Today I would like to talk to you about some of the research
that I did during my early days at MIT.
It's a long time ago. I got my Ph. in the Netherlands on nuclear work and so the tension during
physics and I came over to MIT in nineteen sixty-six.
I was supposed to be here only for one year.
I had a one-year postdoc position.
But I loved it so much I never left.
I changed fields. I joined the research group of
Professor Bruno Rossi. Here at MIT I changed from
nuclear physics to x-ray astronomy.
X-ray astronomy speaks for itself.
You're trying to do astronomy in x-rays.
You cannot see any x-rays from the ground because the earth
atmosphere absorbs them completely.
So you have to go outside the atmosphere unlike optical
astronomy and radio astronomy which you can do from the
ground. When I use the word x-rays I'm
thinking of the kind of x-rays that your dentist would be
using, medical purposes, about one to fifty kilo
electron-volts. And since all of you took eight
oh two you should know by now what a kilo electron-volts.
Uh optical light is two electron volts,
where x-rays, way more energetic than optical
light. Uh during the Second World War
Werner von Braun in Peenemunde developed under Hitler Germany
destructive rockets. They were used to destroy the
Allies. To destroy you and me.
And after the war, around nineteen forty-eight,
the Americans used these rockets to do science.
They also got Werner von Braun over to this country and for
reasons unknown to me he became a hero.
They tried to observe x-rays from the solar system.
And they found indeed that the sun
emits x-rays. The sun is very close.
So you may say well that's not a surprise.
But it's really very unusual because to create x-rays you
need extremely high temperatures,
which we didn't think existed on the sun.
And if you take the power that the sun puts out in x-rays,
this is joules per second now, and you divide that by the
power in the optical and the infrared light of the sun,
this symbol stands for sun, that ratio is about ten to the
minus seven.
So you must conclude that the sun emits largely optical light
and infrared and that the amount of x-rays is a modest byproduct.
Interesting as it is all by itself.
In nineteen sixty-two several scientists here in Cambridge,
Massachusetts, among them Bruno Rossi and
Riccardo Giacconi and Herb Gursky, made an attempt to
observe x-rays from stars outside our solar system.
The odds were strongly against them.
The detectors were not sensitive enough.
If you take the sun and you bring the sun to the nearest
stars, which is a distance say of ten light-years,
then there would be no hope that you would be able to detect
x-rays from an object like the sun.
In fact the detectors were too insensitive by about a factor of
one billion. They tried anyhow and they were
successful. They did indeed find to
everyone's surprise and joy, they found x-rays
from at least one object outside the solar system.
This object was later called SCO X one.
SCO stands for the constellation Scorpio in the sky
and X for x-rays and one for the first source observed in that
constellation. We now know that this object is
a faint blue star. And what is extremely special
about the object SCO X one is that if you take the ratio x-ray
power over optical power then that ratio is
about ten to the third. Compare that with the sun.
This object, we had no clue what it was in
those days, primarily emit x-rays, and the optical emission
is a byproduct. Whereas with the sun it is
reversed. And so the burning question was
in those days what kind of animal is this?
It must be a totally different beast.
Something very different from our sun.
And when I came to MIT in nineteen sixty-six,
there were six sources known outside our solar system.
And they were all discovered with rockets.
The rockets in those days could spend about five minutes above
the earth atmosphere and they would quickly make a scan over
the sky, five minutes, that's all they had.
And I joined the group of uh George Clark,
who is still at MIT, uh he was doing x-ray astronomy
from very high-flying balloons, very close to the top of the
atmosphere, and the advantage of balloons was that you could
observe the sky for many many hours,
if you're lucky sometimes even a day or more.
But on the other hand, since there is always a little
bit of atmosphere left above you, even though there's very
little, there is still some left, the x-rays are absorbed,
almost all x-rays below twenty kilo electron-volts would be
absorbed, and we would not be able to see them.
But of course the compensation was that we could look at the
sky for many many many hours. Nowadays no one is doing these
balloon observations anymore. No more rocket observations.
Everything is done of course from satellites.
So when I came to MIT, together with George Clark,
I developed new x-ray detectors for these balloon observations.
Many graduate students were involved.
Many undergraduates. It would take about two years
to build a telescope. To give you a rough idea it
would take a million dollars in terms of nineteen sixty-six
dollars, and the weight of such a telescope
would be roughly a thousand kilograms.
The balloons in those days would cost about a hundred
thousand dollars to get us up to these high altitudes,
and we would need about eighty thousand dollars of helium,
and you will see some slides of that.
We have to go to altitudes of about a hundred forty thousand
feet. We had huge balloons for that.
You will see one. They have diameters of about
six hundred feet. And the material was
polyethylene. Extremely thin to make them
light-weight so that they can go high.
The thickness of that polyethylene was about half of
one-thousandth of an inch. Which is thinner than the saran
wrap that you have in the kitchen.
It is thinner than cigarette paper.
A very risky business to fly these balloons.
No guarantee of course that they would work.
You pay your money. If they work that's great.
If they don't work that's just tough luck.
There is a good chance that you have a failure when you launch
the balloon. They're very fragile.
There could be damage right at the launch.
But even if they make it up in the atmosphere they have to go
through the tropopause,
near about a hundred thousand feet where it's very very cold,
the balloons get brittle, and then they can burst.
And that of course would be the end then of that balloon flight.
And that could also be the end of a Ph.D.
thesis. Because that all these flights
of course were connected with research and therefore with Ph. in the Netherlands on nuclear work and so the tension during
these early phases of the launch were always extremely high.
Sometimes even unbearable. So now I would like to show you
some slides. Which will give you a good idea
of what these expeditions were like.
Oh, yeah, a classic problem. This is nice that these -- ah,
now they work. All right, so if I can have the
first slide, you see here Jim and Pat who at
the time were undergraduates, they are now both Ph.D.s,
and they are working there, very tedious work,
trying to put the electronics together.
You may think that science is not very romantic.
But I can assure you it is. They fell in love.
They married. They have kids.
And that's the way it sometimes goes in life.
And so here you see the plant in Texas where these huge
balloons were made. Uh balloons are put together
sort of like the -- the way that the tangerine is put together.
At the surface you see these gores of the balloon.
And the sealing of these gores to make up the balloon were --
was only done by women. Only women were allowed to work
there. Has nothing to do with sex
discrimination of any kind. It just turned out that women
were more patient.
They did the work better. They make way fewer mistakes
than men did. That's the way it goes
sometimes in life. Here you see balloon coming out
of the box. Nicely protected.
Plastic cover. And we also have here cloth on
the -- on the grass because the balloon is so thin that it would
certainly get damaged if it touches the grass,
it's enormously thin. This was not my balloon.
Uh we were worried that there was something wrong with it.
You can see here the concern. They thought it was a -- there
was a hole in the balloon. And that if there is a hole in
the balloon there's just nothing you can do about it anymore.
You can't patch it because the hole is almost always through
many many layers. What you're looking at here is
hundreds of layers of balloon that are folded together.
But as I said, since it wasn't my balloon I
wasn't too worried, but of course it's never nice
if you see a failure of your colleagues.
Now I bring you to the desert town Alice Springs in uh
Australia. Right at the heart of
Australia. And now you get a pretty good
idea of what it's like. Here you see the launch truck.
The telescope is there. And then you see this
enormously big balloon. All of it is empty now and most
of this will stay empty. This is the roller arm which
holds this part down. This is the only part that will
be inflated. And here you see the helium
truck. And here you see the inflation
tubes. And we will let helium in from
both sides which will then gradually begin to fill this top
part of the balloon. And you see here the roller arm
in detail. The roller arm is very
important because when this part of the balloon is being filled
it wants to lift, it wants to go up,
and of course you have to keep it down, you have to keep it
under control. And so this roller arm and this
-- this car is loaded down with concrete.
It's very heavy. And then just before the launch
this roller arm by command is fweet flipped over and then as
you will see later then the balloon will make it up.
And here you see the early part of the inflation.
Helium comes in from both sides.
And so we -- we fly these balloons almost always early
morning because then the winds are very calm.
You need extremely reliable winds.
You need to know the direction very well.
And the winds should be uh no more than something like three
or four miles per hour. If they are stronger you would
lose the balloon.
You see here these gores that I mentioned to you earlier.
Where the sun is behind the balloon.
Here the bubble is nearly fully inflated now.
Here it's still going on. Still going on,
the inflation. But we are very close to the
end of the inflation. Here is the roller arm and then
in this direction here five hundred feet or so down is the
payload with the truck. We're now very close to a
launch. We're still in Alice Springs.
This is -- was my graduate student Jeff McClintock at the
time. He's now Doctor McClintock.
Here you see radar reflectors which allows us to follow the
balloon -- radar. Here you see the telescope
hanging on the launch truck. Here is the roller arm.
All this is empty. And here you see the parachute.
We have here a connection between the parachute and the
bottom of the balloon. And we can control that on
radio command. We can separate that so that
the telescope safely comes back to earth.
At least that's the idea on paper.
And so now you see the release of the bubble.
So the roller arm is up and this bubble now takes off.
This is an incredibly fantastic moment.
This is really butterflies in your stomach and ants in your
pants. This is the moment that balloon
can easily fail. Very thin material,
the helium goes up, reflects against the top,
is pushed back again, you get this peculiar mushroom
shape, it makes an enormous sound like a storm.
The idea now is that this balloon will go higher and
higher in the sky. Will pick up all this empty
part. This is not inflated.
As the balloon goes up in the atmosphere the atmospheric
pressure will go down. And the helium will expand and
will fill the balloon. And the -- the trick now is for
this truck to manipulate, to maneuver itself under what
we call the bubble.
And therefore the wind has to be in this direction so that the
balloon comes to the truck. And then the truck tries to get
straight under the balloon. And then the payload will be
released here. Here you see a close-up of this
mushroom. You can actually see this
reflection of the helium going up and coming back.
You can also see these gores very clearly.
It's tedious work. By these women who have to seal
these balloons. Enormous amount of labor goes
into it. Amount of helium as I said
earlier is about eighty thousand dollars.
About the same price of the balloon.
And here it goes higher. We're in Alice Springs.
The cover is falling off. Balloon is going up.
See the engine is already running.
The truck cannot move yet because if it started to move
this part of the balloon would slide over the cloth.
There would be friction and there would be holes in the
balloon. So this truck has to wait until
all of this is off the ground. Going higher.
And I'm now so close to the balloon that I couldn't continue
my picture-taking from Alice Springs.
So I will jump back to an earlier flight in the United
States. We flew these balloons in the
United States from a town called Palestine, Texas.
And so you will see then the remaining part of the flight
from Palestine, Texas.
So the balloon is now completely off the ground.
See a little bit of gas, well it's not so little,
but it looks very little compared to the size of this
balloon. You see the parachute here and
here then is the connection which on radio command we can
separate. So now this is a very crucial
moment. The person in charge on this
launch truck has probably driven the truck to get straight under
the balloon. And when it's straight under
there they will allow the payload to
go free. The payload is attached to this
truck. If the balloon is too far ahead
and the payload is released it will pendulum into the ground.
And if you release it too early then of course the payload will
pendulum back into the launch truck.
Both would be a disaster. If the pull of the balloon is
not enough, for instance if a hole developed during the
launch, so if the tension is not strong enough,
you would release the payload, it would go bang,
back to the ground. So all these factors have to be
taken into account, and then finally the person in
charge commits to a launch. And then there it goes.
All the way empty. Here you see the helium.
The parachute. And you see the -- the payload.
And here you see the balloon at a hundred fifty thousand feet,
forty-five kilometers high in the sky.
The helium has now expanded. The balloon is fully inflated.
And you can look straight through it.
It's only half of one-thousandth of an inch of
polyethylene. And these are huge ducts which
have openings of about ten meters each.
And they are there because the balloon cannot stand any
over-pressure. If there is any over-pressure
the balloon would pop and so when the balloon keeps rising
and rising and rising when it reaches -- reaches its maximum
volume the helium would escape at the bottom.
That's the idea of these ducts. Here you see George Ricker who
was graduate student, my graduate student at the
time. This is in Australia.
He is now Doctor Ricker. He's still at MIT.
He's a staff member. And this is the kind of
equipment that we built, at least partially.
And he is checking the early results during the ascent of the
balloon. The balloon will go up with
about a thousand feet a minute. If all goes well,
there's no leak, it will take about
two-and-a-half, three hours to make it to
altitude. You see me here sitting on the
plane that we used to follow the balloon.
We fly of course at much lower altitudes.
Five thousand, ten thousand feet.
We stay as close to the balloon as we possibly can.
It's not always so -- not always easy, certainly not in
Australia. And so we keep an eye on things
and if necessary we can terminate the balloon flight by
giving a radio command so that the parachute comes down.
Certainly when we get close to the ocean of course that is
necessary if you don't want to lose the
payload. The data -- data come back via
radio, so we wouldn't lose the -- the data.
You get very sick, by the way in these planes.
If you're sitting for eight or ten or twelve hours or longer in
these planes as I have, I learned actually a little bit
of flying which is quite easy with a plane like this.
Here you see a map of Australia.
Here's Alice Springs. Uh we fly pro-balloons weather
balloons at a hundred forty thousand feet every
day to find out in what direction the balloon would be
drifting. And we had all reason to
believe that the balloon would drift somewhere in this
direction. And we alerted these radar
stations. These circles here are radar
stations. Because there are no airfields
here in Australia. And so we knew we couldn't
follow the balloon. We would probably lose it.
And therefore we alerted these radar stations.
They could tell us then when the balloon was in sight.
And that would allow us then to cut the balloon,
cut the payload loose and -- and make the recovery.
Instead, the balloon went straight south.
So the predictions by the weather balloons were not too
accurate. The balloon went straight
south. And here there was sunset and
then we don't know too precisely where the balloon was.
Remember this was in the nineteen seventies.
And so we were uncertain. But here about twenty-six hours
later when we were getting close to Melbourne which we were not
allowed to enter the air space here between Sydney and
Melbourne we cut the balloon loose.
That means we separate the payload from the balloon.
The balloon is very brittle. It's extremely cold up there.
The balloon then fractures and comes down in many pieces and if
everything worked well then parachute opens and brings the
payload safely back to earth. And then comes the big problem
how are you going to recover the payload.
You're in the middle of nowhere.
This balloon came down, this payload came down in the
desert. And there are no airports.
At least the chances are that you
are a few hundred miles away from the nearest airport with
your payload. So what you do then is the
following. You try to find a house close
to where the payload is located. We locate the payload.
We see the stuff come down. Radio beacons on the payload.
And then you fly over that house many many times in a very
obnoxious way. You make a -- a lot of noise.
You fly over very low. And so the people who -- whose
next neigh- neighbor is probably se- seventy miles away from
them know what you're trying to tell them.
You're trying to draw their attention.
And they know what that means is that they want you -- they
want you to meet them at the airport.
Whatever airport means. It's sometimes just a strip in
the desert. You can't land there at night
but you can land there during the day.
And so that's exactly what we did.
We drew attention to the house of this guy Jack.
He was -- he was a complete nut, he was always drunk,
was a crazy guy. And so we went to the airstrip
and we waited and indeed after fifteen
hours he showed up with this truck.
Uh there is no windshield here in the -- in the truck.
And he used to shoot kangaroos there.
He would go sixty miles per hour on the desert floor and he
would -- he would shoot kangaroos.
And he gave me a demonstration. Uh he put his dog on the roof.
He would go sixty miles an hour.
And he would slam the brakes. And then the dog would catapult
through the air, the poor dog.
And then all he would say is oh, you can't teach an old dog
any new tricks. And he seemed to enjoy that.
When we go after the payload the plane, the recovery plane,
is in the air. Takes off from that airstrip
and we have contact with the recovery plane.
They and only they can see and know where the payload is.
From the ground of course you can't tell.
And so they maneuver you to the payload.
And so of course Jack's help was invaluable.
We needed him. That was independent from the
fact that the man was a little strange.
On these recoveries you encounter many animals.
You see a koala bear here. In a l- eucalyptus tree.
Very lazy animal. Unlike most of you.
And then when we came close to the payload there was this
animal, a goanna, six-feet tall goanna.
I'll tell you, it scared the hell out of me.
And uh I didn't want to show that to my graduate student who
was with me and I said to him look you know these a- these
animals are completely harmless, why don't you go first.
This animal was no farther than four feet from the payload.
And so my graduate student went first and the amazing thing is
during the ten hours that it took us to recover the payload
and put it back on Jack's truck this animal never moved.
It was just sitting there completely still.
This is their way of trying not to be noticed.
And so here you see the payload.
This was Alice, was Jack's wife,
this is Tom Brooks, he came from the United States,
he was an electronic expert. And you see here the payload.
It looks heavily damaged but it really isn't.
This is crash pad which protects the payload when it
impacts. And an impact crash pad worked
very well, very little damage to this payload.
And then you come back after several days to Alice Springs.
Alice Springs is a hole in the ground.
Nothing ever happens there. And so obviously you make it to
the front page of the Centralian Advocate.
Perfect balloon launch, thousand watch start of space
probe. They think of this as a space
probe which is fine. Balloon professor is back in
Alice. They called me there the
balloon professor. I gave -- I gave several talks
there for high schools and for uh the Rotary Club.
So I was a sort of a local celebrity.
I talked to the uh news reporter for
several hours. And when you read this story
you won't believe the nonsense but that's all a detail of
course. OK, that's enough for the
slides for now. I had si- about twenty
successful s- flights. Between nineteen sixty-six and
nineteen eighty. Many from the United States.
From Canada and also from Australia.
Where that's a s- southern hemisphere which covers part of
the sky that we cannot see from the United States.
I had two free falls. Two of my balloon burst on the
way up in the tropopause. We were unable to separate
payload fast enough and then the whole thing, parachute gets
entangled. You get a free fall.
Big hole in the ground and that's the end of the telescope.
And it was. Twice did I lose the telescope
completely. But I was lucky and I made
several interesting discoveries during those successful flights.
We discovered very early on five new x-ray sources.
None of them had ever been seen with the rockets.
And several of these sources, that was the really new thing,
were highly variable. They changed their x-ray
intensity on a very short time scale.
We noticed even one source went up by a factor of three in about
ten minutes.
And that of course could not have been discovered with
rockets. Because the rockets were only
above the atmosphere for five minutes.
They would quickly scan the sky and so there's no way they could
discover variability on a time scale of tens of minutes.
But with balloons you can do that.
So it did pay off that we were watching the sky sometimes for
ten, twenty hours. Our -- my longest balloon
flight was actually twenty-six hours.
We also observed x-rays from one source
which we named GX one plus four.
Which stands the one plus four stands for the position in the
sky. And um this showed a periodic
signal in x-rays. About two point three minutes
periodicity. At the time we had no clue what
that meant but later of course we understood the significance
of that. And you will understand that
also very shortly. How significant that was.
So what kind of objects are they?
They are very very different from the sun.
And we now know what they are.
These objects are binaries. Binary stars.
One star is not unlike the sun, it's a normal nuclear burning
star. And it is in orbit with a
neutron star or in some cases a black hole.
They go around each other. And if they are close enough
together it is possible that the matter of this star is attracted
by this neutron star with a larger force than it is
attracted by the star itself.
And if that's the case this matter doesn't want to stay
here. But wants to go to the neutron
star. Now of course the matter has
angular momentum because they goes around.
So it cannot free fall to the neutron star.
But it would spiral in and slowly make its way to the
neutron star. And we call this an accretion
disk. And we call this the donor,
provides the fuel for the transfer of mass onto the
neutron star. And let's assume that the
neutron star has a mass capital M and has a radius R.
And let's assume that we dump some matter, little M,
onto the neutron star. Well all of you should remember
from eight oh one that you can calculate very easily the speed
with which this matter reaches the neutron star.
The kinetic energy, one-half MV
squared, must be equal to MMG over R.
We had this equation on the blackboard last lecture when we
discussed cosmology. It was the same equation.
This is the speed with which the matter will fall onto the
neutron star. If this is the mass of the
neutron star and this is the radius of the neutron star.
You lose the -- the mass as you always do.
And so you can calculate this speed.
This speed is horrendous because the radius of a neutron
star is so ridiculously small.
The mass of a neutron star is very comparable to the mass of
the sun. A little larger.
But not much larger. But the radius is a hundred
thousand times smaller than that of the sun.
It's only ten kilometers. And as a result of that the
speed with which the matter hits the neutron star is about
one-third of the speed of light. When it hits the neutron star
this kinetic energy is converted to heat.
It will heat up the surface layers of the neutron star and
increases the temperature to about um ten
million, a hundred million degrees, and at such high
temperatures, the star would emit almost all
its radiation in x-rays and not in the optical.
Our sun is relatively cold. It's only six hundred six
thousand degrees. And so the sun has most of its
radiation in the optical but when the temperature becomes ten
million, a hundred million degrees, the maximum of the
emission is in x-rays. To give you a little bit
respect, a little bit of insight, for this incredible
power, for this incredible
gravitational pull of the neutron star,
because R is so small, if you took a marshmallow and
you threw a marshmallow from a large distance onto the surface
of a neutron star, at impact the energy that is
going to be released is comparable to the energy of an
atomic bomb as was thrown on Hiroshima and Nagasaki near the
end of the Second World War. Neutron stars have very strong
magnetic fields. And the matter that flows from
the donor onto the neutron star is ionized.
It's plasma. It's charged.
And as you remember from eight oh two when you have a charged
particle in a magnetic field there is the V cross B term.
The V cross B force. And the V cross B force will
then spiral these charged particles around the magnetic
field lines and they would end up near the magnetic poles,
not unlike the solar wind when it reaches the earth,
these charged particles enter the earth atmosphere near the
magnetic poles, giving rise to aurora as we
discussed earlier. And so you end up on the
neutron star with two hot spots where this matter slams into the
neutron star. At the magnetic poles.
And if the axis of rotation of the neutron star is not the same
as the -- [Audience noise] Q: Tall male physicist.
[noise] Hi, Miss Peltier. [laughter] I remember your
name. [laughter] You were in my eight
oh one class. Q: [unintelligible] Q:
[noise] And [unintelligible] physics, is this a physics
lecture? Ask the students.
I don't know. [laughter] Q:
Very handsome and charming. OK.
[laughter] So are you. [laughter] Q:
[unintelligible] I think you know the answer to
that one. Q: OK, I think we have the
right person. [background noise] Q:
We have a song to sing to you. Q: [unintelligible] [tone]
Singing: When I
wisdom, times have changed. Singing: Maxwell's equations
are too hard for me, so please just help me,
help me. Singing: Help.
I need somebody. Help.
Not just anybody. Help.
You know I need someone. Walter.
Singing: When I was younger, so much younger than today.
I never needed anybody's help in any way.
But now these days are gone, I'm not so self-assured,
now I've found my homework's hard, no nothing anymore.
Singing: Help me if you can I'm failing now.
Walter Lewin, can't do physics,
I don't know how. I am on pass-fail,
don't let me down. Singing: Won't you please
please pass me. When I was dumber,
so much dumber than today, I didn't know about Lentz,
Maxwell or Faraday.
Singing: But now those days are gone, I'm not so dumb no more.
Thanks to eight oh two and our daring professor.
Help me if you can I'm failing now.
Singing: Walter Lewin, can't do physics,
I don't know how. I am on pass-fail don't let me
down. Singing: Won't you please
please pass me. Walter Lewin thanks for
teaching eight oh two. Now it's summer and we will all
miss you. Singing: You've been crazy and
we've all had a good time. So in your praise the Muses'
voices chime. Singing: Eight days a week.
I love you. Eight days a week.
Is not enough to show I care. Singing: Thanks for all you've
done, I'll pass somehow. E and M can't get
me down, I know this now. I'll miss pass-fail next year,
this I vow. Singing: Won't you please
please pass me, pass me, pass me,
ooh. I have tears in my eyes.
[applause] So that was very nice.
So now you won't have any time to fill out your evaluation
form. So I was going to um talk to
you about the um show you some evidence for these binary
systems. So we have these hot spots on
the neutron star and as the neutron star rotates then and
the axis of rotation doesn't coincide with the dipole,
magnetic dipole axis, you're going to see hot spot,
hot spot, hot spot, hot spot, and that explains
then the x-ray pulsations.
Uh you can also see in some cases x-ray eclipses.
If the neutron star as seen from the earth hides behind the
donor star which is much bigger then all the x-rays are absorbed
and so the x-rays stop completely.
You go into an x-ray eclipse and a few hours later you come
out of the x-ray eclipse again. And that's what I would like to
show you the evidence for, which came in the early
seventies. With the satellite Uhuru,
the first slide is the basic idea.
No, that was right, uh John, John,
go back to that picture, yeah, so you see here this is
of course a sketch, this is not the real thing.
Here you see a star. Not unlike the sun.
And then here you see the neutron star.
In some cases a black hole. And then the matter is being
sucked off this star because the gravitational force in this
direction is larger. Forms the accretion disk and
ends up on the neutron star. And the next slide is then the
discovery of the early -- convincing discovery of a
pulsating system. The rotation of the neutron
star. This time scale here is about
one-and-a-quarter second. And the data are here,
this is the data, unfortunately in this
publication this -- this very bold line dominates almost the
data, but the idea being very clear that the x-ray signal,
this is the strength of the x-ray signal,
this is time, one-and-a-quarter seconds,
is oscillating one-and-a-quarter second
periods. And that's the rotation of the
neutron star. And the next slide shows you of
the same object,
it's called Hercules X one. You see on a very different
time scale this is days. You see that the x-ray
eclipses, that the x-rays disappear completely when the
neutron star goes behind the donor.
And the orbital period is one point seven days.
The x-rays disappear completely.
And so this picture is well-established.
It's beyond a shadow of a doubt we do know what these objects
are. With no one is flying balloons
anymore I make all my reserv- o- observations nowadays from
satellites using European satellites, Japanese satellites
and American observatories. Lately we have the Rossi timing
explorer in orbit and also Chandra which is the biggest
thing in town. Now between nineteen
seventy-five and nineteen seventy-nine we were so
fortunate here at MIT that we had our own private x-ray
observatory. It was called Sas three.
It was an all-MIT operation. We uh maneuvered it from my
building, Center for Space Research,
Building thirty-seven, twenty-four hours a day,
three hundred sixty-five days per year.
It was at that time that Josh Grindlay and John Heise had
discovered using a Dutch satellite believe it or not --
they had discovered that some of these x-ray sources showed
sudden x-ray bursts. The x-ray intensity would rise
in about a few seconds, would become ten,
twenty, thirty times stronger, and then over a time scale of
maybe a few minutes, the x-ray intensity would peter
out again.
And we had this Sas three observatory which ideally suited
to do research in these x-ray bursts.
And we discovered within two years eight new burst sources.
And I think it's fair to say that it's largely due to our
observational work and also to the theoretical work by
Professor Joss who was at MIT and still is at MIT that we now
understand what these x-ray bursts are.
They are huge nuclear bomb explosions on the surface of a
neutron star. The matter that falls onto the
neutron star is largely hydrogen and helium.
Because that's the matter of this star.
And the density and the temperature on the surface of
the neutron star is so high that you get nuclear reactions.
And three helium four nuclei can fuse to carbon twelve.
And then energy is released. And this nuclear reaction is
very unstable, is extremely sensitive to the
temperature. If the temperature goes up the
reaction rate is higher, more energy is released.
Temperature goes up. When the temperature goes up
the reaction rate goes up and so on, more energy is released.
And the whole thing gets out of hand.
You get a thermonuclear runaway as we call it.
You see a thermonuclear flash. A gigantic bomb explosion on
the surface of the neutron star. These bomb explosions are about
eighteen orders of magnitude more powerful than the most
powerful hydrogen bombs that we can
build on earth. So this -- this layer,
this fresh accreted layer, goes up in one huge bomb
explosion and then new material is accreted and a few hours
later you will see another bomb explosion.
So you can see several of these x-rays bursts per day.
The optical counterparts of these stars, these binary
systems, uh are very faint, but you can see them from the
-- the ground with uh optical
observatories from the ground. And we had reasons to believe
at the time that simultaneously with an x-ray burst you might
actually observe an optical burst.
And I'll give you the reasons why we believed that.
You see here the neutron star and you see the accretion disk.
And let us assume that there is an x-ray burst going on now.
These red wiggles is uh x-rays from the x-ray burst.
The earth is in this direction. So these x-rays reach the earth
first. But there are x-rays which go
in this direction. They are absorbed by the disk.
And then the disk heats up. To thirty, forty thousand
degrees. And starts to emit optical
light. Some of that optical light will
go in the direction of the earth.
And so what this means now is that there is a delay between
these x-rays, these -- this optical light and
these x-rays. Because look.
This path length is longer to us than this path length.
And therefore we were hoping not only to be able to see an
optical flash, which would mean the x-ray
heating of the disk with the optical light from the disk,
but we were al- also hoping to see the delay.
If you can measure the delay, if you see a one-second delay,
it would mean that this geometry was
roughly one light-second from here to here.
And so we were very ambitious. We organized a worldwide
campaign in the summer of nineteen seventy-seven.
With Sas three we were going to observe one particular x-ray
burst source in the sky for two weeks on.
And we were asking observers from the ground,
optical, radio, infrared observers,
to keep an eye on that object and also observe that,
all the time, as long as they could.
Seventeen countries contributed.
Forty-four observatories contributed.
Participated. And during these two weeks we
saw one hundred and ten x-rays bursts from this object with Sas
three, none was -- were observed in the optical,
none were observed in the radio.
In nineteen seventy-eight we tried it again and we were
successful. This was a collaboration with
Joss Grindlay who was at the time at Harvard and my graduate
student Jeff McClintock who was at that time already Doctor
McClintock. He also was at Harvard and he
still is there. It was a smashing success.
We made it to the cover of Nature, which is a rather
prestigious uh scientific journal.
It was covered by the New York Times and by many newspapers in
the world. So we were the first to be able
to detect simultaneously an x-ray burst and an optical burst
from this binary system. Uh the data that I want to show
you are not the nineteen seventy-eight data.
But they are data from a year later because they have better
quality. We learned how to do it of
course. These are the x-ray data not
from Sas three. Because Sas three was no longer
in orbit in nineteen seventy-nine.
This was a Japanese satellite Hakucho.
And so here you see the times in seconds, and here you see the
x-ray intensity and you see here the x-ray burst as we observed
it with the Japanese observatory Hakucho.
And here you see the optical data which were taken by my
friend and colleague Holger Pedersen.
He used a European southern observatory in -- in Chile.
And he observed clearly an optical flash.
This is the intensity of the light before the x-ray burst and
then you see an incredible increase and then you see a
decay. I have plotted these so that
they both have the same height. That of course is artificial.
But it makes it easier to compare the two.
And so now comes the acid test. If now I put one on top of
another then you see very clearly that as we expected all
along that the optical signal is indeed delayed relative to the
x-ray signal. And you can now do the
measurement that we hoped we could do.
You shift one curve on top of another and if you shift it by
about two seconds then they're almost carbon copies of each
other. And so we succeeded then for
the first time to measure the geometry of the accretion disk
around these neutron stars. They have radii of very roughly
two light-seconds. Light had to travel two seconds
more, first x-rays, x-ray heating and then the
optical. Two light-seconds is about
twice the distance from the earth to the
moon. So these systems are amazingly
compact, very small indeed. Needless to say that during the
past term I haven't been able to do any research,
no science at all, eight oh two has swallowed up
everything that I had to offer and more.
And I think you should feel sorry for my graduate students.
And I think you have all the right to feel guilty as well.
Uh you were on my mind all the time and not only at MIT but
also at home, in my living room,
my kitchen, when I took showers, and even very often
appeared in my dreams. Believe me, it was hell for my
-- for my significant other, who is in the audience,
Susan, it wasn't very nice for her.
Frankly, my life will change after today.
I make myself no illusion however.
Most of you will quickly forget all four Maxwell's equations and
you will forget all about induction and about
nonconservative fields. I hope for you though that it
will not be before next week. But surely when you will see a
rainbow you won't be able to resist to check that the red is
on the outside and the blue is on the inside.
It is a disease for which there is no cure.
And if you carry your personal polarizers with you,
then you will want to verify that indeed the bows are
strongly polarized, and if you do that I'll be very
proud of you. And I hope those will be happy
memories. Thank you for
attending my lecture. [applause]