So today I will talk to you about the research that I was
doing in my early days here at MIT which was a long time ago.
I got my Ph.D. in the Netherlands and nuclear
physics. And then in early 1966I came to
MIT. I had a postdoc for one year.
I fell in love with MIT, and I never left.
So I changed fields from nuclear physics to x-ray
astronomy. X-ray astronomy was a very
young field at that time. You can only do x-ray astronomy
from above the Earth's atmosphere because the x-rays
are completely absorbed by the Earth's atmosphere,
unlike radio astronomy and optical astronomy which you can
do from the ground. You have to get above the
Earth's atmosphere. So, I joined the research group
of Professor Bruno Rossi and George Clark.
George is still at MIT. And when I say x-rays,
you have to think about the same kind of x-rays that your
dentists are using, one to roughly 50 kilo electron
volts. All of you know,
I hope, what a kilo electron volt is.
Optical light is only two electron volts.
So, x-rays are substantially more energetic than optical
photons. It was in 1948 that American
scientists started to explore the outer space for which you
need rockets. And they used the German
rockets, which were developed in World War II [UNINTELLIGIBLE] by
Wernher von Braun. These were weapons.
They were launched from France, from Belgium,
and from Holland. And the target was London.
They caused a tremendous amount of misery and death with a
flying bomb. And after the war,
Wernher von Braun was welcomed into the United States.
And for reasons that still puzzle me to date,
he even became a hero. Using these rockets,
these V2 rockets, leftovers from Hitler,
Germany, the Americans discovered x-rays from the sun.
The sun is very close, so you would say,
well, it's not too surprising. Now, indeed,
it's perhaps not too surprising.
And in fact, the amount of x-rays from the
sun is only a minute fraction of the total energy output of the
sun. So if we take the power,
joules per second in x-rays, and we divide that by the power
in optical light [of which?] the sun is also ultraviolet and
infrared. That's all included.
And if you do this for the sun, then you get a number of about
ten to the minus seven, so, one ten millionth of the
energy comes out in the form of x-rays.
It's still very puzzling, by the way why there are so
many x-rays. But that's a different story.
In 1962, scientists from Cambridge, Professor Rossi,
Riccardo Giacconi, and [Herb Goersky?],
and Frank [Poalinni?], wanted to explore the
possibility whether there were also x-ray sources outside the
solar system. And the odds that they would
detect something like that were extremely low because if you
take the sun and you put the sun at the distance of the nearest
stars, there is no hope on Earth with the detectors that existed
in those days that you would detect the sun.
In other words, the detectors I think I
recalled were about one billion times too insensitive to see a
solar like object at the distance of the nearest stars.
Yet, they succeeded. 1962, they reported the first
extra-solar x-ray force. It was later called Sco X1.
Sco stands for the consolation, X for the fact that it was an
x-ray source, and one because it was the
first source in a constellation of scorpius.
And, we now know, it took several more years,
that the optical counterpart of this bizarre object is a very
feint, blue star. It's about a distance of about
1,000 light years. And what is so unusual about
Sco X-1? That if you take the power in
x-rays, and you divide that by the power in optical,
and you do that for Sco X-1, then you will find
approximately 1,000. Now, think about it.
In the sun, the x-ray emission is a minute fraction of the
total emission, one in 10 million.
In this case, the optical emission is only a
very small fraction of the total emission.
It's completely dominated by x-rays.
And Riccardo Giacconi received the Nobel Prize for this two
years ago, the Nobel Prize in physics.
And so, the big question was, what can this be?
How is it possible that any object in the universe can do
this? When I came to MIT in early
1966, about six of these sources were known.
They had been found using rockets.
These rockets would be launched, spent at most five
minutes above the Earth's atmosphere and would reenter.
They would make a quick sky scan, and that led to the
discovery in '66 of about a total of six of these sources.
So I joined the group of George Clark who was preparing to
observe x-rays from highflying balloons.
The advantage of highflying balloons is that balloons can
stay up for hours, in some cases even for days.
So, you have a huge advantage over the rockets.
You have way more time to observe.
But there is a problem. And that is,
you can never get above the Earth's atmosphere because
you're a balloon. So there's always a little bit
of residual atmosphere above you, as little as that is.
It absorbs most of the x-ray spectrum.
In fact, it absorbs everything below 20 kilo electron volts.
So, yes, we had way more time but we were only sensitive to
the energy range roughly above 20 kilo electron volts.
Nowadays, no one flies rockets anymore.
No one flies balloons anymore. We do everything from
satellites 365 days a year. So, I developed and I built in
1966 x-ray detectors with the help of graduate students,
with the help of undergraduate students who take about two
years typically to build such a telescope.
In today's dollars, it cost about $3-$4,000,000 to
build such a telescope. So, you need substantial
funding. And, the weight of the
telescope would be 1,000 to 2,000 kg.
The balloon that we needed, you'll see a picture of it,
with today's dollars, would be about $300,000.
And the helium that is needed to inflate the balloon to
inflate only a small portion of the balloon, something like
$150,000. So this was not really a cheap
endeavor. The diameter of the balloons,
when they are fully inflated, is 500 feet.
They are made of polyethylene, and they are extremely thin
because you need a low weight balloon.
Otherwise, you can't get very high.
In fact, the skin is thinner than cigarette paper,
15 ° thin. You can look straight through
it. It's a very risky business.
You pay a lot of money. There's no guarantee that it
will work. You try your luck.
If it doesn't work, tough luck.
You get no money back. They could easily be damaged
because they are so thin. There's a good probability that
you damage the balloon right at the launch.
You'll see why. It's very difficult to launch
these balloons. But even if the launch is
successful, when the balloons go up they go through the
tropopause, which is near 70,000 to 100,000 feet where the
temperature is very low, -70°C.
And there are jet winds. And so, the low temperature
makes the [NOISE OBSCURES] break the balloon.
And, when that happens, we do have a parachute.
You will see the parachute that is supposed to bring the payload
back. But when the balloon bursts,
in general it interferes with the parachute.
And the whole thing comes down. You have a freefall and you
lose your payload. And with that,
when such a balloon bursts, often the Ph.D.
thesis also pops because there are always Ph.D.
students involved in these experiments.
It's very dramatic if a student has worked two years on these
payloads and then there is a freefall, and you destroy the
payload. And it has happened,
believe me. So, what I want to show you now
is a series of slides that give you an idea of what the balloon
launch is like. So, if we can make it very dark
for this, and we shall turn this off, then you will see here the
first slide. These were two undergraduates
of mine: Pat Downy and Jim Valentine.
They were working on the electronics.
Astrophysics is a very romantic feud.
They fell in love with each other.
They married. They have kids.
They're still in touch with me. And Jim got his Ph.D.
with me. He stayed with me as a graduate
student. And, here you see the plans in
Texas were these huge balloons were built.
This hall is 1,000 feet long. Balloons are put together in a
way like a tangerine is put together with [gores?].
They are sealed. And that work was only done by
women. It has nothing to do with sex
discrimination of any kind. But it just turns out that
women are more accurate workers when it comes to this than men.
They make fewer mistakes. That was the only reason.
You can imagine just a small mistake and you can have a leak
in the bulletin, and that is the end of it.
So, we lay these balloons out on an area in general close to
an airport. You put cloth on the ground
because the balloon is so thin that if the balloon touches the
grass, it would definitely rip. And the balloon is packed in
some plastic. You see that.
That is the pink sheet that you see that the balloon is inside.
There are hundreds and hundreds of layers of balloon inside
there. And it's taken out of the box
very slowly. People inspect it because
there's always a possibility that there is some damage during
transport. And here, I take you now to a
desert town in Australia, the heart of Australia,
Alice Springs, where you see already the first
layout of the balloon. Here's the launch truck.
The payload is there, and the balloon is also there.
You see the parachute shortly. And all of this is empty
balloon. And it's only this top part
that is going to be inflated. We launch these balloons
typically early morning when the wind is very low.
You also need a very consistent wind.
You'll see shortly why. The direction of the wind
should not change by more than 10 or 20°.
If it does, you lose the balloon.
You need the wind to push the balloon towards the launch
truck.
So this is, then, the launch truck and then.
And this is all this empty balloon.
And it will remain empty because as the balloon rises,
the gas will expand. And so, it is only this part
here that will be inflated with helium.
You see one inflation tube here.
And so, that part wants to go up.
And so, you have to hold it down, of course.
And, you do that with a very heavy weight,
tons of concrete. And there is this roller arm
that holds the balloon down. And then, when the balloon is
being launched we slipped his arm up.
We do a little radio command, and then the whole thing goes
up. So this is a very critical part
in a launch. As I said earlier,
you want your payload back. The balloons are not allowed to
fly over certain areas where there is heavy plane traffic
even though the balloons fly way above the planes.
We fly them at 140,000 feet. And airplanes fly only at
30,000 feet. But you are not allowed to fly
over those lanes. So when you get close to that,
you're forced to terminate the flight.
And you do that on radio command.
And there are scripts here that break those cables.
And then, the parachute comes down.
And the balloon is so brutal to cause it's cold there that the
balloon shatters and pieces and comes down all by itself.
And we often, though not often,
we always find the balloon. Also, we terminate the flight,
of course, when we get close to water because we would not be
able to recover the payload over water.
So here, you see the beginning. We are back in Alice Springs.
We see the beginning of the inflation.
So here is this roller arm here, and this part is going to
be inflated, helium truck there. We have to truck the helium and
from the United States: very time-consuming,
very expensive too. It was done by the Air Force
for me. They pay for that,
which was very nice. And here, you see a typical
situation. We fly these early morning when
the wind is very calm and very steady, which,
by the way, happens only maybe a few days per month.
You can't fly every day, so you sometimes wait for a
long time for the right conditions.
And so, you see here the inflation going on,
and then the sun is just about to come up.
You see the [gore?] lines here. I mentioned to you that these
balloons are put together like a tangerine done by the women who
seal them with filament tape to give them strength.
And you see those gore lines here very clearly.
And now, the bubble is up. So the roller arm is right
here. Inflation is still going on.
You can see that, and the sun was just about to
rise. I waited for that moment.
I thought it was very romantic, you know, just at the moment
that the sun comes up, at the apex of the balloon,
you know, terrific. But my real favorite picture is
this one, which I took a little later.
Now, the sun is a little higher.
And the inflation was still going on.
This is still Alice Springs. And so, you see the inflation
tubes, and you look at these gores, this incredible tedious
work to seal a polyethylene together with the filament tapes
hundreds and hundreds and hundreds of these gores.
And here, now, we are very close to the
launch. You see here at the time my
graduate student Jeff McClintock.
He's now Dr. McClintock.
He's at Harvard. You see here the telescope,
and then the empty balloon. And then you see the portion
that is being inflated. This part may be only 80 feet.
And that's enough. That's all you need.
You get enough free lift out of that, and then as the balloon
rises, the gas expands. And then the balloon would
completely inflate. You see radar reflectors here.
So that makes it easy for us to track the balloon.
And here, you see the parachute.
There is the point where I told you we can do this operation.
And Jeffrey was doing some last-minute changes.
This is very shortly before the launch.
And here it goes. This is a moment that your
heart beats, easily goes from my normal 60 to 140.
This is a moment that if things can go wrong,
this is the moment they go wrong.
It's very thin material. It's all of a sudden released.
The helium wants to go up. And it bounces back against the
top of the room. You get a mushroom effect.
You see that. This is the only helium there
is. There's no helium here.
And then, this whole thing goes up.
The launch truck, the engine is already running.
You can see the exhaust. The launch truck has to wait
for the balloon to be entirely off the ground.
And then the trick is that the launch [work?] has to maneuver
itself to get straight under the balloon.
And you'll see why that is necessary.
And the wind is blowing the bubble towards the launch truck
and if the direction of the wind changed during the past few
hours of layout, then we lose the balloon.
And we cannot because the launch truck cannot meet the
location of the balloon at the right moment in time.
And then the balloon is ahead, and then we abort the flight
and you lose all your money. But you don't you lose your
payload, of course. You will still hold the payload
on the launch truck. And so, here you see a close-up
of that bubble. It makes a tremendous amount of
noise. It's like a storm.
Really, it feels like a storm. You can see,
you know, this material 15 ° thin.
This reading of this material is amazing that some of them
actually survive. And so, now it's picking up.
So, the truck is still waiting. Some of that pink material,
the cover, is falling off. You see that here?
And higher, you see the helium. This is only helium here.
The rest is all empty. The balloon is now almost off
the ground. But I was too close in this
case to take more pictures. So, now I will move back to the
United States where we used to make these launches from Texas.
Palestine, TX is where the balloon launching station was.
So, I will continue the launch from a different site,
also a slightly smaller balloon, but it's the same idea.
So, you see the balloon is rising.
And the launch truck is now definitely moving to try to
maneuver itself straight under the balloon.
And the balloon, now, you see now the truck has
managed to get straight under the balloon.
If the balloon gets ahead of the truck, and if now you commit
yourself to a flight, that means the payload is
attached to the truck. And, with radio command,
we can snap some wires, and then the payload is
released. If the balloon is ahead of the
truck, you pendulum into the ground, and you lose your
payload. If somehow the person on the
launch truck commits too early when the balloon is still
behind, then of course the pendulum pushes the payload into
the truck. And that can also destroy the
payload. So, it's very critical.
This moment is extremely critical.
The balloon has to be straight overhead within a few degrees.
And then the person who is responsible for this launch must
also make sure that there is enough tension in the line so
that the payload will actually go up rather than go down and
plunge to the ground. In other words,
have we perhaps lost helium? And all those decisions have to
be made in a matter of seconds. And then the commitment is made
to the launch. Then the balloon goes up.
And you see that here. The payload is being released
from the truck, and this balloon is now going
up typically about 1,000 feet per minute.
It takes about 2 1/2 hours to go to an altitude of about
140,000 feet. And then, it sometimes happens
that the balloon fails. This was a beauty.
Actually, this was a flight from Texas.
You see the telescope, and you see the parachute.
All that balloon is empty, and all that will fill up as
you'll see shortly. It will all fill with helium as
the balloon rises. And then, occasionally,
you see this. You know things went wrong.
Some hole was torn in the balloon during the launch,
and that's, then, the end of it.
In this case, we did not terminate the
balloon on the flight on radio command.
So, we did not separate here because we were so close to the
ground but that was a destroyed telescope.
The parachute would not have opened.
We just let the whole thing come down all by itself.
So, the balloon, in a way, acted like a
parachute. It just came down very slowly
and very softly. And there was no damage on the
payload at all. So, we'd fly it again a few
days later. And here, you see the balloon
at an altitude of 150,000 feet. This is the largest bulletin
that was ever flown successfully,
I should say. I bought this balloon,
which is a 52,000,000 cubic feet balloon.
I paid a lot of money for that. Today's money would be easily
$250,000. And [a larger?] was flown
later, but not successfully. It was not my own,
by the way. So, this is the largest ever
flown. It's at an altitude of 150,000
feet. And you see straight through
the material. It's that thin.
It's 15 °. And you see the payload here,
even. This is the parachute.
And this, from here to here, it's almost 500 feet.
What you also see here are these [ducts?].
We call them ducts. They are huge.
There are about the size of this lecture hall.
They are connected to the balloon up here like the veins
going into your heart. And then they go to the bottom
of the balloon, and then they just open,
openly connected to the atmosphere.
And the reason for that is the balloon is so thin.
It cannot stand any overpressure.
It would pop. So, as the balloon keeps
rising, and the gas expands, there comes a point that the
balloon is fully inflated, and you must get rid of your
helium. Otherwise the balloon would
pop. And then, the helium just
naturally flows out through these ducts.
I think there were five ducts that we have.
You see three here. So, these openings,
these five openings here near the bottom are each about the
size of this lecture hall. So, you see here at the time my
graduate student, George Ricker,
is now Dr. Ricker.
And he's still at MIT, still very active.
A lot of this equipment was built by him and by other
students, graduate students and undergraduate.
But also a lot of it was, of course, owned by the balloon
launching station. And so, we have contact with
the telescope. We received the data by radio.
We can also command the telescope.
And you see me here sitting on the chase plane.
We use a small plane, and resolve the balloon,
of course at very modest altitudes.
In the United States, you hop from airport to
airport. You stay as close to the
balloon as you possibly can. And then you terminate it when
you have to for reasons that I mentioned [SOUND OFF/THEN ON]
over traffic, air traffic,
or you get close to water. In Australia,
that is way hotter because in Australia, you don't have many
airports in the desert. You only have some airstrips.
It's very, very difficult in Australia to follow the balloon,
but we try, and we hop from little airstrip to little
airstrip. But at night,
you cannot do that because there's no way you can land
[SOUND OFF/THEN ON] in Australia, no lights on these
airstrips. So, this is the kind of plane
that we used. We'll call it a chase plane to
follow the balloon as it starts drifting away.
Here is a map of Australia, one of my many successful
flights. This is Alice Springs,
desert town. We launched the days before we
think we are going to have our flight.
We launched data balloons. We tracked them by radar.
They go to a pretty high altitude, 120 5000 feet,
which gives us a good idea the direction that our balloon will
start moving. And we had all reasons to
believe that it will probably go somewhere in this direction.
And so, we alerted all these radar stations in Australia.
They will pick up our payload because we have radar
reflectors, and they could tell us, then, where the balloon was.
And that, of course, we had to do in Australia.
You don't have to do it in the United States for the reasons
that we cannot follow my airplane here.
There's no way that you can land there.
So, we relied, then, on radar stations to tell
us where the balloon was. Instead, however,
the balloon went straight south.
[LAUGHTER] So, our chase plane was perfect
because this is an area where Australia is reasonably
populated. And then, when the sun sets,
we didn't quite know where the balloon was anymore.
There was no GPS in those days. So, we didn't quite know where
the balloon was. But then, in the morning,
we picked it up again. It's gorgeous to see that
balloon in the sky. It's unbelievable.
It is way more impressive than Venus now in the morning.
It is fabulous. When the sun hits the balloon,
you see something that you'll never forget in your life.
Hundreds of calls to police stations about reports on UFOs.
[LAUGHTER] Hundreds of them. And, I can understand that.
These people have no idea how far is away.
You can't estimate that it's 150,000 feet.
Later, I talked to a pilot which was in one of those small
airports, and he said, when I saw this thing,
I'm going to it. And he says,
and I went higher, and higher, and higher,
and higher, and I couldn't get there.
And I said, well, it was a little higher than you
can fly. And, he realized that it had to
be that far. But, of course,
he had never made that connection.
So, for him, it was really a UFO.
I'm not even sure he believed my story when I told him it was
a balloon. So, when we were getting close
to Melbourne, we had to cut it loose because
this is an area where you cannot fly over.
There's a city here. So, this is very heavy air
traffic. And so, you then terminate the
flight. And if everything goes well,
it comes down on a parachute. And then the balloon becomes
brittle, and the balloon comes down as well.
Now, how do you recover your payload in Australia where the
payload comes down somewhere in the desert may be 70 miles away
from the nearest house and maybe 120 miles away from the nearest
airstrip? Well, what you do is the
following. You try to find a house close
to where the payload is. So, you know where the payload
is because you terminated it. You see the parachute come
down. You locate it.
You put it on the map where it is, and so you know the location
so that you can find it again later.
And then, what you do is you find the nearest house.
In this case, it was the house of Jack.
It was 70 miles away from the payload.
And you fly over that house obnoxiously low.
You make a lot of noise, and you make many dives until a
person comes out of that house. And they know what that means
because they've lived there. They know that you want to meet
them at the nearest airstrip. Once they come out of that
house and they wave at you, you just go back to your
airstrip and you wait. And we did.
We waited 24 hours. And 24 hours later,
there was Jack. Jack was this car.
Jack is crazy: always drunk,
and his profession was to shoot kangaroos.
There is no windshield in this car, and this is a piece of
wood. And he would drive 60 mph on a
desert, and would have his gun on that piece of wood,
and he would shoot that way kangaroos.
We are in contact with the plane.
The plane [talks us?] to the payload.
So, we are in contact [with this?] telephone.
Jack had a dog. And he said,
I'll show you something. This is a very special dog.
And he put the dog on the roof. He started driving 60 miles an
hour, and you would slam the brakes.
And if the dog would catapult through the air.
I mean, I felt sick in the stomach.
And then he would say, you can teach an old dog any
new tricks. You must have done it many
times with his dog. He was crazy,
and I think also a little cruel.
But, we got to the payload, in other words,
through the telephone with the airplane overhead.
He managed to get it there. And, here you see,
oh, there's a kangaroo. Yeah.
By the way, on our way to the payload, he shot this one
because the moment he said, he said, sorry,
you've got to wait. And then, he changed direction,
and there he went. And he chased it,
and he's amazing, killed it with one shot.
I felt sick in the stomach more than once during that recovery.
And, we also encountered somewhere a koala bear.
Isn't that nice? Wonderful koala bear in a
eucalyptus tree. Ah, and then we got to the
payload. And what did I see?
A 6-foot iguana 5 feet away from the payload,
maybe 10 feet. But it was so scarily close
that my stomach was turning. And I was with my graduate
student, Jeffrey McClintock, and I said to Jeffrey,,
not that I believed in myself, but I said, Jeffrey,
this animal is absolutely harmless.
Why don't you go first? [LAUGHTER]
And Jeffrey did go first. And not only that,
but these animals don't move because they think if they don't
move that you don't see them. During the seven hours that it
took us to take the payload apart and to put it on Jack's
truck, this animal didn't move at all.
So, I had plenty of time to photograph it.
And here, you see the payload. This was Jack's life.
And, this portion here came with me from the United States.
He was from the Palestine relaunching station,
very competent electronic technician.
You may think that the payload is heavily damaged.
But that's not really true. He protected with a crash pad.
And so, when it hits the ground, it may tumble over.
But the crash pad in general absorption most of the shocks.
So, to his absolutely no serious damage in this case.
And then, you go back to Alice Springs.
You must understand that Alice Springs is a hole in the ground.
Nothing happens there ever. And so, there's a big article
in the newspaper. It says: perfect balloon
launch. And it says balloon professor
back in town. They called me the balloon
professor. I gave talks for high schools
and for the [Lion Club?], your local celebrity.
And then when you read this article your stomach turns.
This reporter had no idea. I explained to him the whole
idea about the absorption of the Earth's atmosphere.
So we had to get very close to the top of the atmosphere.
And all he could think of in his article was,
the reason why we fly balloons: because that makes the distance
between us and the stars a little smaller.
[LAUGHTER] Yeah.
All right, that's enough for now.
I had about 20 successful flights in the period 1966 to
1980. And at 1980I stopped flying
balloons because then the satellites were completely
taking over. I had flights from the United
States, from Canada, and many from Australia.
Australia has the vintage that you can see the Southern
Hemisphere, which is a totally different part of the sky from
the United States. I had two freefalls.
I already mentioned to you, that happens when there is a
balloon burst. And then, the parachute doesn't
open. And, twice did I lose my
payload. I was lucky,
though, and I made several interesting discoveries.
We discovered early on five new x-ray sources,
which had never been seen from the rockets.
And that is perhaps not so surprising.
I mentioned the rockets have only five minutes up there.
And in five minutes, they scan the whole sky.
We have flights 20 hours. The largest and longest flight
I had was 26 hours. And we are very sensitive above
20 kilo electron volts where the spectrum is very low,
very low flux. So, the rockets would never
observe that, would never detect that,
not enough time. But, we were able to find whole
new sources, a whole new class of sources which had a very high
energy spectrum. And now, of course,
we know many of those. So that was an early
conclusion. And, we also discovered the
variability in x-rays as we were observing.
So, we were looking at Sco X-1. I mentioned Sco X-1 earlier.
And, as we were observing with our balloon payload,
we noticed that in ten minutes time, the flux,
the x-ray, the number of x-ray counts per second increase by
about a factor of three. And that was a bombshell
discovery at the time because clearly, how on Earth can an
astronomical object change its intensity by a factor of three
on a time scale of ten minutes. Imagine that the sun would do
that. You look at the sun,
and all of a sudden ten minutes later, it's three times
brighter. That doesn't make any sense.
But, in any case, we now know that x-ray sources
do that. And, that was discovered in one
of my early balloon flights. We also discovered a source
that we gave a name, GX1 plus four.
One plus four makes a connection to where it is
located in the sky. And there was a clear hint of
periodicity. We saw the x-ray signal,
the periodic. The period was about 2.3
minutes. And we didn't have a clue what
that meant. But now we do.
And you will know shortly also what that means.
So, the big problem is, and then remains,
what are these x-ray sources? And I think this is a good
moment to have a break. You can think about what they
may be, and then after the break I will tell you what they are
and a little bit more. So, we'll have a four-minute
break. So, enjoy yourself.
Walk around and stretch your legs.
Two minutes left in the break.
OK, then, the last time you will hear the whistle.
What are these sources? These are binary systems.
Here is a star. It's a normal star burning
nuclear fuel. And it is in orbit with another
star, which is either a neutron star, or which is a black hole.
And it's a binary system. Go around each other like this.
And there is somewhere a point between these two stars why the
gravitational pull in one direction is the same as the
gravitational pull in the other direction.
We call that the [inner/inter?] Lagrangian point.
There is also such a point between the Earth and the moon.
It's very close to the moon. But, there is such a point.
And, if that point is inside that star, so when the
gravitational pull in this direction is the same as the
gravitational pull in the direction of the neutron star,
if it's inside that star, then the matter will flow
towards the neutron star because that's energetically more
favorable. And so, since the whole thing
is rotating, it cannot radially do that.
And so, it spirals in through an accretion disk.
And then, it ultimately ends up on the neutron star.
So, this star is called the donor the name speaks for
itself. And this disc is called,
then, the accretion disk. And this object is called the
accretor [SP?]. Now, let us assume that this is
a neutron star. It can be a black hole.
But let's assume it's a neutron star, and that this is the
neutron star. And the neutron star has a
mass, M, and has a radius, R.
And, I take a little bit of matter.
It's a test particle from a large distance,
a little test particle. And I dump that onto the
neutron star. And
And I want to know, what is the speed with which it
reaches the surface of the neutron star?
All of you should be able to do that in 30 seconds.
Well, not all of you, but most of you.
Right, Emma? All of you who have had 8.01
can obviously do this. It is a matter of conversion,
of gravitational potential energy to kinetic energy.
And so, I can write that down blindly even if you [wake up?] 3
o'clock at night, one half MV squared,
M being this mass, V being the speed as it hits
the neutron star is then MG divided by R,
not R squared, but R.
This is gravitational potential energy converted to kinetic
energy. And, the little mass never
enters into the equation. If you pick for the mass of the
neutron star the mass of the sun, in reality,
they are probably a little larger.
But, take the mass of the neutron star.
And you take for the radius of the neutron star 10 km,
which is enormously small. That is 100,000 times smaller
than the radius of the sun. If you do that,
you get an enormously high speed of about one third of the
speed of light. So, that matter hits the
neutron star is about one third the speed of light,
100,000 commoners per second. And, if enough matter is
transferred onto the surface of the neutron star,
this kinetic energy, of course, is all converted to
heat. You get an enormously high
temperature: ten million, 100 million degrees,
and at that high temperature, the radiation that comes out,
electromagnetic radiation is almost all in x-rays.
And that is what really is the x-ray source.
If you take a marshmallow, and you throw a marshmallow
from a large distance on a neutron star,
when it hits the surface, the energy that is released,
that means the explosion that is caused by the impact is
comparable to the energy that was released when the atomic
bomb was thrown on Hiroshima at the end of the Second World War.
So the x-ray source that we saw from Sco X-1 is really the
neutron star. That's where the x-rays come
from. And the optical emission comes
from the donor. And also, some of it comes from
the accretion disk itself. And so, when you take this
ratio the way I did it for this system, this is really what
comes from one of the two objects, and this is what comes
from the other object. So that's why you get such an
absurd ratio. It is not just one object.
But, it's a binary system. The neutron stars are formed in
a supernova explosion. When a star burns its nuclear
fuel like our sun, then there is a heat source
which is the nuclear furnace inside, burning hydrogen and
burning helium, nuclear fusion.
So you have heat production here.
And that obviously will make the star grow bigger,
expand the star and the heat source.
And then, there is of course gravity, which holds the star
together. And there is always that
equilibrium between gravity and the pressure due to the heat
that determines, then, the final size of the
star as an equilibrium situation.
But, when all the nuclear fuel has been exhausted,
there comes a time that the nuclear fuel is just gone.
Then you get an implosion because gravity takes over.
So the red goes away, but the green always is there.
Gravity is very patient, and ultimately gravity takes a
chance, and it pulls the whole thing together.
And that's a supernova explosion.
We have about one of those every hundred years in our own
galaxy. They are extremely rare.
Our sun will actually not undergo a supernova explosion.
So, our sun will not become a neutron star.
Stars have to be at least 10 times the mass of the sun,
10 to 20 times the mass of the sun for a neutron star to be
formed or a black hole, for that matter.
But our sun will also die at some point when the nuclear fuel
has been used up. That's going to happen about 5
billion years from now, so you don't have to worry yet.
In 5 billion years, the sun will become a white
dwarf. It will become the size of the
Earth, very small. And for that,
it will actually first become very large.
And it will swallow up the Earth.
It will swallow Venus, and all kinds of misery.
So you don't have to worry about that either.
When a neutron star is formed, the magnetic field of the star
before it became a neutron star. It may have been very modest,
but in the formation of the neutron star,
that magnetic field goes up staggeringly.
And that has to do with 8.02. If you make a very na°ve
calculation, you get the right answer may be for the wrong
reasons. And the very na�ve
calculation is done the following.
If you have a star, which has a magnetic dipole
field, and you shrink the star, say, by a factor of a million
in terms of radius, then the surface area goes down
by a factor of 10 to the 12th. But the magnetic dipole field
doesn't change. And so, at the surface of that
new star, you get the magnetic dipole field which is about 12
orders of magnitude higher than what you had originally when the
star was this big. So that very na°ve back on the
envelope calculation tells you, then, that the magnetic field
would increase by a factor of 10 to 12 x 12 orders of magnitude.
So, you have strong magnetic fields.
That's 8.02. Then, we have 8.01,
which says, yeah, but you cannot shrink a star
without spitting it up. There is this idea of
conservation of angular momentum.
I omega is conserved, I being the moment of inertia.
And so, if you make the radius of a star 100,000 times smaller,
then the moment of inertia which goes always with R
squared, right? I hope you remember that.
Yeah, you do. OK, 100,000 times smaller
radius means that the moment of inertia goes down by a factor of
10 to the 10th. And that means that omega goes
up by ten to the tenth because I omega remains constant.
So, the star spins up by a factor of 10 billion.
So, we have a strong magnetic field.
The neutron star is spinning like crazy.
And then, there is something else.
And that is, if you have here that neutron
star, and here you have this magnetic field,,
this magnetic dipole field it is enormously strong.
Then, there is matter that comes from the side from the
accretor, and that wants to fall onto this neutron star.
That matter is highly ionized because it's hot.
But, ionized matter cannot just go through like this because
there's 8.02. And, 8.02 says that the force
on a charged particle, Q, is Q times V cross [B?].
This is the famous Lawrence force.
And so, you cannot cross these field lines.
It is a cross product between the velocity and B.
And so, there is a problem. And what happens now,
the same thing happens, by the way with the Earth's
magnetosphere that as the matter gets close enough to this
neutron star, it begins to spiral around to
field lines that it can do, and then it ends up on the
magnetic pulse. And so, most of the matter,
then, ends up here and ends up there.
And so, you get onto the neutron star two hotspots.
They may be no larger than maybe a few kilometers in size,
not even that. They may be as small as a
soccer field. So, that means you have two
hotspots. Now, imagine that I'm a neutron
star, and that here is one hot spot and there is another
hotspot. So, these are my magnetic
poles. But this happens to be my axis
of rotation. So we rotate like this and you
will see a strong x-ray signal. Now you see less,
and now you see another strong x-ray signal.
And now, you see another strong x-ray signal.
So now, you will see pulsations.
So, now you expect that the x-ray intensity varies because
you were looking at these hotspots, these magnetic poles.
And then, there is, of course, the possibility in
these binary systems. As seen from the Earth,
it may be that when they go around each other is that the
neutron star hides behind the donor star.
And when that happens, the x-rays from the neutron
star are completely absorbed because the donor star absorbs
the x-rays. So, then you don't see x-rays.
That's called an x-ray eclipse. And all of that has been
observed. And I will show you the
evidence for that so that you can fully appreciate what's
going on here. So let me then go back to the
slides, make it dark. And, I'll show you the idea of
a binary system. So, this is,
of course, just an artist's conception.
You see on the left side the donor.
And then you see here the location of the neutron star,
or in some cases, a black hole.
And then the inner Lagrangian point is apparently inside this
star. And so, the matter will flow,
makes an accretion disk,, and gradually finds its way.
And then the energy is released the way that we discussed
gravitational potential energy is converted to kinetic energy.
This data is from one of the early x-ray satellites called
[UNINTELLIGIBLE]. It's data from 1971.
Horizontally, it's time.
This is 1 1/4 seconds. So, that gives you an idea of
the timescale. And, this is the x-ray
intensity, how many x-rays per unit time were observed.
And, the actual data are the very faint lines in the back.
And they immediately notice the pulsations.
And so, then they did a fit to the data, and they came up with
this shape. And they unfortunately made
that very bold. And so, you can hardly see the
data anymore. But in any case,
it is very clear that you have here a signal of the rotating
neutron star. And, the neutron star in this
case goes around, spins around in 1.24 seconds,
so from here to here it's 1.24 seconds.
This object is called Hercules X1.
Spinning like crazy, you think about the sun.
The sun rotates about its own axis in 25 days.
You can see that very well because there's nothing to
recognize on the sun. But, it goes around in 25 days.
This one goes around this neutron star in 1.24 seconds.
Here we see data from the same source, but on a very different
timescale. From here to here is 1.7 days.
And I show you this so that you can see the eclipse is that I
mentioned, the x-ray eclipses. It's a very different
timescale. You can't see the pulsations
anymore. And so, we see that every 1.7
days, the neutron star hides behind the donor,
and is invisible until it emerges again on the other side.
And then, the x-rays come back up again.
And, this is not a set of data. So, this is many,
many days of data. And so, there you see the
overpowering evidence for these objects being binary systems.
The eclipses, you see the pulsations,
and what more do you want? I am no longer flying balloons
these days. Clearly, satellites have taken
over. I have flown observing every
conceivable x-ray observatory that you can think of.
[UNINTELLIGIBLE] was a European satellite.
[UNINTELLIGIBLE] Japanese satellites.
[UNINTELLIGIBLE] was also a Japanese satellite,
was the x-ray timing explorer, which is still up and running.
[Chandra?] is the big thing in town nowadays.
I make observations with Chandra.
And, XMM Newton, very important observatory also
in orbit: European observatory. So, I have been doing satellite
work for most of my time here at MIT.
And, in the period, 1975 to 79, we at MIT had our
own small little x-ray observatory.
It was called SAS-3, Small Astronomical Satellite
number three. And, we were running in from my
building, the Center for Space Research.
George Clarke was the principal investigator.
24 hours a day: data come in 365 days a year.
Compare that with a five-minute rocket flight,
and with a 20 hour balloon flight.
It was at that time that an astronomical satellite,
which was, I think it was a Dutch satellite.
It was called the astronomical Netherlands satellite.
It was a collaboration with the United States that they
discovered a phenomenon which we now call x-ray bursts.
The person who discovered it was Josh Grinkey [SP?],
who was at Harvard. And, John [Heisey?] at that
time was in [UNINTELLIGIBLE] the Netherlands.
What I noticed is that when they look at some of these x-ray
sources, one in particular, they all of a sudden in a
matter of a few seconds, seeing the x-ray intensity go
up by a factor of 10 or 20. It was very bright,
and then a few years later it's back to normal.
And that's referred to nowadays as x-ray bursts.
And with SAS-3, with our spacecraft,
we were ideally suited to search for these x-ray bursts
even much more so than the Netherlands satellite.
Our system was perfect. We never thought of that when
we designed it, but it just turned out to be
perfect. And in the first two years,
we discovered a new burst sources.
And it is largely due to those observations,
and also due to the theoretical work by Professor Paul Joss,
who is still at MIT, that we now know what causes
these crazy x-ray bursts. These x-ray bursts are huge
nuclear bomb explosions on the surface of the neutron star.
What happens is the following. You get accretion onto the
neutron star. And the matter that comes from
the donor is largely hydrogen, maybe a little bit of helium,
largely hydrogen and maybe some helium just like our sun.
And so, that matter falls onto the neutron star,
and it releases than the x-rays due to what we just discussed,
the gravitational potential energies converted to kinetic
energy, but nevertheless, it's still hydrogen as it
reaches here. And so, it forms a layer onto
the neutron star. And the densities are high.
And the temperature is high, and there is a thermonuclear
reaction, and it turns the hydrogen into helium.
And then, in the temperatures are even higher and the
density's just right, the helium starts to go into a
nuclear reaction informs carbon 12.
And that is a very peculiar reaction.
You need three helium nuclei, helium 4, and they merge,
then, to carbon 12. And you get energy.
This reaction rate is enormously sensitive to
temperature. I forgot the exact number but I
recall something like, that is proportional to the
temperature to the power 30, temperature in degrees Kelvin
to the power 30. So, when this reaction starts,
energy is released. So the temperature goes up.
When the temperature goes up, the reaction rate goes up.
And when the reaction rate goes up, the temperature goes up.
And that's what you call a runaway.
You get a runaway process, and we call that,
then, a thermonuclear flash. So the whole system gets
completely out of hand and you get a thermonuclear bomb
explosion on the surface of the neutron star,
which gives the immediate increase, then,
in your x-ray signal. If you wait a few hours,
then you build up a new layer of hydrogen, and that goes to
helium, and you get another x-ray burst.
So, when you look at the sources, you may see one every
hour, one every two hours, or sometimes a few per day.
These bomb explosions are about 18 orders of magnitude more
powerful than the hydrogen bomb that we can make on Earth.
So, the optical counterparts of these x-ray binaries are rather
feint. You see the donor.
I mentioned that to you in the optical from the ground.
Then you see the accretion disk.
And x-rays, then, come from the neutron star.
But you can see from the ground with an optical telescope,
you can see the optical counterpart.
And we had reasons to believe that if there is an x-ray
bursts, that somehow it should be followed by an optical burst.
And I can explain that to you best by showing you a
transparency. When the x-ray burst occurs,
the x-rays go off in all directions.
So, here you see the neutron star.
And you see there the accretion disk.
And when the x-ray [bond?] takes place, you see the
direction to the Earth. This is the direction to Earth.
That's where you happen to be. So you see the immediate
increase in x-rays, but the x-rays also go in this
direction. And then they hit the accretion
disk. And they get absorbed by the
accretion disk. And they heat up the accretion
disk. They can heat the accretion
disk locally up to maybe 30,000°, a much higher
temperature than it normally had.
And so the accretion disk, now, begins to radiate an
optical light, at least more light than it did
before. And then, when this becomes a
source of optical light, it goes in all directions.
The optical light, and some of it goes in the
direction of the Earth. And so, then that result is
that you first see the x-ray source, and then a little later,
the x-ray bursts. And then, a little later,
you'll see the optical burst because there is extra travel
time from here to there. It may take a second,
and then there is extra travel time from here to here.
It may take another second. And so, don't be surprised that
the optical flash, the optical burst,
then comes a little later. But if you can measure that
delay, then you have succeeded in getting a reasonable idea of
the size of these accretion disks because if the delay is 10
seconds, then you would know roughly that the radius of these
accretion disks might be five to six light seconds.
But if the delay is only two seconds, that tells you,
then, that this distance is probably only one light seconds.
And you have to add, of course, this extra distance.
So, this was, for us, the reason to start a
worldwide campaign in the summer of 1977.
We committed SAS-3 to serving one and only one of these stars,
binary systems, and we ask everyone in the
world to have a telescope, optical telescope,
radio telescope, infrared telescope,
to watch the optical counterpart.
And then we would tell them later when the x-ray bursts were
observed. And they would look at the data
to see whether there was any change in the optical light from
that star. And I remember,
the summer in 1977, there were 17 countries that
contributed. 44 observatories contributed.
We saw 110 x-ray bursts from the source.
And not a single observatory saw an optical increase or in
the radio or in the infrared. So, we decided we needed larger
telescopes because we were not going to give up.
So, we tried it again in 1978. And then we succeeded.
It was a collaboration with Josh Grinkey from Harvard,
and also Jeff McClintock, my ex-graduate student,
who was also at Harvard at the time.
We succeeded. We saw an optical burst a
little bit after we saw an extra burst.
It was a smashing success. And we made it to the cover
page of Nature, which is a very prestigious
scientific journal. It was covered in the New York
Times, the usual thing, Boston Globe,
and even more. I want to show you an x-ray
burst and an optical burst, but not the data from 1978.
I have data of much higher quality, which we got to in
1979. In other words,
we pursued this research very heavily.
We worked with Holga Patterson, who was in Chile,
the European Southern Observatory, and with Michael
[Laver Beyon?] from [Paradise?]. We often went there and we made
the optical observations. By that time,
SAS-3 was no longer functioning.
So, we had the Japanese satellite [Hocho?],
looking at the particular object that we were interested
in. And, I want to show you first
the x-ray burst that was observed by Hocho.
Here is the x-ray burst. You see the timescale.
The [rise?] of this burst is unusually slow.
It's just the way it is. It happens occasionally.
See, this rises. It takes about five or six
seconds. Normally it goes up in one or
two seconds, but nothing I can do about that.
So, you see here the data before the burst.
So that's the accretion. That is the x-ray it becomes
available because of the metal falling onto the neutron star.
And then you see the thermonuclear bomb explosion,
and then afterwards here, there is still a little bit of
afterglow. But it settles very quickly to
the pre-burst level. And then, Holga Patterson in
Chile came up with this optical burst.
You'd think it's almost like a carbon copy.
And, I plotted it for you in a way that the peak is at the same
height. That is just an artificial way
that I can scale that, of course.
And, if now, I put the x-ray data on top of
the optical data, then what do you see that
indeed there is a delay? The optical data are a little
bit later than the x-ray data. And all you actually have to do
is just slide it by two seconds, and they almost become carbon
copies of each other. And so, indeed we have
succeeded. And we are the first to measure
the size, the rough size, of the accretion disk at the
point where the x-rays were absorbed, and then converted to
optical, which then was seen by the Earth.
And we concluded that the accretion disk was something
like one light seconds in radius, which is about the
distance from Earth to moon, one light second.
Oh my goodness, I recognize you.
What are you coming here for? I'm getting flowers?
Oh, my goodness, what have I done to deserve
them? [APPLAUSE]
Wow, I recognize some of you. This is fantastic.
[SINGING STARTS] Thank you very much. [APPLAUSE]
I'm very impressed. Thank you very much.
And I gather that the text, which I couldn't always follow
was very much connected to my [OVERLAPPING VOICES].
So, in that sense, I appreciate it double.
Thank you very much. Let me thank you,
because I recall you from, you were in my class last year
right?
Well, I don't know whether those much time left for you now
to fill out the evaluation forms, but that's the problem
that you have. The orbital motion of neutron
stars and the donor stars give rise to Doppler shifts,
which is something that we covered in 8.03.
So, we have a rotating neutron star with pulsations.
[It's a clock?] in the case of Hercules X1, 1.24 seconds.
And, when that clock moves to you, the period is smaller,
blue shift. And when it moves away from
you, the period is longer, red shift.
And you can measure that. You can actually see as the
neutron star goes around, and even comes towards the
Earth, that you see a different period then when it goes away
from you. And then, of course,
you have the donor itself. The donor has an optical
spectrum, and you see absorption lines in the spectrum which come
from the atmosphere of the donor.
And then, you see the shift of the absorption lines.
And you worked on a particular case of [sickness?] X1.
You perhaps remember that, your problem set,
whereby we use the Doppler shift of the donor to calculate
the mass of the accretor. And that turned out to be a
black hole. That was the first x-ray binary
that was discovered in 1972 by [Mervyn Vesper at Webster?],
and by Bolton, a 5.6 day orbital period.
And they derived from the Doppler shifts that the black
hole was something like 15 solar masses.
So, this has led to a whole new industry of mass determinations
of neutron stars and black holes.
And then there is Fourier analysis, which we covered very
heavily, or at least sufficiently,
I hope, in 8.03, which plays a central role in
uncovering these periods, radio people,
x-ray people, optical people.
They all for you analyze their data.
The fastest rotating neutron star that is a single radio
pulsar, it's not in a binary system.
It has a period of 1.5 ms. Imagine, it goes around more
than 650 times per second. Think about if you're at the
equator of the neutron star. You go around more than 650
times per second. You would have a speed of
40,000 km per second, 14% of the speed of light.
And, it just so happens that only a few days ago or four days
ago, an announcement was made that an x-ray pulsar in a binary
system was found with a spin. Of the neutron star of 1.7 ms.
All of this could not have been done without Fourier analysis.
This was done in a binary system with an orbital period of
2 1/2 hours.
We've now come to the end of 8.03.
Needless to say that it has swallowed me up completely.
I lost 10 pounds, and it has also taken a lot out
of Marcos, whose dedication was fabulous.
[APPLAUSE] And you should feel sorry for
my graduate students and my postdocs who didn't get to see
much of me. I'm not saying that you should
feel guilty, but perhaps you should.
[LAUGHTER] you are on my mind all the time,
and not only at MIT but also at home, living room,
in the kitchen, in the shower,
and even in my dreams. It was a very difficult time
for my significant other, Susan, who is here.
And Susan at times said to me it was pure hell.
Did you forget, Susan?
Good. So, life will become normal
again, but I think I will miss you.
And of course in it myself no illusions.
You will quickly forget most of what we covered in 8.03:
Maxwell's equations, Fresnel equations,
[railey?] scattering, the [Limar?] equations,
and even the resonance frequencies of sound in a closed
box. All that will fade away in a
matter of months. I just hope for you that it
doesn't fade away before next week.
But surely, from now on, when you see a rainbow,
you will check that the red is outside.
You will look for the primary, you will look for the
secondary, you will look for the bright light inside the primary.
You can't help it. It's a disease.
It is a disease that no one can cure anymore.
It will be with you for the rest of your life.
And I take full responsibility for it.
You cannot resist when you see a rainbow to make those checks
even if you try. I will be very proud of you
when you use your personal polarizers, which I'm sure
you'll all carry with you because you have one now to
check that the bow, indeed, is polarized.
And occasionally, don't forget the blue sky 90°
away from the sun. It's also 100% polarized.
And those who do not have your knowledge will enjoy the
rainbows. Of course they will see the
pretty colors. But you will now see so much
more then just the pretty colors.
It's like with art. Knowledge only adds.
Knowledge never subtracts. And if that is all you will
ever remember of 8.03, I will have achieved something
that has enriched your life. And you'll remember me.
And I hope those memories will be happy ones.
I have immensely enjoyed this term.
And thank you for being such great students.
And thank you for coming to my lectures even when we had no
many quizzes like today. Thank you.