Riccardo Giacconi at MIT – 2004 MA Space Grant Consortium Public Lecture

Riccardo Giacconi at MIT – 2004 MA Space Grant Consortium Public Lecture


YOUNG: I’m Andrew Young. I’m the Director of the
Massachusetts Spacecrafts Consortium. And this is one of
our enjoyable events that occurs annually, to
invite a distinguished lecturer to come up here to Cambridge and
talk about an aspect of space research. There are many other activities
of the Massachusetts Spacecraft Consortium. I urge you to keep up with us by
consulting the website, MASGC, which is listed on the back of
the program for today’s event. All of these events
are open to the public. And you’re welcome
to participate. We are also the host to a spring
seminar course, Modern Space Science and Engineering, which
meets every Wednesday at 3:00 and those lectures are
also open to the public and the schedule is
on that same website. And incidentally, for
those students here who are registered in the
course, would you, at 4:30, congregate up where Professor
Hoffman and I are sitting now and we will go over
some course business. With that I would– as well as take attendance. OK. With that, I would
like to turn over for the introduction of
today’s distinguished speaker, the co-director,
turn this over to the Co-Director of
Massachusetts Spacecraft, Professor Jeffrey Hoffman,
who had hands on experience in space with one of Professor
Giacconi’s key contributions. Jeff. HOFFMAN: Thank you. As Larry Young says, this is one
of the more pleasurable events of the year when
we get to invite distinguished speakers
to share with us some of their experience. And today’s speaker is more
distinguished than most, if I may say. I don’t want to go
through year by year his entire life history. Hopefully all of
you, when you came in did get a copy of the
program, and there are two full pages detailing
Professor Giacconi’s contributions. I will say one or two things
by way of highlight however. First of all, the work for
which he received the 2002 Nobel Prize in physics
was really involved with the birth of a new science,
which is X-ray astronomy. As most people here know,
astronomy for centuries was limited to what we
could see with our eyes. When the Nobel prizes
started to be awarded early in the 20th century, for
almost the entire century, no Nobel Prize ever
went to an astronomer. And there are
numerous urban legends as to why this is
which I won’t go into. But that tradition
was happily broken when Martin Ryle and Tony Hewish
won the Nobel Prize for work done in radio astronomy. And now we have
another Nobel Prize for work done in
X-ray astronomy. However, Professor
Giacconi of course, was not satisfied
to remain in one narrow region of the spectrum. His work as the first director
of the Space Telescope Science Institute, which managed
the Hubble Space Telescope, both through thin
and through thick, involved visible
and ultraviolet. He went on in his work as
the director of the European Southern Observatory
to work on ground base, visual observation. And now as president
of AUY, he’s responsible for managing
the National Radio Astronomy Observatory. So he really has spanned
the entire spectral range of astronomy. And in fact, their
newest project is the Atacama Large
Microwave Array, so yet another region of the
spectrum with which Professor Giacconi is becoming acquainted
and lending his expertise. It’s a unique opportunity
to have somebody who has really started a
field and has turned it into one of the most
productive areas of astronomy. And Professor Giacconi, I’d like
to ask you to come up and share with the audience your
insights into what we’ve learned about the
background of X-ray astronomy. GIACCONI: Do I have– no. HOFFMAN: It’s working? GIACCONI: Yeah. HOFFMAN: OK. Thank you. GIACCONI: Thank you,
thank you, Jeff. It’s nice to be
back here, see a lot of familiar faces, old friends. I spent a lot of my career
here in Boston, in Cambridge. And so it’s always a homecoming. What I will try
to do today is, I will not try to give you
a historical correct– how do I get it lower. I mean I hear, I hear, HOFFMAN: They’ll
adjust it for you. GIACCONI: OK, fine. I mean, I am hearing myself
with echoes and so forth. So what I would
try to do today is to take a thing, a
problem in X-ray astronomy and which actually
happened to be discovered, the X-ray background,
happened to be discovered in the very first
rocket flights, which revealed the existence of
celestial extra solar X-ray sources, and then
tried to follow the progress in understanding
what this background was, until fairly recently, I
mean this year, last year. And that allows me
to go through also the technological development,
which was involved and the progress of the field
with respect to sensitivity, angular resolution and so forth. So by discussing
the X-ray background we touch on a lot of topics. So the first thing
is the 1960, ’62, when we started looking at the
possibility of doing science in space. And in particular
it had become quite clear by then through
the work for us those old Herbert
Friedman and his group at NRL, who had used captured
German V2 rockets to put on instrumentation and study
the X-ray emission from the sun, that there was great
advantage in working in space and the ability
to look in regions of the spectrum in which a
light would not reach the Earth. So there was a general
interest in looking at ultraviolet radiation
gamma ray radiation and X-ray radiation. I didn’t know anything,
happily, about all of this until I met
with Bruno Rossi and he suggested– he was
chairman at the time of Space Science Board, discussions that
occurred there which included many participants, but he
transmitted this information to me. And for some reason
or other, that seemed to be a very
nice thing for me to do. However my background was in
elementary particle physics. And I detested loss
statistics experiment. My thesis was– I was up at the Plateau
Rozier, a wonderful place to ski, 3,500 meters,
and it took me two years to collect 80 protons,
in which I was studying interactional protons in lead. And I barely was able to
confirm a fireball model. I thought, forget it. I mean, I don’t want to get
into another field, which is like that. And that’s what
you would conclude as soon as you start studying
the emission from the sun, and you tried to put
it at the nearest star. That is, we were talking
about sensitivities, which where five
orders of magnitude or six orders of magnitude
below that kind of experiment and which had been
done up to then. So I actually considered
for a while giving up, and then two things happened. One is, we went through,
and by we now, I mean George Clark, Bruno Rossi– George Clark, [INAUDIBLE],,
Bruno Rossi and myself, a brief review, as
physicists will do, right? What is the problem? What are the expected fluxes? How do I do a
crucial experiment? And what we found out was,
that what people have been trying to do was not adequate. Because the brightest sources we
could conceive were in the sky would give us something
like ten to the minus seven, the flux of the sun, 0.1
photon per square centimeter per second. And so the question
was, could you start building
detectors that would be sensitive to that
kind of radiation? Turns out the answer was yes. But also at the same
time, by chance, I mean, since I had been thinking
about focusing my protons there on the cloud chamber, by
chance by reading Pfleuger, I came up with the idea that– well, I learned that
it was possible to have a total external reflection
from surfaces of X-rays. And the immediate thing, as soon
as you know that, and then you have had a mother
like I did, who is an expert in
projective geometry, you come up with a paraboloid
as a concentrator for the X-ray flux. And that was actually
done before we started any real detailed instrument
design, because it was important psychologically. Because it told
us that even if we had to go six seven
orders of magnitude, ultimately we could do it. And so, it wasn’t useless to
start with simple experiments and then a long technological
development for the X-ray telescope. And both started at essentially
at the same time, 1960. Now, this was the payload,
which was first successful, the payload this is the
inside the rocket shroud. So imagine a shroud around
it with three windows. OK, one, each, one, two, and so
on and one that you don’t see. There were counters. And to make it
simple in my mind, I think of this triple-A
changes between this instrument and what had gone before. The change number
one is the area had increased by a
factor of 10 or 20 with respect to what
had been done before. Then two, second
day, anticoincidence. It turns out that the
background of x-rays is about one particle per
square centimeter per second. And if you’re trying
to find sources which are 0.1 photon per square
centimeter per second, then obviously, if you can
eliminate the background due to particles,
that would help you by a tremendous amount. And the third way, which
had not quite been realized was the following. If I tried to scan the
sky with three arc– three degrees field of view,
then my probability if I launch a rocket, my probability of
finding a source anywhere in the sky with this small
field of view is about 1%. So in principle, I should fly
100 rockets to find one source. I mean that doesn’t– Furthermore, even if I
find it, so to speak, meaning if I cross it,
then because I only have a small field of view
and the rocket is rotating, I would not collect photons. And so in effect, you could show
that those instruments would never have been able to
detect Sco X-1, the brightest source in the sky. So when we finally
flew this instrument, there were two
characteristics which then introduced the subject that
I want to discuss in the rest. One was that we were able to
see the first star, so to speak, or stellar system
that emitted X-rays. And two, that it
revealed a background, a uniform background,
which we could decide was a background due to
radiation and not a particle. Because we could actually
measure the spectrum by having two counters,
whether this actually shows the anticoincidence
without the counter, no this has the
counter in it, which had different windows, one
thicker than the other. And so you could,
roughly speaking, measure the spectrum
of the X-ray radiation that went through. So this is all to introduce
this picture, which is, what we adopted was the
anticoincidence, as I said, the larger area and a
very wide field of view. This field of view
is about 120 degrees. So essentially there
was no baffling. There was no attempt
to reduce it. And what we found was
this very strong source, which I’ll move along and not
spend too much time on that. That’s from my [? fed. ?]
And then a uniform Bell curve all over the sky. And we concluded from– we were able to
conclude from this, that this was a Bell curve
which very probably had an extra galactic origin. OK, now, I went through
all of the arguments. There was no evidence
of increase in– as you went through
the [? eclipic ?] or when you went through
the Milky Way and so forth. That is the background which
remained to be explained for– thank you. Hey, I didn’t know. All right. OK. That was the background that
remained to be explained and took about 40
years to explain. Immediately upon finding this,
a contribution to cosmology was given, because– which was interesting,
and negative, so to speak. This background, big as it
was, and it was very big, because there is no trace over
the cosmic ray component here– was absolutely much
smaller what one would have computed
in the hot universe model of the continuous
creation theory. So Hoyle immediately
realized this. And one of the things
which is fantastic here is that, just because he
hadn’t thought about X-rays, he hadn’t predicted it. Otherwise he would have
predicted that the sky was going to be awash with X-rays. But it’s interesting that just
a negative statement essentially killed that particular theory. The other question,
remember historically, that this is the time of finding
the three degree background radiation. Then there was the measurement
of the gamma ray background from the galactic center. And somehow, the idea that
the background carried information of cosmological
significance was around, let’s say. So in effect in 1963 when we
had these data, we thought– we tried to think about what
the background was all about. Burbage was the first to suggest
that this background could be due, not to a
diffused emission, but the combined radiation
from the distant galaxies, in which since we didn’t
have any angular resolution, then the flux would all
be confused together. And so it dictated the
design of the X-ray telescope we ultimately wanted. I’m trying to
interweave the fact that finding
interpretation or finding and design of the
next experiment are very much a function of what
you think that you’re doing. So what I thought I was doing
in ’63, I proposed at one point to meet that X-ray telescope
which had the virtue that it could image the X-ray
background with– if you– was uniformly
distributed with an angular resolution of 1
arcminute so that we would decide whether it was done
with individual sources or not. OK, so this was the first step. Now we didn’t do
much more than this. There wasn’t any major sorry– any major change
with this until 1970, we flew a simple X-ray
satellite, which if you wish, it’s just a bigger counter to– one on each side
of the spacecraft. The field of view is
the important thing. Here is one half of the square
degree by five square degree. Then a five by five field. The satellite was
launched by Kenya. And since it was
launched on Freedom Day, Uhuru was called Uhuru. Also, because there
was so much better than the NASA SAS
or ABCD or whatever. Uhuru, that’s what
we really felt. So what is the purpose
of the satellite is to do a scan
of the entire sky. As the plane of the– as the days go by, the
plane of the sky changes. So you have intersection. You could find the
position of sources. And this was done over
two years or four years. Now here is one scan. And I mean this is the Sco X-1,
the first star that we had seen and that prevails
on the whole sky. This is in galactic coordinate. Here you see now
that scale expanded and you see how many sources. We see it’s over– what I want you
to do is to ignore all the sources for a moment. And here’s the
background, still there. OK? And is in fact, the
largest background the largest contribution
of the background to the detection of sources. Now, the results, I want to
introduce two concepts, which then allow me to go on. This is the result of the
catalog from the all sky search. And it’s a galactic coordinate. This is the Milky Way. OK? And most of the sources
that you see here, which are represented by
dots of increasing size depending on their intensity. This is Sco X-1, the most– the brightest one. Our galactic. And most of the other ones, or
almost all, are extragalactic. They are diffused around. Two fundamental findings of
this satellite were, one, the discovery of binary X-ray
system, which involved the– well, sorry. I inverted the order. This is– the first
one was the discovery of hot intergalactic gas. What is this gas? Here is a photograph. This is taken much
later, not in Uhuru. This is a ROSAT
picture, the X-ray. And what underneath is a,
I think, it’s a [INAUDIBLE] picture or something. Here you see galaxies. There are thousands
in this cluster. And here you see
the distribution of X-rays, which
are emitted, not concentrated on each galaxy,
but in the space pervading the galaxy. Now that sounds interesting. However, the most salient point
is that more, 10 times more, mass is in this gas at
very high temperature, which cannot be seen in visible
light than is contained in all the galaxies and stars
represented in this picture. OK, so this was finding
a new state of matter in which very high temperature
gas was emitting X-rays by thermal bremsstrahlung. And as we go along will see
why I wanted to mention this. The next thing– and this
I won’t go through this. The next thing was the
discovery of this binary system containing a neutron
star or a black hole. And the reason why I’m bringing
this up, that this was not in any particular
[INAUDIBLE] was that we discovered binary
system in a collapsed star. In a normal star, gas would
fall from a normal star up to the collapsed star. And it, the collapsed star
was emitting pulsations. Now the question that arose
was are these pulsations emitted at the expense
of the rotational energy of the compact star as
is the case for pulsars. And the answer is no. In fact, this
X-ray source is not slowing down its
rate of rotation, it’s acquiring energy. And it is acquiring energy
because of the inflow of gas onto its surface. And this mechanism,
which is shown here, if you wish in a
diagrammatical way, this is gravitational
equipotentials. Gas from the normal star falls
in the deep potential well of a neutron star for
instance, and acquires more energy per proton that you
can obtain by nuclear fusion. So this is a very
powerful energy mechanism. And it’s now been extended. In Uhuru, as a result
of Uhuru, stop. These are not– this was one of
the sources, which is periodic, which I mention now. Other sources show nothing. And then there are
very erratic sources. And by bringing it
to the attention of the astronomical community
this unusual behavior, then very good
positions were obtained, some from the MIT
group some from Uhuru itself, MIT group using rockets. And then an optical
identification was made by Webster and Murdin. He determined that the mass
was ten solar masses, six solar masses. There are very rapid variations
which involve the whole star and they are very
rapid, because these are at the level of milliseconds. Therefore, the object must be
very small and is very massive. And it had been shown
by Rhoades and Ruffini that no neutron star could
have a mass greater than three solar masses And therefore this
was by definition a black hole. And what it means
is we simply don’t know any physical law
that prevents this star from indefinitely collapsing. Now, the point that I
am trying to make here is that, without going into
all of this, that there was a new source of energy due
to gravitational infall, which is very efficient and a model
for what is happening naturally in the nucleus of
active galaxies and quasars which explains why
the tremendous amount of energy which is emitted by this object. These two facts,
that is the idea that very energetic
emission in X-rays comes from the nuclei of
active galactic nuclei and is powered by
gravitational infall, gives you one
potential explanation of the X-ray background. The possibility that
hot gas diffused throughout the
universe gives you another potential
interpretation. And the difference
between the two is that one will be a
diffused continuous emission and the other one would be
a point source emission. OK, so there were some– now again coming back
to Uhuru, there was– here is the background,
as you see, is dominating. I mean, here is the cosmic rays. And these are very strong. These are very
different than what you see in the visible
light sky, right? It’s not like extra galactic
component or the visible light sky dominates the sky. I mean that would be that it’s
light all the time so to speak. But it isn’t. But in X-rays it is so. Then there was a finding, which
I thought was very suggestive. If you take the sources which
are at low galactic latitude and you plot how
many of them as you go weaker and weaker,
fainter and fainter, you find at a certain
point, you stop increasing the number because
you are running out of galaxy. If, on the other
hand, you do this for extra galactic
sources, their number keeps increasing as you go to
fainter and fainter magnitude as if the cosmic
expansion were balanced by evolutionary increases. Now, this actually
was done by Matilsky, who was, I mean a real ’60s
kid, I mean, motorcycle type. But that was a very
profound finding. OK. So here one could be
led to the conclusion that, OK, so if the extra
galactic sources keep going up and up and
up, then we have the exponential
background, right? And in fact, this was suggested
naturally by [? Bolger ?] and [? Sutton. ?] If all sources
extra galactic were emitting like a quasar, then we would
explain the background. And here again,
I’ll jump this away. Now, the other
question was, well, what was the spectrum of
emission of the background? And that was– it
looked somewhat simple. This is a composition by
Gorenstein, I believe, of, you know, the emission
from different sources. And it was clear that
you could explain it either with the thermal
bremsstrahlung spectrum, which is shown here or by
two parallel spectra. And this was about what we knew. Now, the next piece of
important information or that seemed extremely
important at the time, was this one. That is, this is now 1978. Uhuru was 1970. This is 1978. Here, one flies. It’s a very large counter. And it finds the spectrum
of the background and here are different fits. And as you see,
it fits perfectly the thermal bremsstrahlung
spectrum at 40 kilowatts. And this became ingrained
in everybody’s mind, and therefore there was
the conclusion, well, this had to be thermal
bremsstrahlung from a hot gas filling the whole universe. Now, never mind
that this actually require more gas than the
total mass of the universe. I mean this was– and that if it existed, you
would have to explain how, after cooling it at
three degrees, which is what we see in the
microwave background, then you were able to reheat
that 40 kilowatts, which were a little problems. But never mind. So this was OK. And I think that
beyond that I stopped going with the known
imaging experiments, not using the telescope. Because there was no more
progress that was possible. And here, I just
wanted to show you why X-ray telescopes can be
important for X-ray astronomy. Here is the absorption
in the atmosphere. So if you go up, you
see more, because this is absorption at I
think one half, here. 100, 110, 1/2 and so forth. Here is a spectrum of
potential X-ray source. And I think it was
Conconi who pointed out that in normal high energy
sources, the number of photo decreases as the energy goes up. It’s an empirical law if
you wish, but it worked. And here is the
limit in which you can reflect with total external
reflection X-ray efficiently at about a kilovolt
or up to 10 kilovolts. And here is the window of
detectors that we can build. I mean, it’s difficult
to have detectors which go to softer
and softer wavelength. So in this
particular, the reason why X-ray telescopes
are useful is because they work here and
maximize in a region n which you have the maximum statistical
number of photons coming in from a source, OK? Now why wasn’t this clear? Because it took a long technical
development to come up with telescopes. And OK, this was in 1961. I mean, a statement of what
a telescope would look like. You need a double
reflector to make an image. This had been done by
Wolter in 1952 or something. Because he wanted to
build microscopes. He ultimately never
could build them, because they were so small
he couldn’t actually do this. But for telescope where you are
talking about large surfaces, then you could polish
these inside surfaces to very high accuracy. And the trick was that you have
to have extremely good surface polish, much better
than what you normally need for optical telescope
down to three photons from all the surface roughness. This was hard. It took many years,
little telescopes, OK, replica telescopes, [INAUDIBLE]. This was the first
serious telescope I think for solar
research, and which already had some of
the features that would be necessary for
stellar research, where you need more area. So you make many surfaces
one inside the other. This particular task flew
on Skylab, the mini space station, 1973. It took a picture of the sun. And I use this to show you,
yes, the technical progress, but also the fact
that it’s amazing, that just this picture tells
you the very important role of magnetic fields in the
containment of the hot gas and in the production of flares. So that revolutionized solar
X-ray astronomy, and actually the study of the corona. This was Vaiana
and Van Speybroeck. Now here’s the first
application or attempt to use an X-ray telescope
for stellar astronomy. Surfaces had to be much
better polished, more area. Here was the interest
of doing many things. Four groups were involved. Goddard, MIT, America Science
of Engineering, Harvard, and Columbia. And spectrometers were put here. And you could turn,
these lazy susan, and had a solid
state spectrometer, imaging proportional counters,
full [INAUDIBLE] increase, et cetera. I will just focus, because
you know, as a mono maniac, I was still trying to find
the source of the background. So what I did was to
choose a region of the sky where no X-ray source was known. And we pointed to this for
10 days, a million seconds. And, this is mostly background. There are three sources
here, one, two, three. And to give you an idea of
what we are dealing with, the fainter sources
that we were seeing was something like
10, 11 counts. So it was one count, one photon
a day that we would collect. That is also the
limit at which we are going with Chandra,
exactly one photon per day, but now to much fainter sources. But X-ray astronomy is not
easy is what I’m trying to say. Here was the same field. Unfortunately, I
should reverse it. But those three objects
are identified now with stellar looking object,
two of those are quasars. One of them is a
star, a normal star. So one can draw the
conclusion, and here I’m going to the very
early Uhuru data. And then you see this is
where the galactic stuff, extra galactic stuff, Cox1. And the deep survey goes
down by two or three orders of magnitude in sensitivity. And yet this trend
of the increase in sources as you go fainter
and fainter continues. Now, the problem
with all of this is that these
measurements are made in a specific region
of the spectrum, one to three kilovolts. And so, there
succeeded long debate, because of what was the spectra
content of these radiation and whether this was just a
fluke, where we were measuring a background that really
wasn’t the background that one would
like to understand. Now that was very difficult to
deal with because at the time, we just couldn’t go to higher
energy, despite the murals were not good
enough and so forth. Now however, there were
some interesting points. And this was one
of the attempts we made to put some rationality
in this research. What had been found was
that spectra of the– so the claim for the proponents
of a discrete source origin for the background,
basically we’re saying that, active galactic nuclei, C4
galaxy quasars, C4.2 et cetera, would make up the background. For the few sources for which
the spectra had been measured, spectra indexes,
which were very steep, that is much more low energy
than high energy stuff were found. And it was very difficult
to explain therefore, how the background that
looked much harder than what each individual sources
could be generated without assuming evolution
or changing the spectra. So what do we try to show was a
kind of reasoning of absurdum, which I think had to come from
a Jesuit education or something. I don’t know. Which was like this, if you
assume that 50% of the sources are due– or 70%, depending
on what you like to do are due to these
individual sources that we have measured, whose
spectrum we have measured, then the spectrum that is
left has to be very hard. Because we are taking a
spectrum, which is soft, spectrum that is
left is very hard. Now what is the– could the spectrum be a thermal
bremsstrahlung spectrum? The answer is no. There is no way
you can reproduce the slope of the spectrum
with thermal bremsstrahlung from a optically thin gas. So in order to explain
by diffuse emission, what are the residual
background is due to, you have to imagine
that you have gas, yes. But that you have to have
absorption in the gas, such as the low
energy suppressed and you allow high
energy to come through. Now in order to
do that, you need balls of gas of a
certain optical depth. And once you– and you
can do it one of two ways. Either you have
enormous balls of gas with very low density or small
balls of gas with high density. So what we said, well, how
do we decide between these? Well, we decided on two grounds. One, that we wanted to
have enough optical depth to get the absorption
effects we wanted. And second, that the total
number, the total mass containing all of these balls
of gas, which had to be millions because we were
seeing the background to be very
homogeneous, could not exceed the mass of
the universe, which seemed a reasonable constraint. Well if you put that constraint,
that immediately collapses, says, that all of these balls
of gas that you’re talking about that should fill
the universe uniformly have to collapse down
to 100 kiloparsecs. So that means that would
be the size of our galaxy. And therefore, it
shows ad absurdum that these postulated sources
of the continuous background actually have to
be point sources. Now, this was not– this was a paper, I think one
of the better papers, I ever. But actually it wasn’t
understood at all. My referee said,
well, why don’t you talk about a physical
characteristic of these objects you’re describing? And I tried to explain, but I– [LAUGHS] Anyway, interesting. So in any case, I claimed
that by 1987, the problem with the background
was solved, that is we knew it had to be due
to discrete sources. However, we had to prove it. And we had to show that
the spectrum, et cetera. So now– and I won’t– there was a flight of a
German satellite, York and [INAUDIBLE] built ROSAT. We went with deep surveys. I was fortunate
enough to be involved. I mean, this was the group. And we found 70% to 80%
now the background– again, remember at low energy. The spectrum of sources, et
cetera, et cetera et cetera. And still– and the composition,
what were the sources, each one identified. 39 at GN, one galaxy, three
galaxy group, three stars, four unidentified objects. Still there was this
problem on the spectrum. So finally, it only got
solved with the advent of AXAF which only happened to be
launched in the year 2000. Now, AXAF has the
angular resolution. This is not allowable to
discuss, pulsar emissions, but it has the
angular resolution to give you this direct picture. This is no artist conception. This is the picture of the Crab
Nebulae, showing the shockwaves and so forth. There is a movie of it, but
I am technically challenged in showing it to you. And I just wanted
you to understand the power of the instrument. This is as good a picture
as you can take with Hubble, essentially of the Crab
Nebulae with the invisible eye. Naturally I didn’t
take that picture. My time I devoted for
looking at the region of the sky with no
source was known before. And I went for and asked
for a million seconds. I got a million seconds. And here now is the
field of view of the CCD, which looked at this region. And we are finding
a source density, which is really
enormous with respect to what we were finding before. Because there is something
like 300 sources containing a tenth of a square degree. So this is not as
high as the source certainly we see in Hubble. Because we are there, we
are looking at galaxy. But basically 90% of the sources
are black holes emitting energy by the mechanism
which I was explaining to you before in full
from an accretion disk onto a supermassive black hole. And the rest are interesting
in their own right. This is an attempt
to show in a collar– I will go through this. Here is a summary of where
we are now in sensitivity. So this was my
chance to show you that we went from 10 to the
minus seven 10 to the minus 17 or several times
10 to the minus 17. And this was in 40
years, which is not bad. It’s equivalent to
going from the naked eye to [INAUDIBLE] or
VLT. So it’s OK. The field is more. Now here is the contribution
of those sources. Now you can sum
all those sources. OK, sum all those sources. And this is big arrows,
and this is the [INAUDIBLE] of the background. So you see that
one of the problem saying, how much
of the background is due to individual sources? We don’t know, really, we don’t
have any good measurements of the background. But, OK, there could be some
diffuse background still there. And that would be
very interesting. If one could actually
prove that it exists. Now the next thing
which was interesting is, remember we are
having very few photos. So when we talk about
spectra, we have to sum. So this is the summed
spectrum of all of the sources with something
of the instrument’s signature, and then a fit to the
background spectrum. And as you see it essentially
reproduces the backer of spectrum completely. Now this is a conundrum, right? Because when we
measure a few sources they seem to be different. Now we’re measuring
them all and they fit. So something is happening. And here is the
different increases by the number of sources. And we can go now to
10 kilovolt, which is kind of nice so that we don’t
have this problem that we don’t see the high energy sources. And you see here that
at the low energy, the number count of sources
seems to be turning over here and here. And at the high energy,
five to 10 kilovolt, it seems to keep going
and is going faster than in the other spectra region. Here is another attempt to
show, is an attempt, I mean, to show why what
is happening here. But, each one is a source. We don’t have enough to
measure the spectrum precisely. So we do take what we call
a hardness ratio, hard versus soft ratio. And what you are
seeing is that you go fainter and fainter here,
the hardness ratio increases. So what it is, is that
the sources are changing their spectrum or you’re
picking up different sources than what you were picking
up when you were looking at– this we can– when you were
looking at bright ones. And here, for instance, the
same trend, the spectral slope, which is given by these
gamma is going down, as you go to lower
and lower fluxes. So the spectrum is
becoming harder. And now the next step, OK? What are these sources? So this is the overimposed– Chandra, as you can
see, contours in x-rays. On the Hubble,
SES, large camera, deep survey of the same field. And what you see is two points. One is that it’s
really important to have that very
high angle resolution. Look at this, for instance. I mean, at the very high angle
resolution of both Chandra and of Hubble to
identify sources. Now once you have
identified the sources then you can go to
spectral measurements. And there, it’s important to
have even larger telescopes than Hubble. So much of what I’m
going to discuss now has been done with Hubble,
Chandra, and the VLT, which is the European southern
observatory four emitters telescope. It’s the largest aggregation
of telescopes in the world. So what do we find? OK, the first thing
we find is that– OK, this was simply said, the
hardness ratio seems to go– has a dependence on the
brightness, increasing brightness of the source. If you measure redshift you
can get increasing brightness. These objects, which
are stars are quasars type two, which one
had been known before. So they are objects in
which presumably you’re looking at them, face in, I mean
without the absorbing effect of the of the gas
or their own galaxy, increasing absorption
of the galaxy. So these have softer
sources because you’re seeing them like this. These are the harder
sources because you see them through the gas of the galaxy. And they tend to be somewhat
of lower emissivity. Increasing emissivity. Here our sources which are
very, very soft, minus one, and they’re galaxies. OK so what we are seeing here
is C41, C42 active galactic nucleus but they are all
active galactic nuclear. And here we are seeing
possibly some clusters. And here we are seeing galaxies. OK, let’s go on one more minute. The ratio between
visible light, here, in magnitudes, R
magnitudes and X-rays, is more or less
what had been found before, these lines, one,
one, 0.1, were found even with ROSAT. Here are the galaxies again. Here are those objects that I
was telling you that are soft. And here are the hard ones. And here are objects which
are seen in X-rays, even very bright X-rays but
are seen in visible light down to 28th magnitude. And I can assure you
that observing objects at 28 magnitude
becomes challenging, not only for ground based
telescope, but for Hubble. So we have been able to
basically– the reason why you’re seeing the sources
identified up to here is because it really is an eight
meter telescope or ten meter telescope to measure
the spectrum. And you can try morphological
identification, et cetera, et cetera, et cetera. But basically what
I’m trying to say is that the X-rays
have allowed us to look at objects, whose
nature we do not yet know. We need to a bigger– a visible light telescope
or possibly ALMA in the millimeter wave
to actually identify these objects. The next thing was,
OK, so, what I’m trying to say in the closing
remarks of this talk, over long, I’m sure
is the following. That the problem of the
origin of this background is gone away. Obviously, it was a non-problem. It was a fiction of
the theoreticians. I mean it– we just
needed an angle resolution and powerful
instruments to show. But what has happened
here is that now– actually I want to
go back here once. Now what do we have
engaged in, no, no, no. Here we go. We have broken through this fog. We are seeing the
individual objects. And now we study them. So the first thing we did for– I mean, the reason
why I went back, I wanted to pick up this
group, which is galaxies. And the first big group
we were able to do is to measure the
rate of star formation in X-rays in galaxies at
fairly large distances from us. And it’s an interesting problem. It was compared to the known
or measured [INAUDIBLE] rates in other wavelength. It was 90 degrees. But this is the stuff of measure
rates or binaries it turns out. Here, another
aspect of the result is that when you
distribute these sources, in redshift bins– you see these clumps, which
have been seen in visible light. These are clumps
which are observed in X-rays, which means these
are active galactic nuclei. They are not clumps
of galaxy, they’re clumps of active
galaxy or black holes or call it super black holes. And finally, the study of
clusters and the emission of clusters, the largest
in assembly of matter, that discrete matter
in the universe, this is a picture taken in one
of these very deep fields, of a very faint cluster. And this was done not
only by us, but also by the people, Bower, and
[INAUDIBLE] and so forth, who had been doing the same. But this is our work
and this is their work. But basically we have
extended the region in which we see clusters by
several orders of magnitude. Again, a little dig to
the theoreticians, which have been a pest in my life. We had a wonderful experiment
which we had proposed, which was turned down not
only by NASA but by ESA and by Italian Space Agency,
on the ground then no cluster existed at this early
epoch in the universe. Now I could kill them. But OK, that’s their job. Now finally, something that we
have found out just a couple of months ago was, OK, now
we have all of these sources. Do they vary? If they are accretion
disk providing matter on a supermassive
black hole, then as this amount of
matter changes in time, then you should see changes
in the emission of the X-rays. And what we find is,
we find variability. Now again, this was done
in a very stupid way. That is, we plotted out
the actual intensity that we would measure in the
different observation types. And we found out that there
was a difference from one observation time to the other. Now why– two other groups
that tried to do that and didn’t succeed
because they were applying overall algorithms
that would measure this and that and so forth, very
sophisticated in fact, software, but not as simple
minded and revealing as this very simple
kind of approach. So I’m saying
sometimes that you’re too smart for your own good. Here, we did find sources
which are valuable. And the most– here
is the plot I mean, I’m showing you very little. But the plot has to do
with hardness ratio. So it says that the ones
which are most valuable are the one which had the lowest
hardness ratio, which has to do with type one and type two. That is again, you
will not see absorption in the line of sight, because
in the Seyfert type stew, you have to have
scattering, and therefore you have dilation
of [INAUDIBLE].. And this is the ratio
of what happens. And this is very strong. And again, the next
slide shows this done now as a function of redshift
variable and unvariable. And it shows that
the most valuable one are they want in which the
increasing absorption is lowest, which means that the
hardness ratio is lowest. So this is just a
repetition of that set. So to conclude, I
think that I can say, well, we have solved the problem
of the background, I think. There may be an
experiment to find out, is there really a new,
some diffuser component. Second, we find many of these
type of sources, clusters, GNs, galaxy, and then 10% are
identified, which I am– we are convinced that
it may be something new that we haven’t understood yet. But we will need more powerful
optical and radio telescopes and so forth to see it. I hope to see it
within my lifetime. I mean, you know, I’ve
been waiting 40 years. It seems real. OK, very high source density,
the possibility to study. OK, one third of the
sources are these sheets, just like it happens on
the galaxy distribution. And basically what the
backgrounders don’t do is this, is the deep study of
evolution of galaxies, of GNs, and large structure. But I’ll stop here. Thank you. [APPLAUSE] YOUNG: Not bad. HOFFMAN: We’ll have a
little time for questions. First of all, I want to
mention kind of an underlying theme in the series of lectures
we give in this Modern Space Science And Engineering
seminar series, is that there are fascinating
scientific questions which can only be answered with
recourse to space observations. And in doing this,
we have to solve a lot of very sophisticated
and fascinating engineering challenges. And Riccardo, I think your talk
has illustrated this perfectly. Fascinating
scientific questions, and you’ve also
taken us on a tour of the history of the
development of X-ray technology. If I could ask the
first question, what would you like to see
as the next technological development, maybe to
take us the next step? GIACCONI: Well, I
would like to, we need a little another
factor of maybe 100 to make a real difference. So there have been– HOFFMAN: You’re always
thinking big, I love it. GIACCONI: I mean,
it’s you know it’s three magnitudes in the lesson. Also I think that it’s important
that we maintain this– I mean the thing that
makes me happiest is in this last
experiment that I was able to use Chandra,
which I proposed together with Tannenbaum in 1976. I mean, and the Hubble that
I was involved in directing. And then VLT, which I
was involved in building. So to me, they are all
part of the arsenal. It’s very important that we
keep this capability in balance. I personally feel that the
millimeter wave interferometry array, which will come
in being in 2007 or 2008, the first manifestation is going
to be a very powerful tool here to carry out this research. I think it’s important
that we go on in X-rays because every time
we turn around and we get new discoveries. I mean we absolutely
did not know that there was this kind of
spectral evolution in GNs, which still is not clear. I mean what– no model we
had on the X-ray background actually explained the
data that we have seen. And so we need more area. Unfortunately that has to
be coupled to a high angular resolution. Because I mean you already saw– first there is the
Chandra spacecraft is more sensitive than the XMM
spacecraft or the Europeans, even though XMM is
larger because XMM gets confusion limited. So if you’re looking
to a nearby source where you don’t care
about confusion limit, then you can get
a lot of photos. You measure spectrum more. But if you’re really looking
to very distant sources, then you really need
angular resolution to know what you’re doing. So angular resolution, spectra
resolution, and area, right? That’s what we need. YOUNG: Some questions
from the audience? And I’ll mention that we’ll
have a short question period and then we do have
some refreshments and we invite you all to stay
and enjoy the refreshments, talk informally with Professor
Giacconi and with one another. And I’ll remind
the students that were involved in
the seminar at 4:30, when Professor Giacconi
will have to leave us, please come up to the front and
we’ll continue with our work. So, some questions. Yeah. AUDIENCE: Riccardo, if you’re
trying to look at a background, of course, it’s like a radio
problem in the background. You can generate it
in your equipment. You can generate it in a
photo array from cosmic rays. And both of those
problems were overcome. Were they relatively
straightforward? Did the require clever
engineering or what? GIACCONI: Well,
the cosmic ray is– we really don’t have very
effective anticoincidence against cosmic rays right now. But the cells of the
CCDs, are so small, they are 20 by 20 microns, right? That the rate of cosmic
rays is very low. And also, the DDX, which
you see is different. I mean the position
of energy comes in a slightly different way. So you can try poor
shape discrimination, which is being used,
and also seeing if you have more than one
pixel involved and so on. But it’s rough I mean, we
still have signal limited, which is an important point. So that is, we go
100 times and we have the adequate resolution,
we are going to go to 100 times in sensitivity. That’s a big advantage for
a telescope with respect to simply counter. Because a counter, you just
increase the sensitivity of the square root of the area. But to have the
sensitivity of Chandra would require a mile
by a mile Uhuru. So and I don’t think
you could have done it. So that was a problem. The other one you
mentioned was– AUDIENCE: Generated insight. GIACCONI: That again, we used
pulse shape discrimination on Uhuru. There were two counters side by
side so you, OK, I mean, yes. But what not– it wasn’t
a physics problem. I mean, it’s not like, you know
tried to invent an accelerator. No. It was OK but, like. Yes. AUDIENCE: Hubble
Space Telescope is not going to be serviced
in the future. That’s what is being
mentioned in the news. How’s that going to
affect researching incident background and, for
that matter, space research? GIACCONI: But if
I had my rather, I would, and I think I
express the view of much of the community,
that it would be nice that if we could be assured of
a continued operational space telescope. At the moment, we have
absolutely nothing with that kind of refinement. For instance, to
look at morphology pf some of these objects,
which we are doing now. Or to make the
statement that there is no object
corresponding to this one, you have to add extreme
procedural position and sensitivity. OK? On the other hand, you know that
the well-known discussion, it is a general problem with NASA
that they start the discipline, but they don’t support
it in a steady state. They find it very difficult
to support a steady state operation of anything. So for instance
this problem will occur with Chandra
and X-ray astronomy unless we do something about it. As soon as Chandra– Chandra case cannot be
serviced because it’s too far, elliptical orbit. So as soon as that dies, then my
estimate is it takes 20 years. I mean, ten to sell,
and ten to build, right? It takes 20 years. You know, my– so,
it’s going to be tough. And I think that the only
thing that sustains us really, is that altogether, astronomy
is at a tremendous progress, right? And if you become frequency
agile, so to speak, you know, to me that’s happened
I’ve gone from high energy to low energy as I
became older and older. So now I’m a radio. I mean I can’t– but anyway, the
problem is that what should be driving you is the
problem that you wish to solve. And you can solve it
in a variety of ways. And you have to be
reasonably agile in the sense that if you wish to use
observational materials, you’ve got to use the
observation of a material that either you can produce
by building an instrument that is within your means. Or you get it from the
virtual National Observatory, from the data bank, or Hubble. I mean half the
research on Hubble right now is done on
archival research, not direct observation. And this is unfortunately,
a prevalent trend. Unfortunately,
because it creates a generation of
astronomers who have never had to build anything. They just have to analyze data. And the question is, who’s
going to build the next? and? It’s a real issue about training
and passing on a culture. But that’s as it is. So I don’t know. I answer you with too long. I mean, [INAUDIBLE]. Yes. No? OK. Are we done? Refreshments? YOUNG: Well, just before
we break for refreshments, we want to thank you once
again for taking the time to come and talk to us. You do have a certificate
of appreciation. Maybe, maybe not quite
as significant hanging on your wall as
your Nobel Prize. GIACCONI: I don’t hang
anything on the wall. YOUNG: But it’s heartfelt.
We appreciate it. GIACCONI: Thank you.

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