Celebrating 50 Years of X-ray Astronomy:
The Past
What was going on in 1962? Dr. Peter Serlemitsos was there.
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."
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.
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."
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.
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.
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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