NASA is about to launch a new spacecraft to look at the universe in X-ray light. The Imaging X-Ray Polarimetry Explorer, IXPE, will look at extreme objects such as black holes, neutron stars, and supernovae, asking fundamental questions about how high-energy light gets produced. The mission’s principal investigator, Martin Weisskopf, based at NASA’s Marshall Space Flight Center, has been studying these objects for more than 40 years with other telescopes including the Chandra X-Ray Observatory. He discusses some of the fascinating objects Chandra has looked at, and what IXPE may soon reveal about them.

Jim Green:To reveal important secrets of the universe, we use light that humans cannot see. But our spacecraft can.

Jim Green:Let’s talk to an astrophysicist who has X-ray vision.

Martin Weisskopf:X-ray astronomers’ main interests — they’re mostly interested in supermassive black holes at the centers of galaxies and how the universe evolves.

Jim Green:Hi, I’m Jim Green. And this is Gravity Assist. We’re going to explore the inside workings of NASA in making these fabulous missions happen.

Jim Green: I’m here with Dr. Martin Weisskopf. And he is the project scientist for NASA’s Chandra X-Ray Observatory and the chief scientist for X-ray astronomy in the Space Sciences Laboratory at NASA’s Marshall Space Flight Center in Huntsville, Alabama. I have known Martin since I started working at Marshall Space Flight Center in 1980. So it’s a real treat for me to have Martin on gravity assist. Welcome.

Martin Weisskopf:Thank you, Jim. It’s nice to see you again. I got there in ’77, three years before you.

Jim Green: We’ve been long term friends and long term NASA employees, and you’ve got so much experience. But your field of interest is really studying the universe with X-rays. How did you get interested in doing that?

Martin Weisskopf:I went to Columbia University as a postdoc, because I wanted to switch fields. So I did my PhD in atomic physics. And I was going to switch fields every five years, and they were doing X-ray astronomy at the beginning. And so it was very exciting. Sounding rockets!

Jim Green:Wow, sounding rockets. Yeah. And we still use sounding rockets for many, many purposes. Where do we find X-rays in our universe?

Martin Weisskopf:The amazing thing is, and very surprising over the years, is that you find them everywhere. X-rays are light at extremely high energies. And they’re found in regions where there’s extremes of matter at high temperatures, millions of degrees, super strong magnetic fields, things like that.

Jim Green:You were the project scientist for the Chandra X-Ray Observatory mission, one of NASA’s great observatories.

Jim Green: Martin, when did Chandra launch and what were some of its goals?

Martin Weisskopf:Chandra launched on July 23, 1999. It’s supposed to launch two days or three days earlier and every day there was some reason we had to postpone, but the third time was a charm.

I was hired in 1977, after Headquarters decided it was a great idea, this kind of X-ray telescope that could look at X-ray objects with much higher sensitivity and better angular resolution than before. I was hired at Marshall in 1977. They all thought I was too young to be the project scientist. Now they think I’m too old to be the project scientist. But, just, you can’t win.

And its scientific goals were really huge. They were to try to understand how the universe works, especially through its X-ray emission. Exploring the universe to try to see what kind of X-ray sources were out there. Were all classes astronomical objects, X ray sources? And if so, why? Why is this happening, you could understand how neutron stars might be in a binary system, might get their energy from gravity, kind of wave your hands, normal stars, magnetospheres are something we’ve been trying to understand for decades. And we’re still trying to understand, they’re very complex. But these are the kind of goals to really nail down the emission mechanisms of astronomical objects and to understand the evolution of the universe.

Jim Green:What has Chandra been finding out recently?

Martin Weisskopf:Well, Chandra has made, most recently, a fantastic discovery. It’s discovered evidence for a planet in another galaxy. Isn’t that amazing?

Jim Green: Indeed, I can hardly imagine that. And it’s a beautiful spiral galaxy too. How did that happen?

Martin Weisskopf: Well, it happened because the, the scientists who wanted to take the observation wanted to study that galaxy. And they knew about these various candidate stars that have possibly planets around them. And they just stumbled into the right information.

Jim Green: So for them to be able to make that fantastic measurement, they actually had to make many observations over and over again, waiting for the right time for the planet to move in front of a very active X-ray star. Isn’t that right?

Martin Weisskopf: That’s right. And there, it’s the active X-ray star and the galaxy itself.

Jim Green:Well, what kind of star was it that has to emit these huge high energy X rays?

Martin Weisskopf:This is one of the great amazing, interesting things. Not only stars like the Sun have solar flares, but other stars flare and we found that we see X-rays for various different reasons, from essentially all categories of stars. Although that’s not the X-ray astronomer’s main interest. They’re mostly interested in supermassive black holes at the centers of galaxies, and how the universe evolves.

Jim Green:Well, what’s been one of your favorite Chandra discoveries?

Martin Weisskopf:Oh, my. I would have to say, because I’ve been interested in this target since I did my first experiments in 1970, is the Crab Nebula and its pulsar. This is a source where a star exploded and left and nebulosity around where the material of this star is running around and crashing into the interstellar medium and getting very hot and it left a compact object.

Martin Weisskopf:When I say compact, I mean compact, about the size of a city like Huntsville, Alabama, but weighs as much as the Sun. The density on the surface of this star is like 10 billion people per raindrop. So these are really cool stars and we want to study them. And I have been studying that object, the Crab and its pulsar, since the beginning of my career and for one reason or another, then with Chandra I did some discoveries, with Hubble, with various different things. And I hope to with this new instrument that I’m fortunate to be principal investigator of IXPE, the Imaging X-ray Polarimetry Explorer.

Jim Green:Those collapsed stars, those neutron stars, as you say, that are emitting enormously intense X-rays. How are they doing that? And what do we know? And how can we call them pulsars? Does the radiation turn on and off?

Martin Weisskopf: It does. That’s one of the exciting things the radio astronomers discovered the first pulsars and X-ray astronomers quickly followed with X-ray pulsars, some of which are also radio pulsars, some of which are not. The X- rays do pulse, like that one in the Crab pulses 33 milliseconds is the period, it’s very fast. And we even have pulsars that are sub-millisecond in rotation. Where does the energy come from? Well, the quick answer is from the fact that these objects are spinning. So if you’re spinning, you have angular momentum, you store energy, and we watch the systems slow down. So they’re losing energy, that energy goes into producing charged particles and X-rays.

Jim Green:One of the properties of all light is that it has a polarization to it. What exactly is polarization and why is that so interesting to us?

Martin Weisskopf:Light is electromagnetic wave. And that’s a fancy word by saying that in addition to the direction of travel at right angles to that direction, there’s an electric field and a magnetic field.

Martin Weisskopf:And if each X-Ray has all the electric fields lined up, we call it 100% polarized. If on the other hand, all of the electric fields are at different orientations, it’ll average to, their net direction will average to zero, it’s unpolarized. So the question is, we want to measure polarization from the X-rays from objects and see what it is and then we have a theory that explains it. And in preparing for our missions, we have done a lot of theoretical work to try to anticipate where we might see polarization and some of these things are really neat.

Jim Green:When I think of polarization, I think of going out onto the lake and light coming down and reflecting off the surface of the water. And then that produces a glare. And that’s polarization too.

Martin Weisskopf:Yes, what’s happening there is when the light comes in and reflects off the surface, that reflection only allows one orientation of the electric field, the one that’s parallel to the surface to come through.

Martin Weisskopf:But what we’re trying to do is we’re trying to do is to measure the glare. If you like. Not to get rid of it, we want to measure it. The light that we’re seeing from the lake and the glare is polarized. And if you put it a polaroid into, to suppress certain directions of the electric vector, then you get rid of the glare. Now we’re not trying to get rid of the glare, we want to see how much glare is there. And which way is it polarized?

Jim Green: The more the glare, the better.

Martin Weisskopf:That would be nice. That would be nice.

Jim Green:I understand that you had an experiment many years ago to measure the polarization of X-ray light. What was it and what did you find out?

Martin Weisskopf: Now that’s amazingly enough, that in 1971, on February 22, flew a sounding rocket from Wallops Island, Virginia. And we looked at that source, the exploded star, the Crab Nebula, and its pulsar, and lo and behold, in that little rocket experiment, which was five minutes above the atmosphere, we measured the integrated polarization from that system. And that, at the time, was extremely important because how were the X-ray is being produced? The answer was synchrotron emission, a type of emission where electronic gets accelerated in a magnetic field. And if that was the correct theory, we would see strongly polarized light. And we saw about 20% polarization, which is very strong in astrophysical terms. And so yes, we nailed it. And then we did a follow up experiment on a satellite called Orbiting Solar Observatory 8 in the mid-70s. And measured it, 20 plus or minus 1%. So we nailed it.

Jim Green: Wow, that’s fantastic to be on the ground floor of using an important wavelength that we can’t see normally, and making new and exciting discoveries using these concepts of polarization. Now, most recently, you became the principal investigator for the Imaging X-ray Polarimetry Explorer or IXPE. What’s IXPE going to do?

Martin Weisskopf:Well, IXPE is the first mission that’s dedicated to X-ray polarimetry. That’s what it does. It has a beautiful, incredible technology that was started at Marshall Space Flight Center, and then developed independently and by their colleagues in Italy, which provided polarization-sensitive detectors, and we at Marshall built X-ray telescopes to put in front of them. We have three optics and three detectors in IXPE. And we’re going to spend all of our time looking at the bright sources and trying to measure the polarization for the first time. And confound the theorists.

Jim Green: I’m sure that will happen. But what are some of the objects that you’re going to look at with IXPE?

Martin Weisskopf:Yes, well, one class of objects is what we call stellar mass black holes. These are black holes, they weigh about 10 times to 20 times as much as the Sun. And they’re in a binary system, they are orbiting around a normal star and from near the black hole, we can’t see the black hole, but from near the black hole, the conditions are right that X-rays are produced. Now, one of the things that our simulations in theory tells us is that the polarization as a function of energy depends on the spin. So we will not only measure the polarization as a function of energy, just to understand what’s going on, but measure the spin of the black hole in a way that’s never been done before. That’s one of the many cool things that IXPE will try to do, and I’m sure will do.

Jim Green:Well, you know, I’m really excited about other things that IXPE can do, such as looking at active galactic nuclei. What do we expect to see when we do that? The, the center of galaxies that are not ours?

Martin Weisskopf: Yes. So we come back to first of all, we come back to black holes again, because we find it that the center of galaxies are supermassive black holes, millions to billions the solar mass. And often, as part of the way these physics of how these things interact with the galaxy, they form jets. So there’s jets of X-ray emitting material that’s pouring out from this object. And we’re trying to understand how that happens. And that will give us some further insight into exactly the details of how do these beasts produce all this energy, not only X-rays, but visible light, radio waves, etc.

Jim Green:So the launch is coming up soon. And once you get it on orbit, how long does it take to check it out before you really start observing things?

Martin Weisskopf:Yeah the launch is in early December. Right now we have the target date of December 9th. And we have a 30-day period from the time of the launch to check everything out. A very important aspect of the, that sequence is a week after launch, we have a boom, an expandable boom that separates the telescopes from the detectors. That has to work. We’ve tested it up a lot, you can imagine, but it has to work. Because if it doesn’t work, we’re in trouble. But I’m very confident that it will. And that happens a week into the launch and then we turn on the instruments for the next three weeks, check them out, and then we start taking data a month into the mission.

Jim Green: Well, you know, since Chandra didn’t measure X-Ray polarization, IXPE is a huge advance. Are you going to be using the same targets that Chandra did, or even more?

Martin Weisskopf:We’ll be using many of the targets that Chandra has done, and we’re, especially for the imaging part where we’re looking at polarization of extended objects, we will be using the Chandra images because Chandra can see a dime at 12 miles. IXPE can’t do that. Chandra is subarcsecond resolution. IXPE is 30 arcsecond resolution, and we will be using Chandra images to guide our images.

Jim Green:Well, will there be opportunities to look at the same object at the same time between IXPE and Chandra?

Martin Weisskopf:Yes, in fact, some of my colleagues have already proposed such and we’re looking at the galactic center at the same time where, with Chandra, as we are with IXPE, so yes indeed, a lot of science gets done, as you know where well, by using the whole suite of instruments, scientific instruments that NASA has provided — things like Chandra, the NuSTAR, which is a higher energy experiment, Hubble, and then hopefully JWST in the future. So, very near future I might gather too. So that’ll be very exciting.

Jim Green:Yeah, that’s fantastic. Now is the launch out of Kennedy Space Center?

Martin Weisskopf:Yes, it’s at Kennedy Space Center. But the orbit will end up going around the equator. We do that to, to keep our charged particle background low. And so the Falcon 9 will take us into orbit, maneuver us down to the equator and then let us go.

Jim Green: That’s fantastic. Well, what are you personally most looking forward to about IXPE observations?

Martin Weisskopf: One is the Crab Pulsar, as a function of pulse phase. Polarization as a function of pulse phase. I tried to do that years ago, we just didn’t have sensitive enough polarimeters, want to see that done. The other one is one of the magnetar experiments.

Martin Weisskopf:Magnetars are called that because we think their magnetic fields are 10 to the 15th Gauss, 1000 times more than a conventional neutron star. And it those field strengths, the physics changes where you have to worry about fancy things like not classical electricity and magnetism. But stuff like quantum electrodynamics, that is, the quantum theory of the fields is very important.

Martin Weisskopf:There was a physicist who in 1934 wrote a paper on what happens to propagation of light when magnetic fields get beyond the critical field of about 10 to the 13 Gauss. That was my uncle Victor. And so I would just love to be able to have, do an experiment that says yes, quantum electrodynamics is right in this context of a magnetar, and I quote Vicky’s paper and it just feels so good, it feels so good. I would have loved to have done that while he was alive. But still, I’m looking forward to that.

Jim Green: Oh, wow. I understand completely. Well, Martin, I always like to ask my guests to tell me, what was that event or person, place or thing that got them so excited about being the scientist they are today? I call that event a gravity assist. So Martin, what was your gravity assist?

Martin Weisskopf:It’s a very tough question, in the sense I’ve been fortunate enough to have several. But I think that first experiment we talked about when I was a young postdoc at Columbia. After doing the data analysis in my office, I realized at that moment that I was the only person ever alive that I had ever existed that knew that the Crab was 20% polarized.

Martin Weisskopf:And it was just, the feeling of awe came over me. I thought I was in church for a few minutes, and that was my first such moment. And being able to be project scientist, which I still am for Chandra, to have been one of the people to build what we call, well, one of my scientists called, a scientific cathedral, one of the great observatories of NASA has been another moment that actually keeps going. We built it designed for three years with a goal of five. We celebrated our 22nd year this year, and the observatory keeps putting out fabulous new unexpected results.

Jim Green:Yeah, indeed, it does. But it also sounds like your family has been involved in astrophysics over the years. What’s that been like?

Martin Weisskopf: Well, I have a, I have a family of intellectuals that are all smarter than I am. My uncle was a physicist. My father was an economist, my mother taught romance languages. My aunt was a psycho, psychologist, taught that at university. So it’s just nice, little frightening. Can’t read my father’s papers, because he uses words that are longer than I can pronounce. I can read my uncle’s papers, because the math is way beyond me. But I’ve done a few things too. And I’m experimentalist, and I love building hardware.

Jim Green:Well, that’s wonderful. And congratulations on being the PI of IXPE. Martin, thanks so much for joining me in discussing your fantastic career, and the opportunity to look forward to even more results.

Martin Weisskopf:I hope so. Just takes a little bit of luck and a lot of hard work by hundreds of people throughout the world. And it’s showing. And NASA has played such an important role in this. If you young person want to get into something exciting, no matter whether it’s from the engineering, management, science or any other aspect of it, come to work with us at NASA. You’ll love it.

Jim Green: Well join me next time as we continue our journey to look under the hood at NASA and see how we do what we do. I’m Jim Green, and this is your Gravity Assist.

Credits

Lead producer: Elizabeth Landau

Audio engineer: Manny Cooper