Host: What do black holes, neutron stars, and the most extreme explosions in the universe have in common? They all shine in X-rays — and thanks to a special space telescope called IXPE, the Imaging X-ray Polarimetry Explorer, we’re now seeing those cosmic fireworks in a brand-new light. 

IXPE is giving scientists an entirely new way to study some of the universe’s most powerful and mysterious objects — by measuring how their X-rays are polarized. That polarization reveals hidden details about how black holes feed, how jets form, and how exotic stars like magnetars behave — questions that have puzzled astronomers for decades. 

In this episode, we talk with Dr. Steven Ehlert about how IXPE is pulling back the curtain on these extreme environments, what it took to make this mission a reality, and how simple measurements can lead to breakthroughs in understanding. This is Small Steps, Giant Leaps. 

Welcome to Small Steps, Giant Leaps, your podcast from NASA’s Academy of Program/Project & Engineering Leadership. I’m your host, Andres Almeida.  

Steven is here with us to talk about this novel mission, the first of its kind in 50 years.

Host: Well, welcome, Steven. It’s nice to have you here. It’s great to be here. What is the primary science goal of the IXPE mission? 

Ehlert: So, IXPE stands for Imaging X-ray Polarimetry Explorer. So, the answer is really right in the name. 

 So, it’s the polarimetry aspect of IXPE. That’s really what the mission is designed to do. It’s designed to look at X-rays and to measure the polarization of those X-rays. And polarization basically has to do with the property of all light that describes the direction with which the electric fields are facing, relative to the, relative to the propagation of the light itself. 

So, the electric fields are always perpendicular to the direction the light’s traveling. But oftentimes, where along that circle, where along that plane it’s oriented, has useful information for us, both in sort of what fraction of it is all distributed in the same direction, as well as what that particular angle actually is.
And IXPE is the first mission in almost 50 years, and the first one with imaging capabilities that’s been able to measure any X-ray polarization for any of these signals. And that gives us a lot of new information about how X-ray emitting sources operate. So, that’s really what we’re interested in doing. 

Host: And X-ray emitting sources, meaning it could be black holes? 

Ehlert: Yes. So, we’re talking black holes, neutron stars, supernova remnants, really kind of the, the fireworks of the universe, the really powerful explosions. I, I will say that a lot of it has always, it’s not exclusively doing this, but in general, we’re kind of looking at nonthermal emissions. So, we’re looking at particle acceleration. We’re looking for systems where the concept of a temperature doesn’t necessarily make any sense. Think of it is the difference between heating something up to where it’s bright in X-rays versus like a laser source or a laser beam. 

Host: So how does IXPE complement other X-ray observatories like Chandra? 

Ehlert: The biggest difference, of course, is that polarization sensitive detector, so IXPE, like Chandra and XMM [XMM-Newton space telescope], can take beautiful images of the sky in X-rays like those, like those two, like those two telescopes can also look at the, the timing structure and the energy structure of of those sources. It has a slightly different energy band to which it’s sensitive. So, each of those three instruments is a little bit more or less capable at different energies. 

And while [IXPE] certainly has imaging capability, it’s not at the same level of detail. It’s not the same sharpness as Chandra. So, Chandra can see less than an arcsecond. So, that’s a very, very small, that’s a very, very small fraction. It’s…1/36 hundredths of a degree is one arcsecond. So, thinking about half of that, that’s 72 hundredths of a degree. That’s an extremely small, that’s an extremely small angle. 

Host: That’s smaller than, smaller than the Moon, correct? Much smaller?

Ehlert: Oh, vastly smaller than the Moon. So, the Moon, when the sky is about a half a degree (and the Sun, for that matter, is about a half a degree) in the sky. So, we’re talking dividing each of those little sections, dividing the Moon into 3,600 sections, and then roughly one of those is the size of a half an arcsecond. 

So, IXPE does not have quite the sharp imaging of Chandra. And while it certainly is capable of doing spectra for reasons related to the detector, it’s really that polarization information, which gives us a lot of really important insights into the geometry of these, of these objects that we’re observing, and because of the way polarization signatures originates from the from the processes that generated the X-rays themselves, that gives us a lot of insight on scales that we just would never be able to observe with Chandra or XMM or with any X-ray telescope planned for the foreseeable future. 

We’re talking about a hundredth the size of what we can resolve with Chandra. We can see those, we can figure out what’s operating on those scales based off of the geometric information we can ascertain from a polarization sensitive detector. 

Host: So then, how do you prioritize which cosmic objects or phenomena IXPE observes?

Ehlert: For the first two years of the mission, there was what we call the prime mission. IXPE had gathered a very large team of collaboration members to help plan out the observations and write all the papers. 

So, there was a big group within the mission, and they weren’t. They all were volunteering to do this, except for those who were contributing to the project more directly in sort of, you know, development support, project development that that they were volunteering their time to help figure out what the right sources to observe were. And we had seven topical working groups, which spanned everything from radio galaxies and black hole jets coming straight at us to supernova remnants and accreting neutron stars and magnetars, which are these extremely highly magnetized neutron stars. And all of these groups were working together, and they came up with their own target lists, and we, we figured out how to get them all into the schedule. 

These days, IXPE operates on a General Observer Program, which means that just about anybody can propose to IXPE with the catch that it’s once a year. It’s a once-a-year call where you write your proposals, and then you ask for your source to be observed. And you have to come up with a very convincing case from both the science and, and the operation side, like saying, “Well, we know that this is how much time we need. We know these are the observing modes we need.” But if you make a good enough pitch, and it convinces a review panel, then it gets approved, and then it gets into the schedule for the next year. 

So, it’s a challenging process, and practically nobody is able to sort of circumvent that, only in very particular circumstances, but it’s a very rewarding one, when things get accepted. 

And as I mentioned, anybody can come up with a good enough idea. It doesn’t necessarily have to be somebody involved with the mission, who’s ever worked with the data before. 

Host: Since IXPE’s first light, what have been some notable discoveries or observations? 

Ehlert: All right, well, how much time do I have? 

Host: It’s hard to pick a favorite child, right? 

Ehlert: Right! So, what I will say is one thing is I’ve, I’ve had to look over and sort of come up with greatest hits of IXPE results from the last few years, several times over the last few years. And one of the things that I think is really compelling about IXPE is that many of its many of its test cases and many of its results can be described very simply, but also be very decisive in terms of entire classes of models that have been proposed and discussed and tested against the data we had pre-IXPE. 

One of my favorite examples has to do with black holes and how they generate X-rays. There’s been a long-standing question about the presence of a corona in these accretion disks that’s responsible for generating the X-rays, and there have been a lot of questions about where it’s located. Is it located along the, in the plane of the accretion desk. Is it located above and below the accretion disk? There are a lot of questions about where it was, and IXPE has made a lot of observations of black holes, both sort of the stellar mass galactic black holes, and also the supermassive black holes at the centers of other galaxies and in almost every single one of them, we were able to say just based off of the polarization angle with respect to what we see as the Jets, which are ostensibly, well, which are all but certainly perpendicular to the accretion disk, we were able to rule out all sorts of models. 

So, IXPE observations really strongly disfavor models where the corona is sort of above and below the plane of the accretion disk, which are often known as lamp post models, whereas models where the corona is located within the accretion disk are very strongly favored. And this is really just a question of whether it’s parallel or perpendicular to the radio jet. So, it’s very simple. It’s a very simple question to ask, at least at face value, is a very simple measurement to see when plotted up against an image. But it’s a very strong constraint on the physics, one that we never had any, any observational evidence for. 

So, there had been arguments about whether it was what the geometry was for a long time, and IXPE was able to answer it. 

Another one of my favorite results has to do with the jets coming off of black holes. My personal research involves a lot of observations of these particular AGN known as blazars. Those are, those are AGN where the jets are pointed straight along the line of sight. 

Host: I remember reading about that. 

Ehlert: Yeah, and they’re very cool. And there was just a press release a few days ago that, two weeks ago, actually, that came out where we could, where we could see all where we were talking about that. And again, in this case, just based off of the instead of looking at angles, in this case, we’re looking at the relative polarization between IXPE and the X-rays, and then optical observations taken with telescopes on the ground, and radio polarization taken with different telescopes on the ground. And those ratios are actually able to say something, oftentimes put very, very strong constraints on what it’s generating the X-rays. What’s the structure of the jet that’s required to generate this kind of polarization structure across radio and optical and X-ray wavelengths? 

So, that’s been very, very interesting and powerful and important to see, to see play out. 

A fairly simple measurement, it gives us an enormous amount of information to rule out whole classes of models that were been proposed for what’s going on in these jets. 

Host: When you talk about this, it it astounds me that we haven’t had something like this, or, you know, we haven’t looked this way in 50 years. 

Ehlert: My neighbor in the office next to me is Martin Weiskopf, who was the original PI of IXPE, principal investigator of IXPE, and was the project scientist for Chandra for over 40 years…there was just, there’s just a little ceremony talking about IXPE here at Marshall and it turns out that he had proposed for an X-ray polarimeter like IXPE, with different technologies. Basically, as soon as he got here. And at least 13 times between proposals to NASA, proposals to the European Space Agency and elsewhere, and to other organizations. 

So, he has been trying, and it took him 40 years, but the science has just been incredible. The science return has been incredible, and I think all X-ray astronomers are greatly indebted to his willingness to keep trying for that. 

Host: Can you talk a little bit about international collaboration with this mission? 

Ehlert: I would say it’s a poster child for international collaboration with NASA, in particular, with the Italian Space Agency (ASI), which, it’s not an ESA mission, but ASI and the INFN [Italian National Institute for Nuclear Physics]. There are many institutions in Italy that have provided the detectors for IXPE. So, the actual polarization sensitive detectors have a long development history in Italy, and they were kindly contributed, graciously contributed to the IXPE mission as part of the proposal. 

The Italian team has also provided the one of the ground stations for communications between IXPE and the ground, so almost all, a huge fraction of the telemetry data comes through, comes through the Italians ground station in Malindi, Kenya, which also helped dictate the orbits. They’ve been so, so helpful in helping with calibration efforts, which have been run both at Marshall and in Italy, as well as contributing to the science. And it’s really been a tremendously fruitful relationship, and I am young enough in my career that I hope to continue to work with the Italian colleagues I’ve met and learned, learned about through this mission so far, well into the future. 

Host: How did the proposal for IXPE come about in the first place? 

Ehlert: Every proposal starts with an idea that pops into your head. And for me, before I before I joined the IXPE team, a lot of what I did were studying galaxy clusters with X-rays. And galaxy clusters generally, because they’re thermal emission and not nonthermal emission, really don’t have a lot of prediction, you know, don’t have a lot of easily predicted polarization signals. 

But when I first started, I was like, “Oh, eventually we should figure out how to get a galaxy cluster involved. I feel like that’s my job, because that’s what I have to do, because I’m working this mission, and I have, I have a good amount of galaxy cluster experience.” So, I started reading up on it and thinking about it and, and I was talking to some of my old colleagues about, you know, some possible predicted polarization signals. Turns out, somebody I worked with in grad school had written long before I had met her a paper predicting a very specific polarization signal in the Perseus cluster, which is the target I ultimately chose. 

So, the Perseus cluster is a very famous galaxy cluster. It has this very big, active galaxy at its center, and it’s a very bright object. So, I started asking her a bit about this and this particular signal. And then after a few days of, you know, working through some numbers, we came to the conclusion that it probably wasn’t a great, a good enough selling point as, “Oh, we’re going to look for this very unique signal by itself. So, I waited a couple of years. I sort of accepted that galaxy clusters probably weren’t going to be observed with IXPE anytime soon. And then somebody else came to me with another prediction for an interesting polarization signal that you could find in a galaxy cluster. And this one is sort of an exotic particle physics signature for a fairly exotic dark matter candidate. And we started working through what it would take to observe that. 

And between that exotic particle physics signature and this potential astrophysics signature that you can see of polarization in a very specific place in the cluster, and then combined with the fact that we have this somewhat unique, for IXPE, an active galaxy that we could observe basically for free by staring at it long enough, that ultimately convinced me that if we could do all, if I could make a strong enough case for all three, that could, that could be a viable, big program to do with IXPE. 

And it didn’t win the first year. So we’ve done two cycles of general observers, and this is very typical, and this is something I hope the audience really appreciates, is that lots of us work on proposals for a long, long time, but don’t get accepted the first time, and oftentimes the second or third or fourth time either. But there’s still, you know, strong ideas that sometimes you just need other things to help you out with. 

Host: Or maybe the timing is wrong, right? There are other priorities. 

Ehlert: Yep, sometimes the timing doesn’t work, and just we need to observe these things now, instead of something, you know, something that is very long. I don’t think I’ve mentioned this yet, but this the time I was asking for, which was two and a half mega seconds, or about a month of actual observing time is kind of what we’re talking about, was actually the longest exposure that was ever asked for, for IXPE so far. Part of what makes it challenging, is you need lots and lots of time. 

But the other thing that happened is there were a few papers that came out in that year between when it got rejected the first time and when it got selected the second time that I was able to sort of redevelop the case based off of some comments. Astronomers are good at giving comments to proposals saying why they, why it wasn’t chosen, or what reasons they had for why they couldn’t accept it this time. So, I was able to incorporate some of those the second time around, not that the panel’s the same, but still, some of the comments can often be the same. 

And then there was some new science, in particular with the central active galaxy, NGC 1275. So, there was a new result that came out over the course of that year that I was able to incorporate, which made things work a little bit better for my proposal, and ultimately, it’s all been accepted. A bunch of my collaborators (there are lots and lots of people on the team who helped me write this) and I are in the process of figuring out how to analyze this data. 

It’s also a very challenging source to analyze, because there’s lots of signals, we don’t care about to test these particular signals. We do, so we have to do lots and lots of tests to make sure that we understand what the rest of the cluster is doing before we look at our particular regions of interest. But on the other hand, there may be some surprises. There may be something exciting that we see in those regions we didn’t ask to look for. So, we’re in those sites that we didn’t think there was going to be anything. So, we have to test those too, because maybe there’s something extremely exciting there. It’s a fun job, and it’s really fun to get something selected. It’s very hard to get something selected, even for professional astronomers who’ve done this many, many times. 

Host: Yeah, it sounds dynamic, like both stressful and extremely rewarding. 

Ehlert: It’s only stressful in about the 48 hours before the deadline, when you’re sort of scrambling to figure out what it’s going to look like, and if you want to change everything. I’ve gone through so many proposal cycles with telescopes, where the entire observation plan seems to change somewhere between 48 and 72 hours before the actual deadline. And then it’s like, “Ah, I gotta run it all over again!” 

Host: And just going back for anybody unfamiliar, can you explain briefly what a mega second is in science. 

Ehlert: Every telescope has its own units of time for which they measure when they’re asking for things. So, James Webb’s is in hours. Hubble’s is in orbits. For X-ray telescopes, it tends to be in kiloseconds or megaseconds, so either 1,000 seconds or a million seconds. 

So, when I say two and a half mega seconds, that’s just 2.5 million seconds on the actual source, which is about a month. You know, it’s approximately a month of time on the source. But given IXPE’s orbit, you there’s also overheads and other things that go along with it. So, the time, it’s actually kind of pointed in that direction, or the time is sort of set aside on orbit for, for Perseus was probably something closer to like four or five megaseconds. It took about two months between sort of January and the end of March to get all the data. 

Host: What kind of impact do you hope IXPE findings will have on our understanding of the universe? 

Ehlert: IXPE has so much, has already given us so much new insight as to how particle acceleration works and supernova remnants and black holes and the jets of black holes in neutron stars and in these magnetars. And I really hope that it becomes clear that there’s really crucial information in there, and it’s very, it’s very reassuring to know that for, for many of these target types, there actually is an answer, which I think that’s, I think that’s very cool. It’s that there are multiple, you know, there are multiple options, and IXPE is able to answer, oh, it has to be Option A or Option B, whatever, however you sort them, it’s like it has to be a very particular option, a very particular model that works. 

And I think that these systems are complicated enough and they’re interesting enough in general, that being able to spend our time, at least with the one, you know, with, with a better starting point as to how the whole system operates is just going to move sort of the rest of the questions, or help us develop answers to the rest of the questions that much faster. And I will also say that sometimes, even though there’s an answer that doesn’t necessarily make the answer you know, the questions you ask right after that easier, or even the questions you originally had easier. So, while it definitely makes things a little bit easier to deal with, but you don’t have quite as many models to worry about, it doesn’t mean they necessarily, like everything else, pops out as clear as day. 

Those other models, you know, survived for so long because there were, there were things that they appeared to do better at the time. So, there were certain aspects of it that they appeared to do better. Maybe it looked better. And, you know, this is too general for me to get into the specifics about it, but there were reasons those models survived as long as they did, and if it turns out that they just don’t work for the polarization data, that doesn’t mean that it’s easy to answer anything else about spectra or timing or the other observations that we’ve already collected. 

Host: What do you consider to be your giant leap? 

Ehlert: Probably the best answer I can give to this is when I was an undergrad, when I was, when my first year at university, I was, I knew I was going to do physics. I knew I was going to be a physics major, but I hadn’t really pinned down what kind of physics I wanted to study. 

I knew I wanted to do research, but wasn’t entirely sure which direction I wanted to go, so I just started emailing all my professors and basically said, “Hey, do you have space in your lab?” Or, you know, “I’m, I’m a, I’m a 19-year-old, 19-year-old freshman looking to, looking to do some science. Is there, is there a spot in your lab? Do you have, do you have some work that I could do?” And the first person who wrote me back and said, “Sure, yeah, come on over. We’ll, we’ll talk about it,” was, in fact, an X-ray astronomer, and I was doing X-ray astronomy work, and I had, I was working a little bit in the lab. I was working a little bit in the machine shop. I eventually was able to, you know, he offered to help me fund a summer and stay out at the university so I could, you know, do research over the summer when I didn’t have classes. 

And I’ve done lots of other types of research since. I did a couple of stints in gamma ray astronomy. I spent five years at Marshall doing meteoroids, but I always ended up finding my way back to X-ray astronomy. So, I would say that would probably be the giant leap. [It] was sort of that initial X-ray astronomy work that ultimately sort of kept coming back to me throughout all the other steps I’ve made in my career up to this point. 

As soon as you can think of somebody to contact about doing research, contact them, even if you’re in high school or even if you’re in middle school. I would say, figure out how to do research as early as you can. And if, if you’re an interested and motivated student, there’s definitely somebody out there who will find a way to give you some work to do.  

Host: Thank you, Steven, thanks for all your time and your expertise with IXPE. We’re looking forward to learning more about what what IXPE observes and discovers. 

Ehlert: Well, I am looking forward to it as much as anybody, so, thank you so much for having me on and for anybody listening, I hope you’ve enjoyed it. 

Host: If you did enjoy this episode and want to learn more about IXPE, visit our resource page at appel.nasa.gov. That’s A-P-P-E-L dot nasa dot gov. You can also check out IXPE’s mission page at nasa.gov/IXPE for the latest news and findings. Want more NASA podcasts? Check out Houston, We Have a Podcast, Curious Universe, and Universo curioso de la NASA. As always, thanks for listening to Small Steps, Giant Leaps. 

Outro: Three. Two. One. This is an official NASA podcast.