most to see something, you know, at this level, if we were lucky, if the crab nebula actually admitted as we were hoping it would, and so forth. And this enormous peak was totally unexpected. As it turns out, the reason why it was so large and it was unexpected, is that we were seeing a nucleus of objects. These were binary x-ray stars. I now want to skip along and say, ok, so I told you about the ability to slow down the rotational rate. This is slowed down a lot. Distance here is 4 seconds. So looking at this particular source, Centaurus X-3, in May of ’71 we found what we believed to be Now this was somewhat unexpected, because pulsars had been discovered by Hewish. But we didn’t expect the radio pulsars to be emitting x-rays at this rate and with this kind of periods. So in that sense it was strange. The other thing that we notice is - and this is where the long time given to you by a satellite rather than where the satellite stayed up for years. So for every hour that you were up, you were doing as much x-ray astronomy as had been done until then. Here was a use of time. Here are the days of May’71, and what we notice was that the intensity of the x-ray source that we were seeing was going up, staying steady for a while, then going away, then coming up, steady for a while and going away. Now the thing that became very interesting – that I think is fundamentally important - was that when we actually measured this period over 3 years, we found that the period was decreasing. The pulsating source was acquiring energy rather than losing energy. And how could this happen? Well, the way this happens is that here is a cut in the gravitational plane of the 2 sources. This shows what is called a Rho …, that is an equipotential. The normal star has gas in its atmosphere. It can fall, after appropriate rotations in order to lose angular momentum, onto the compact object and as this is a cut in the vertical plane, as it does this, a proton will acquire more energy in the infall than it can actually produce by nuclear fusion. So this has become the explanation for all of the compact sources that we are seeing, and then extrapolated to very large dimensions to supergalactic, supermassive black holes. I won’t go through that - except to say that the magnetic field or rotation or the compact object for neutron stars explains why you see pulsations. I’ll go to the next one which is, we saw another source, which was very different from the regularly pulsating one. That was Cygnus X-1. It created some excitement, there was optical determination of the position, I mean, there was x-ray determination of the position, radio, then optical, in an identification with a source. Webster and Murdin measured the mass of this object to be something like 6 solar masses. Now it had been shown that neutron stars cannot have mass that large, so what we were seeing was an indefinitely collapsing object, which we call a black hole. For lack of understanding, or what physics goes on into it, this is an object in which density has much exceeded the density of even a neutron star or 10 to 15 grams per cubiccentimetre, and it is a black hole. That has meant that - I will just show you 2 pictures - this is a picture in x-rays, real life - there is also a movie of it - of the pulsar in crab nebula. You see the acceleration of the jets, you see shock waves being propagated in the interstellar medium. So this tells you that with this resolution now you can do dynamic studies of plasmas and shock waves in galaxies, in clusters of galaxies, in supernovas and so forth. But the last slide I wanted to show you is this one. This is one of the longest exposures, not the longest anymore, but one of the longest exposure done on a fixed field, which was not known to contain any x-ray source and no particular visible light object. It’s in the south, Chandra Deep Field South. It was obtained in the year 2000, and what you are seeing here is a collection of objects which is very high density. It’s 3000 objects per square degree, so you're looking at something that fills the night sky at the level of a hundred million objects or something. What are these objects? To be brief, they are all super massive black holes accreting from accretion discs around them. We are seeing these objects at distances which are greater at time than those, with which you can follow them in visible light. So we can study them early in their evolution and formation. And just to close I’ll show you one picture which tells you where we are in this kind of studies today. This is the x-ray contour plot of a source overimposed on the Hubble - one of the deepest exposures of Hubble There was none to be seen. So that means that this object here is less than 27 magnitudes. When you look at it with Keck you can’t see it, when you look at it with VLT you cannot see it. But with the arrival of Spitzer, the new satellite that works in the infrared, we can now see it very clearly. And we conclude from this that what we are seeing is a QSO, a quasi-stellar object, active galactic nuclear, which is very absorbed, very darkened by gas and dust around itself at the redshift of about 6, which is fairly early on in the life of the universe. So this is simply to say that there is interesting physics to be done in x-ray astronomy and there is tremendous power for further observations, which are of relevance to evolution, cosmology and so forth and I’ll stop here.


In 1959, after teaching at the University of Milano and spending a Fulbright Fellowship at the Universities of Indiana and Princeton, I joined American Science and Engineering, a private 30 person corporation in Cambridge, Massachusetts. There I started a group to develop space science initiatives, particularly in x-ray astronomy (which can only be carried out from space). We developed new detectors and techniques that permitted us to discover the first x-ray star during a rocket flight in 1962, carried out under the sponsorship of the US Air Force. The detection of this star required an improvement of the techniques used up to then, by a factor of 100.

The star system we had observed was of extraordinary astronomical interest. It represented a new class of celestial objects previously unknown, a thousand times more luminous than the sun and emitting 99.9% of its energy in x-rays by some unknown process.

This finding stimulated the development of more sophisticated satellite-born instrumentation, under NASA sponsorship, which culminated in 1970 with the launch of a satellite “UHURU”, a 1000 times more sensitive than the rocket experiment (Fig. 1.). With this satellite more than 300 x-ray sources were discovered (Fig. 2).

To summarize the findings;
• We found the first x-ray binaries, normal stars with a neutron star or a black hole companion (Fig. 3);
• We found a hot (millions of degrees) gas pervading the spaces between galaxies in clusters, containing more mass than the galaxies themselves. We realize now that this gas may provide most of the normal matter in the universe;
• We found the first x-ray emitting galaxies and quasars containing super massive black holes.

These findings changed our view of the Universe and underlined the importance of high-energy observations to all of astronomy. This in turn, stimulated further technical development. The concept of an x-ray telescope initiated at ASE in 1959 and developed by the corporation for more than twenty years came to fruition as a NASA facility costing more than 1 billion dollars. The satellite CHANDRA was launched in 1999 (Fig. 4) and can see sources in the Universe a
billion times fainter than the star first discovered in 1962 with an angular resolution comparable to that of the best optical telescopes (Fig. 5).