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Celebrating 50 Years of X-ray Astronomy:
The Past

What was going on in 1962? Dr. Peter Serlemitsos was there.

picture of Peter Serlemitsos Peter Serlemitsos

"In 1962, I was in my graduate school at the time. I think I started at NASA a little bit later than that – I was a graduate student at University of Maryland. I hadn't picked up a subject to do my thesis on yet. The subject I chose dovetailed nicely with my part-time job, which is the first thing that happened to me when I got picked up at Goddard. From that point on, I got embedded into the program and did my thesis on data from space and so forth – the low-energy particles from Earth's magnetosphere. This was Explorer 12 and Explorer 14, and they went halfway to the Moon. They were also some of the first satellites launched by the United States. So my advisor, Frank McDonald, was leading the group at Goddard at that time. He gave me a part-time job, and that eventually became full-time when it became clear that I was interested, and wanted to do research with the data from the satellite."


Photo of the Explorer 12 spacecraft

This small spacecraft – only measuring 4 feet by about 1.5 feet, weighing 38 pounds – was built at Goddard Space Flight Center and launched in 1961, and was one of the earlier missions to study energetic particles. X-ray astronomer Peter Serlemitsos was doing his graduate work at University of Maryland at the time this craft was launched.

Since the beginning of the field, Serlemitsos has had a hand in many X-ray projects during his time at Goddard Space Flight Center.

picture of Peter Serlemitsos Peter Serlemitsos

"I started here, working with satellite data, and I did my thesis here in 1966. Then almost immediately after that, I joined Elihu Boldt on the X-ray group, and there were two of us in the beginning. Then [Richard] Mushotzky came afterward. In fact, I was the one who interviewed Mushotzky in Hawaii – we met over there, and he had applied and written me a letter. I met him in Hawaii, and he decided to come. I was working half the time on instrumentation, and half the time on data analysis. Richard Mushotzky was primarily data analysis.

In my case, I was just as much enamored with the lab work as with data analysis. There were many balloon flights, all over. That's the way the group started, with rocket and balloon flights. Our first rocket experiment in space was a small instrument that Steve Holt – who was a third member of the group – put on Ariel-5, in England, and OSO-8, that's the one that was the combination of my work that tried to get multiwire proportional counters with low background and uniform, better resolution, which was the mainstay of the group for the next several years. XTE, for instance – the last instruments were decommissioned about a year ago."


Artist's conception of the OSO-8 spacecraft

OSO-8's primary objective was to observe the Sun, four instruments were dedicated to observations of other celestial X-ray sources brighter than a few millicrab. OSO-8 ceased operations on Oct. 1, 1978. "Millicrab" is a unit of intensity that equates to 1/1000 of the intensity of the "standard" Crab Nebula.

What was the most interesting thing going on in the field in its infancy? Dr. Richard Mushotzky, who is now a professor at University of Maryland at College Park, said everything was exciting.

picture of Richard Mushotzky Richard Mushotzky

"When I entered this field, things were changing incredibly rapidly. The first satellite, Uhuru, had been launched the year before. Before Uhuru, there had been rockets and balloons, so results were scattered. With a rocket, you get about five minutes of data. With balloons, you get a much longer time span, but they're less sensitive. Everything was fuzzy. You had these things going on, and you didn't quite know what it was. This bizarre thing, whatever it was, was emitting X-rays. It was very exploratory.

Photo of the Uhuru satellite during preflight checks

Uhuru in preflight checks with Bruno Rossi and Marjorie Townsend. Dr. Townsend named the satellite Uhuru, which means "freedom" in Swahili.

When the first satellite was launched, there were a small number of things that were well-known, but not many. The most famous is Scorpius X-1, the first X-ray source. The question is, 'What are these things? Are they near? Are they far?' Back then, things like black holes were not accepted, and neutron stars had just been discovered as the source of radio pulsars, so everything was new. With Uhuru, suddenly results started coming out. We discovered the first accreting neutron stars. There was an indication that Cygnus X-1 was a black hole. They found that there were a very small number – like two or three – quasars, active galaxies that were X-ray sources. There was the very first indication of clusters of galaxies and supernova remnants. Everything was exciting. That was 1971, 1972. At that time, I was a graduate student, and my thesis advisor had gotten an experiment on a NASA satellite called OSO-7. I started looking at the data from OSO-7, and we had to focus on something, so I decided to focus on active galaxies.

The first thing that is very strange is: why should any given object emit X-rays at all? Back then, we were trying to figure that out, and it was completely not obvious. The only things that were understood by the end of my graduate career were that we had accretion onto neutron stars, that is, matter falling onto the neutron star. The potential well of a neutron star is very deep, so if the matter comes into equilibrium with the potential well, it gets very hot – tens of millions or billions of degrees – that means they go into X-rays. Got that down, more or less. Then there were supernova remnants, and what happened there is that the gas is shocked, moving very rapidly at a few thousand kilometers per second, and if you come into equilibrium with that velocity, the energetic material emits X-rays. With active galaxies, even today, 30 years later, we don't really know why they emit X-rays, but they do. What we were just starting to learn back then is that the X-rays in active galaxies come from very close to the event horizon. We now know they come from within about 10 Schwarzschild radii.

The second enormous surprise, which is after I got my post-doc and came to Goddard, was a large class of X-ray sources called clusters of galaxies. These are literally the largest things in the universe – the largest thing that knows about itself. It is bound; it is an entity, gravitationally confined, as opposed to saying, 'It's a big chunk of the universe.' They're enormous – about 3 million light-years across, and they have a mass 100 trillion to 1,000 trillion times the mass of the Sun. They were discovered as X-ray sources, and again Uhuru did a lot of the pioneering work.

Just when I got to Goddard, it was proven that the source of the X-ray emission was hot gas trapped in the cluster potential well. If you did some simple numbers, you found that the amount of material in the hot gas was much larger than the amount of the material in the clusters of galaxies. This is radical. Like, 'What do you mean there's a chunk of the universe where most of the material is 100 million degrees hot, and emitting X-rays? This makes NO sense!' When I got to Goddard, that was one of the things. I was fortunate that there was a satellite that Goddard participated in, and did an experiment on, called OSO-8, and that was what was exciting. And 40 years later, I am still working on galaxies and clusters of galaxies."




Publication Date: June 2012

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