Episode 40: The mystery of the Fast Radio Burst

with Dr Marisa Geyer

In this episode, we speak with Dr Marisa Geyer. She is a pulsar astronomer and Commissioning Scientist at the South African Radio Astronomy Observatory (SARAO) in Cape Town, South Africa.

Marisa tells us what it’s like to be a commissioning scientist for the MeerKAT telescope. She also explains her research on pulsars and mysterious Fast Radio Bursts (FRBs).

FRBs are sudden, bright flashes of radio light. Usually they only appear once and then are never seen again.

The mystery is that we don’t know what is causing these bursts of light! All we know is it’s something powerful, and probably something very far away.

Marisa talks us through the mystery and reveals some enticing clues about what might be causing FRBs. Hint: it involves something called a magnetar!

Artist’s impression of the messy magnetic fields surrounding a highly magnetised pulsar, called a magnetar. Image from EarthSky, via Carl Knox/ OzGrav.

The picture above shows the dispersion signal which helps astronomers to figure out how far away an FRB originated. This particular one shows the Lorimer burst which was the very first FRB to be discovered. It was found in 2007 by a group led by Duncan Lorimer.

One famous pulsar we discuss is the Crab pulsar. The image below is the Crab Nebula, which is the remnant of a supernova explosion.

The Crab pulsar is the remaining core of the dead star that went supernova. That supernova was witnessed in 1054 CE, and is the earliest ever recorded.

(Image from the Hubble Space Telescope. Credit: NASA, ESA, J. Hester and A. Loll (Arizona State University))

Daniel also talks about a once-off event seen in the skies over South Africa and Namibia, which you can see in the video below. It shows the upper stage of the Chinese Yuanzheng-1S rocket being de-orbited and burning up over South Africa.

Video credit to the original source.

This week’s guest

Acknowledgements

Social media by Sumari Hattingh.

Transcript

[00:00:00] Dan: Welcome to The Cosmic Savannah with Dr. Daniel Cunnama

[00:00:08] Jacinta: and Dr. Jacinta Delhaize. Each episode, we’ll be giving you a behind the scenes look at world-class astronomy and astrophysics happening under African skies.

[00:00:17] Dan: Let us introduce you to the people involved, the technology we use, the exciting work we do, and the fascinating discoveries we make.

[00:00:25] Jacinta: Sit back and relax as we take you on a safari through the skies.

[00:00:33] Dan: Right, welcome to episode 40.

[00:00:35] Jacinta: What a milestone? We were racking them up. Yeah, we’re getting up there. Season four, episode 40. I like that.

[00:00:43] Dan: So for this episode, we spoke to Dr. Marisa Geyer, who is a commissioning scientist at the MeerKAT radio telescope. And she’s going to be talking to us a little bit about what that entails, what it entails to be a commissioning scientist, and also some of her research on pulsars and fast radio bursts.

[00:01:04] Jacinta: Yes, the enigmatic, mysterious, fast radio bursts.

[00:01:09] Dan: But before we get into that, Jacinta we are recording across the Indian Ocean at the moment.

[00:01:15] Jacinta: Yes, we are. I am coming to you from Australia. From my small bedroom in quarantine at the Howard Springs Center for National Resilience in Darwin, in the Northern territory of Australia. I finally managed to get on a repatriation flight so I could come home, see my family.

I haven’t been here for, I haven’t seen them for over a year and a half and also to do various administrative things that I had to come back to Australia to do. So, yeah. Recording from the other side of the world.

[00:01:47] Dan: And how’s quarantine going, or as you like to call it in Australia “quazza”.

[00:01:53] Jacinta: Yeah. So Australians do like to abbreviate everything.

So “quazza” it’s going really, really well, actually. I mean, I’m so lucky to be here. At this particular facility. The staff are amazing. The facilities are amazing. So it’s kind of as good as it can possibly be for two weeks as a semi-inmate. We don’t have any yard time. We get our food delivered every day, but they also deliver our shopping for us.

So in some ways it’s actually a little bit of a holiday, a little bit of a break, and I’ve got some nice neighbors, so we’re kind of enjoying ourselves. So as much as we can.

[00:02:29] Dan: But you can’t interact with your neighbors. Right?

[00:02:30] Jacinta: You can talk to them, you can’t touch or go near each other or pass each other or anything. But yeah, we can talk to each other

And what’s been happening? What’s the latest over there in South Africa?

[00:02:41] Dan: We had a very exciting day yesterday. I was very busy with interviews and TV. There was a Chinese upper stage rocket, which came down over South Africa on Monday night. It’s burned up in the upper atmosphere and was a very spectacular sight.

Spotted across Botswana, and then the north of the country. A few people got some wonderful videos of it. I wasn’t fortunate enough to see it myself. Didn’t get to Cape Town. But yeah, there was at first, a lot of, sort of speculation about what it could be and some clever people who managed to track these sort of objects, worked out what it was and yeah, very exciting.

[00:03:23] Jacinta: What did it look like?

[00:03:24] Dan: So when it burns up, it looks a little bit like what you think of as a meteorite, it’s moving a little bit slower and it broke up in the sky as it came in. So you first see the single fireball and then it slowly breaks up into various smaller fireballs and then burns out and disappears.

[00:03:42] Jacinta: The video looked really impressive. I was quite envious that we didn’t get to see that.

[00:03:47] Dan: Yeah. I was very jealous too, especially, you know, talking so much about it, but not having had the chance to see it. But, you know, we’ll post the video on the, on the blog so that anyone can have a look if they want.

Yeah. It was a special, special sighting. Always love seeing something up in the sky. Any sort of event, even if this one wasn’t natural.

[00:04:08] Jacinta: True. But speaking of things, going flash in the sky, that brings us to our episode today. As you said, we’re speaking with Marisa about fast radio bursts, which are exactly as the name sounds. They are bursts that are fast and they appear at radio wavelengths.

So you can pick them up with radio telescopes. And it’s a big mystery as to what is causing these things. They were only found, I think it was what? I think Marisa told us that it was in 2007 was the first one was found. And only a few dozen known until very recently, as Marisa will tell us more about. And they are using, along with MeerKAT, there’s another telescope called CHIME that is being used to pick up many of these new fast radio bursts. That is the Canadian Hydrogen Intensity Mapping Experiment. C-H I-M-E, CHIME.

[00:05:00] Dan: Yeah, so very cool. And I mean, the conversation with Marisa was fantastic, which you’ll be able to listen to now. Very interesting objects, well, events. Fast radio bursts. And Marisa will give us some clues as to what they could potentially be.

[00:05:15] Jacinta: All right. Well, without any further ado, let’s hear from Marisa.

[00:05:21] Dan: Alright. So today we have Dr. Marisa Geyer. Marisa, welcome to The Cosmic Savannah.

[00:05:31] Marisa: Hi Danielle. Thank you. It’s really good to be here.

[00:05:35] Dan: Yeah. Good to have you on. Uh, I think we’ve been meaning to have you on for a while. As we mentioned already, Marisa is a commissioning scientist at the South African Radio Astronomy Observatory. And Marisa, maybe you can just talk a little bit about that, that role and how you got to be a commissioning scientist.

[00:05:53] Marisa: Yes. Let me talk a little bit about what a commissioning scientist does. So I work for the MeerKAT telescope and basically there are various aspects to what you need to do to get the telescope to work nicely.

So there’s the obvious bits of the engineers that need to build it. And all the software pipelines that need to run so that we can point it at particular astrophysical objects of interest. But then there’s also the bit where we need astronomers on board to make sure that the data that’s coming in from the telescope is of science quality.

And so we want to ensure that we’ve basically calibrated and set up the telescope in such a way that when I point to my favourite galaxy, the data appears the way I expect it to appear. So I am part of a particular sort of subsection of this team. So we work on the beam-forming for the MeerKAT telescope. I think we’ll talk a bit more about this down the line in the podcast, but we use beam forming to do time domain astronomy.

And so, you can imagine that we’re all quite familiar with these beautiful radio images that MeerKAT has produced. Think back of like the launch with the galactic center. And they were really looking at like the details of what we can see in the radio sky in sort of an image domain.

But there’s another bit of science that’s quite interesting. And that’s to make sure that you can actually use the telescope in such a way that you are sampling the data very fast. So you are learning a lot about events that happen on a short timescale. So I’m part of the group that work towards making sure that the telescope can do very high precision timing science.

And I think you asked how did I get into this. So it’s quite typical for people in the commissioning team. And sort of on the science side of MeerKAT to be astronomers. So like Daniel, I have a background in astronomy. But I only really got to have an astronomy exposure a little bit later in my academic studies.

So shall I tell you about my path through the academic ladder?

[00:08:03] Dan: Absolutely. I also have something of a chequered past. I came from theoretical physics and got into astronomy only in my PhD.

[00:08:12] Marisa: Oh right. Okay. Okay, cool. So we have that in common. Exactly. So like Daniel, I started out studying theoretical physics.

I grew up in Stellenbosch. And so it was quite a natural choice to go to Stellenbosch University. I wanted to keep my options quite broad, to be honest. So I had settled on studying science. So a BSc. And I was still kind of thinking about whether it’s going to be genetics or physics or which of these components I would find most interesting.

And then I think, as you kind of do a three year and then fourth year is your Honours, right. And as I was going through the BSc ranks, I really started liking the physics approach to problems. I think I found it quite fundamental. So those of you have studied other fields like biology and such, you’ll know that though there are many logical rules even in all these other sciences, it does feel like you have to spend quite a bit of time memorizing things.

And maybe that wasn’t my forte. I don’t know. But I really liked how in the physics department it felt like, a pencil and a piece of paper and you should be able to sort of unravel most of the answers you need. And so maybe that should also tell you that I wasn’t initially so fond of like experimentation. The lab sessions sometimes got the better of me.

And so I think along the line, I grew more and more towards the mathematics/theoretical physics side of things. So by the end of my fourth year, I had to figure out what to do next. And I think weirdly it would have been standard for me to go into a Master’s straight away. But I think just after four years, you know, you’ve had quite a lot of science exposure, and I was starting to be in the mood for something more creative.

And I thought, well, if I don’t give it the chance or a shot now, maybe I wouldn’t come back to it. So I did a bit of journalism for one year. So you can kind of do like an honours in Journalism.

[00:10:18] Dan: This is pre-podcasting?

[00:10:21] Marisa: This was before the days of podcasts.

[00:10:24] Dan: So you wrote with a pencil and paper?

[00:10:26] Marisa: Yeah it was me with my pencil and paper again. Well I’m mostly not much of a journalist now, so maybe that was more like an indulgent year, to be honest. It included things like photography and, you know, all sorts of creative writing and an interesting reading list to boot. And obviously there was a science journalism component as well. Where we got to pick projects that was kind of my focus. And actually this was a lucky coincidence that it was 2010. So it was the world cup year. So what a great year to be studying journalism.

[00:11:09] Dan: To take a year off.

[00:11:09] Marisa: Exactly! I have to watch this game now. It’s part of my work.

No, so from the science side, 2010 was really the year that the SKA awardships were being finalized and it became ever more clear that, you know, part of it was going to come to South Africa. And so I actually got to spend some time in that year, writing about that project and learning about how South Africa will play such a prominent role. And straight after that, I then went back to a Masters and I think this the turn where I start looking at astrophysical applications. And in my Master’s, I had a project that had to do with black holes and orbits around black holes, all still very theoretical, but really me getting into the stars.

And fast forward. I did a PhD abroad. And then when I came back, I joined the commissioning team here in Cape Town.

[00:12:07] Dan: And that’s where we are now.

[00:12:08] Marisa: That’s where we are now, before it gets too long.

[00:12:13] Dan: No, that’s great. So where we are now. A commissioning scientist for meerKAT. So MeerKAT is, sort of, recently established and obviously there is still some commissioning going on, making sure the data is of high enough quality.

But you do some of your own research too.

[00:12:32] Marisa: Yeah, that’s right. And maybe if I can just quickly interject. Because you were talking about an interesting trajectory of commissioning and where that goes. So MeerKAT was inaugurated 2018 shortly after I started. But even in that short span, 2018 to where we’re now, the focus of my own commissioning team is already changing. So I just wanted to make sure to include that, that while commissioning is still an important part, MeerKAT is now so well operational in many of its modes. So we’re only really commissioning a last couple of modes.

That we also tend to play a science support role now. So you are more involved with the PIs that have, you know, time on the telescope and ensure that the data that is being taken for their projects is of high quality. And yes, it just, it opens up some more time also for your own research.

So my research on MeerKAT is, I guess it flows from what I did for my PhD. So, for my PhD, I worked on pulsars, on neutron stars. Daniel’s told me that there has been a podcast sort of fleshing out that topic a little bit.

[00:13:43] Dan: Yeah, we’ve spoken about neutron stars before and some pulsars. So pulsars are essentially neutron stars spinning very fast and beaming out light at sometimes regular intervals, right?

[00:13:57] Marisa: Yeah. That’s right. So actually incredibly regular intervals. That’s why we liked them so much. The ones that spin really fast, we call them the millisecond pulsars. The precision with which we can know when the next pulse will arrive is kind of rivaling atomic clocks.

So if you stare at a millisecond pulsar for long enough and you time it using an instrument that’s geared to and commissioned well for high precision timing, then you can really time these things where you know the period of the spinning star up to the 10th or 12th decimal.

That’s really exciting. And it’s exactly that aspect of MeerKAT that I’m interested in. Ironically, lately I’ve been working on a pulsar that’s a horrible timer. So maybe I should just give the three sentence intro. So like Daniel said, they are neutron stars. So pulsars are neutron stars and they are born through supernovae collapse.

So if people listening are familiar with the life and death of a star, then basically what you have is, what is it? It’s kind of a live, hard die young scenario, right? So the big massive ones party hard and blow up fast. And they are the ones, say if you start out with a star several times the mass of our Sun. And when that kind of burns through its fuel, it will collapse to a neutron star. And like Daniel mentioned, some of them beam radio emission. And so as they spin this beam of radio light emitted at their poles can kind of sweep through our line of sight. And so you get this passage, this pulse of light coming across the MeerKAT telescope that you can see as a blip. And it’s these blips that we observe and time.

And so the one I was going to talk about because I’ve just completed some, well, you never complete research. I’ve just completed a paper on this pulsar. It’s a pulsar up in the Large Magellanic Clouds. So that’s cool if you’re a pulsar astronomer, because you know, most of the pulsars we observe are within our own Galaxy. So while the Large Magellanic Cloud isn’t that far away, that kind of is one of the most distant pulsars we observed. So it’s at 50 kiloparsecs.

And I was saying it’s not a good timer and it’s really a bad timer. So much so that I need X-ray data to actually help me time the pulsar well. It just seems to emit these giant radio pulses every now and again. So what we did with MeerKAT is we basically stared at it for a couple of hours. I think we, the original work had three sessions of two hour observing, and then you try and find when this pulsar is emitting it’s giant burst.

And we picked up about 800 of these pulses. And they know you get to play all sorts of games. It’s like having a new camera. You switch on MeerKAT and the sensitivity is just, you know, it’s so much better than for example, what we were used to looking at Parkes data with this pulsar. So all of a sudden we’re just seeing new features. And we see these pulsars have got interesting frequency structure that we hadn’t seen before. And so, yeah, I’m looking at what we can learn about this pulsar and its environment, looking at these giant pulses and their properties.

[00:17:23] Dan: Why do you think it’s irregular. What’s going on?

[00:17:27] Marisa: Yeah, let me be a bit more clear. So it’s regular in the sense that it’s got a 50.8 millisecond period, meaning I know the pulsar is rotating regularly. But I don’t for every rotation see emission from the pulsar. So that’s kind of the irregular bit is not in every rotation is there a bright enough emission that I can pick it up with the telescope. This is this a little bit similar to, or actually I guess quite similar to what we see in the Crab [pulsar]. So Crab also has these giant pulses, but for Crab we see the regular emission as well.

So with every rotation if your telescope is sensitive enough, you see a pulse from the Crab pulsar. But for this pulsar, which is also in a supernova remnant just like the Crab, you only see the very bright ones. And so I guess one of the obvious questions now is, is there regular emission with every rotation and I just can’t yet see yet? Or is this pulse are really only emitting the giant pulses from time to time?

[00:18:33] Dan: Well, we’ll report back as soon as you’ve figured it out.

[00:18:37] Marisa: Indeed stay tuned.

[00:18:42] Dan: So the other thing you work on, which we wanted to chat about today was something called fast radio bursts. Now, that’s something which isn’t regular.

[00:18:51] Marisa: No, no. And in fact, I snuck this pulsar of mine into a fast radio burst conference recently, for exactly that reason that we think there are some overlaps between the bright end of what pulsars are able to emit, be it these irregular giant pulses, and sort of the lower end of what we see from what you’ve just introduced. These fast radio bursts.

[00:19:16] Dan: So talk us through it. What is a fast radio burst?

[00:19:19] Marisa: Right. Okay. They are enigmatic.

[00:19:22] Dan: Ah, lovely.

[00:19:25] Marisa: Who knows? I guess, podcast over! No, in fact not. We are learning more and more about them, which is exactly why this is a exciting time to talk about it. They were discovered in 2007 and I think then probably they were most enigmatic.

So the original fast radio burst, though it was discovered in 2007, it was actually just found in archival Parkes data. I think the data itself was taken in 2001. And so basically one of the things to note is fast radio bursts is a signature that we’ve had in our time data maybe all along, but it wasn’t very easy to find.

And I think there’s two reasons we didn’t find them before we knew how to look for them. One is that they are once off. So we were joking about pulsars sometimes being regular, sometimes being not so regular. But they’re regular enough so that, you know, if I point my telescope for long enough even on an irregular pulsar, at a point in the sky, I will pick up pulses from the pulsar.

Now this is not the case for this fast radio burst. It was a millisecond radio flash that came and went and was never seen again. And so the way in which we search, for example, for pulsars, isn’t tuned very well for necessarily finding this.

And then the other thing that makes them completely different to our “Galactic” family of pulsars is that we could tell from the very first discovered FRB that they were coming from very far. And so this is what we’ll call from cosmological distances. And you might ask, well, what does that mean? That just sounds like forever away. And I think the point is kind of that yeah it’s forever away.

It’s not from our galaxy. It’s not from even the local groups. So if you zoom out of the Milky Way, the Milky way has got, you know, sort of its neighborhood systems, which is the Local Group. And you can kind of zoom out and then galaxies can form clusters.

And you can continuously zoom out until you’re at what we call the observable universe. And so most of the FRBs, or maybe I should say many of the FRBs that have been discovered, seem to be coming from these across the Universe distances.

And now you should ask, how do we know that?

[00:21:55] Dan: Firstly, I want to ask…so you saw the first one in 2007. How many have we seen since? Because presumably, you can’t look for these things. It’s just luck.

[00:22:10] Marisa: No, well, we have gotten quite good at looking at it, or particular instruments have gotten quite good. So the theory is thus at the moment. So your biggest chance of finding them is just by staring at the whole sky and picking off the brightest pulses. Right? So if you’ve got a telescope and it’s like staring at a particular point in the sky and you just hope that at some point there will be a bright pulse, it’s quite unlikely that you’ll find it.

But now there are telescopes such as CHIME. This is probably the most prominent FRB telescope running at the moment, which has a 200 degrees field of view. So it’s this cylindrical form telescope that kind of just sees a… think of it as like it’s staring at a slice of the sky all the time. And as the sky rotates, that slice is kind of moving across star space. And so in a very short amount of time, you basically can see most of the northern sky.

And so, yeah, the logic is if you can just stare everywhere and if we know that these bursts are sort of happening isotropically, then with luck, you’ll find a bright one.

And you were asking, where are we now? So yeah, this is actually incredible. This telescope I was just talking about in Canada, CHIME, they published their first, what they call FRB fast radio burst catalogue. And it contains 535 FRBs.

Now that’s incredible. Just a year ago in the 2019 version of this conference I just attended, I think we only had about 60 sources, 60 FRB. So now it’s almost a 10 fold increase just through that one catalog. And that’s just the published catalog of this one particular telescope.

If you add what we know from other telescopes, then we have about 600 FRBs. And if you add all the unpublished sources that live in the hallways of Universities, then we’re across a thousand. So in just one year, we’ve kind of gone from the tens of sources to the more than a thousand FRBs known.

[00:24:27] Dan: That’s cool. A couple of weeks ago we were talking about big data. So this is like the opposite of big data because we’ve only got a thousand objects.

[00:24:34] Marisa: It used to be. Back when the first one was discovered, did I say that was 2007 when it was published, it was such a new phenomenon. People now had to start thinking, what is this bright radio burst coming from across the cosmos. That for a while there, the joke now goes is that we had many more theories for what they were than we actually have bursts to kind of support any one of these theories running wild.

And now we’re at the point where, you know, if you’ve got 600 to 1000 of a type of thing, some of them repeating some of them not, you can start doing statistics and you can start asking questions.

How are the bursts different from one another? Or what qualities do they share? So I think that’s quite exciting.

[00:25:20] Dan: So before we get into how you know how far away they are, and then we can start talking some of the remaining theories of what they are. I wanted to ask about something else. There was a Nature paper in 2015. I’m sure you know the one I’m referring to. About fast radio bursts at Parkes where they discovered that the…

[00:25:45] Marisa: Lunch time fast radio bursts!

[00:25:47] Dan: Lunch time fast radio bursts, right! And they were coming from people heating up their food in the microwave.

[00:25:54] Marisa: That’s right. So, gosh. Yeah. It’s funny how I’ve actually forgotten about it. We have a name for that. They’re called perytons. So perytons are now the FRBs that are not FRBs but rather microwave induced FRBs or human induced FRBs.

Yeah, you can imagine that with a signal that’s just once off, you’re limited in the amount of ways in which you can double check that it’s astrophysical. If a source is repeating and you can always point your telescope at the point of sky and, you know, reconfirm reconfirm that it’s there, it’s easier.

And so when this field started out, this was a real issue to learn how to make sure that we know whether they’re true or not. And yeah, exactly. In Parkes, having discovered the first one, when they started looking through more data and recording more data looking for fast radio bursts, they noticed signatures that looked at not too dissimilar.

And they could tell that it wasn’t quite astrophysical, but it wasn’t clear what it was. And so finally they realized if you opened up the microwave door of the canteen, because the Parkes is such that the telescope itself is up on the roof of the building.

So you’re kind of working and eating and doing your research in a three story building and the dish is upon the roof. Yeah, lunchtime. People were too impatient to wait for the microwave to finish. They’ll just, you know, you hit the escape button and the door flings open. And in that process, you make an interference signal that looks not too unlike a fast radio burst.

[00:27:30] Dan: Wow, I mean, let that be a lesson to you, then you wait for the microwave to go beep beep beep.

[00:27:36] Marisa: Yeah, exactly. Oh that lesson! Yeah I thought this was going to be a lesson in double checking your science.

[00:27:42] Dan: Ah yes. No, no that too. So, I mean, presumably the Australian radio astronomers now have to sort of have cold food for lunch.

[00:27:52] Marisa: Yeah. Down to salads only.

[00:27:55] Dan: Salad only. Ok. So we managed to work out those ones and eliminate them from our sample.

Now, how do you work out… well, firstly, if it’s a single occurring event, how do you work out where it is in the sky and then how far away it is?

[00:28:12] Marisa: The easier of your two questions is how far away it is, ironically. Oh, man. I wish I could show you a plot, but now I’m going to try my very best.

[00:28:21] Dan: We can stick it on the blog.

[00:28:24] Marisa: Yeah, maybe we should afterwards, but this is a good exercise in language. If you think of a light burst coming from whatever is generating this FRB. It’s a single shot. And this shot of radio waves is made up of multiple frequencies. This is how we know nature produces radio waves that contains many, many frequencies within. Actually, that’s why we build telescopes like MeerKAT to have a broad band. It’s running from 800 Megahertz up to 1.7 Gigahertz because we want to see everything that these astrophysical objects are sending out across all these frequencies.

So you’ve got this pulse, this broadband, multi frequency, pulse going off from whatever it’s generating the FRB.

And now I don’t yet know how far it is. But wherever it is, it now has to travel to me. I am on Earth with my telescope waiting for this signal. However, it needs to travel through many media to get to me. Most notably through things like the interstellar medium. If it’s in our own galaxy or even the intergalactic medium, if it were to come from others.

And this medium… though we like to think of space as a vacuum because it’s so much less dense than what we’re used to, like the air on Earth. It’s not a vacuum. It’s got a lot of free electrons whizzing around. And it’s these, we as astronomers tend to call the environment within our own Milky Way, a cold plasma. It’s not really cold. It’s just partly ionized. And so there’s some free electrons associated with this plasma through which any radio wave has to travel.

So back to the pulse. The pulse was emitted from this fast radio burst, and it’s now traveling through a medium that has free electrons. And these free electrons scatter the light. So in the same way that you can send white light through a prism and you get the rainbow out on the other end. In that same way, these frequencies will now be scattered differently through the interstellar medium or the intergalactic medium.

And so long story short, by the time I get this pulse, that used to be a single shot, multi-frequency pulse, I actually don’t get it all at once. I get the highest frequencies first because they are less disturbed by having to travel through the plasma. They don’t get scattered as much. But the low frequencies, they interact a lot more with the medium they have to propagate through. And I get them a bit later.

What was a single pulse is now sort of this smeared out across frequency shape. And this dragged out pulse that I finally detect at my telescope, the level to which it’s been stretched apart, that tells me how far it’s been traveling.

[00:31:17] Dan: Aha!

[00:31:18] Marisa: Yes. So in a nutshell, if a signal is coming from my Milky Way, I can go like, oh, that’s not too bad. I know the Milky Way really, really well. I’ve spent all these years researching it. I’ve looked in every single direction. I roughly know how many free electrons, or what electron density, to expect, depending on where I’m pointing my telescope. So I know, well, if I point at the Galactic centre, ooh, that’s very dense. There’s a lot of electrons there. Things are going to be a lot more smeared out. Or we say “dispersed”.

If I get a signal coming from that point in the sky, well, if I look up, well, there’s not that much, there’s a lot less free electrons there. Maybe I’m looking through a line of sight that’s not even hitting a spiral arm, then the amount by which my signal is going to be stretched is a lot less. And so I’ve got this perfect, well not perfect, but I’ve got a reasonably good understanding of how electrons across the Milky Way, free electrons, looked like. And by how much they will therefore stretch out any radio pulse trying to propagate through it to me.

So basically when this very first fast radio burst was discovered, it had a stretched-outness that was like 10 times larger than what you would expect just if it was coming from our own Milky Way.

And so you’d be like, oh, no signature could arrive at my telescope looking like this if it had just come from somewhere here in the Milky Way. This thing has traveled very far, very long to have been pulled apart in this manner.

So the quantity we use to say how pulled apart the signal is, is “dispersion measure”. And so whenever you read about FRBs you’ll come across this DM or dispersion measure quantity a lot. And it’s the value of these dispersion measures that tell us that they’re not from close by, they’ve traveled a long time.

[00:33:14] Dan: So of the thousand, most of them are all from far away right?

[00:33:17] Marisa: Yes, absolutely. Although, yeah. I’m not sure if this is the point to reveal another piece of interesting information…

April this year, we had a signature coming from within our galaxy. And it was picked up by CHIME, this telescope that was designed to find all these distant cosmological fast radio bursts. And it was coming from a source within our own galaxy. A source we know reasonably well. Of course, now we know it very well because everybody is now trying to learn more about it.

And so for the first time we had something that looked like a fast radio burst from a source in our galaxy. And as you can imagine, that has really, really driven where models of what we think they are, is going to next.

[00:34:04] Dan: Okay. Give it to us straight. What’s the source?

[00:34:08] Marisa: Okay. So the source is a galactic magnetar. So I don’t know if magnetars have been mentioned here,Daniel?

[00:34:17] Dan: Let’s do it!

[00:34:18] Marisa: Let’s do it. Okay, so magnetar. It’s in the name. It’s basically just like the overly magnetised pulsar. So it’s again, it’s a neutron star, but it’s this different neutron star in the sense that it’s got a magnetic field that is, oh, I don’t know, just so much more strong than what we have for a pulsar.

And so we think that in a pulsar where you’ve got a neutron start spinning, it’s the rotational kinetic energy that’s really allowing this pulsar to produce emission. But in the magnetar, we think it’s this incredible magnetic field that is allowing… it’s kind of the reservoir to send off these giant bursts.

So this particular magnetar in our galaxy, it’s called SGR1935+2154.

[00:35:11] Dan: Wow. Good memory. Or did you have that written down?

[00:35:14] Marisa: I had to look into the left corner of my eye, wherever the memory sits.

Right. So it emitted a pulse that was a mega-jansky pulse. So if most of pulsar astronomy kind of lives on the milli-jansky scale. Maybe like a hundred milli-jansky scale. This pulse was mega-janskys.

So we’re talking about something that’s 10 million times brighter than what we’ve seen from this particular source before. And so we’ve seen the source emit gamma rays and x-rays and so on before, but we’ve never seen it emit in the radio and we’ve never seen it emit anything like this. And there were two bursts coming in and even at the same time, it was detected in x-rays.

Sort of all of the news I’ve given you up to now about fast radio bursts and the fact that we now have hundreds of them, that’s all radio information. And now for the first time we also have sort of coincident x-ray emission. Now of course it’s a lot harder to observe x-rays, you know, at cosmological distances. So that’s kind of why. But this, the fact that we’ve got x-rays coincident with this galactic magnetar and the fact that it’s a magnetar, that kind of really fuels, I think the direction in which we now tend to think of what is generating most or at least a subgroup of the FRBs we observe.

[00:36:45] Dan: And do we have an idea why it’s a once-off?

[00:36:49] Marisa: Okay. Actually very, very good point.

So from the group of 600 FRBs, there’s maybe like two dozen of them that do repeat. In 2016, the first one was found to repeat for a long time. That was just called “The Repeater” because it was the repeater. Now, of course you can’t just say The Repeater anymore because there’s two dozen of them.

But that was already before the discovery of this galactic Magnetar. That was already a critical shift in understanding what could be causing these bursts. Because now you’ve got most of them happening once off. And then some of them repeating. But the ones who are repeating… when I say repeating, it just means that on more than one occasion we have observed bursts from this fast radio burst or this point in the sky.

It doesn’t mean that they come in every second, like a pulsar, a spinning neutron star does. So there’s only two or three for which we now see something that we can call periodicity. But even in that case, you’ll see from the numbers I’m going to use that it’s sort of more slow varying processes.

So, “The Repeater”, the original repeater for which we probably now have over a hundred individual pulses, that comes and goes in flares of activities. So it kind of has a period of 160 days. So roughly every 160 days you expect it to flare up again, and then you get pulsations from it. But there’s no short term period that sounds like something like a neutron star spinning.

So we still don’t really know what does this half a year period mean? Is that a process building up and then finally, you know, reaches some sort of threshold and you’ve got all this emission? Or is it some long-term period? The way we know stars can be in binary and you can have some sort of process through which at a different part in the orbital phase, you’ve got emission. There’s all sorts of models really still up for grabs for these things. But at least we have some form of repetition. Not all of them are once off.

But those sources where there is repetition, we know that it can’t be a destructive cause anymore. So back in the days when we had more models than sources, many of these models were destructive or cataclysmic. So you’d have like, you know, neutrons stars bashing into one another and creating this strong pulse of emission. But that can’t repeat. You can’t have stars like smashing into each other every 160 days.

So at the moment, the field is in a position where from several hundred FRBs, you’ve got some repeating and some not. And this can mean that either there’s completely two classes of objects. There’s also some sort of defining characteristics that are different between the pulses from these either repeaters or non repeaters.

Or it’s still the same class of object. Maybe some sort of magnetar in a specific state. Often people are talking along the lines of very young magnetars, so maybe they’re still erratic. They’re not emitting quite regularly. The ones that don’t repeat, we will either still see to repeat, or they’re just sort of in a different evolutionary phase to the repeating ones.

So yeah, definitely still many models up for grabs. But the fact that we’ve had some repeat means that, okay, compact objects such as neutron stars are definitely a good model to gamble on. Especially because the pulses have got such short duration, like only milliseconds. And especially because they’re so bright. So we think it must be some sort of coherent emission. And we know that pulsars emit coherently.

Coherently means that the radio waves that the pulsar is emitting is in phase. So we know that the bursts that we observe are so bright, that it’s quite likely that it’s coming from a coherent source. What are the other coherent sources we know? Neutron stars, mmm but neutron stars aren’t typically bright enough.

Ooh. But now we’ve seen this Galactic magnetar. And so, you know, none of this is settled, but there’s a lot more links pointing towards that the source could actually be a magnetar.

[00:41:13] Dan: Alright. Very interesting. So just to wrap up, is this something which your work at MeerKAT is going to help answer?

[00:41:21] Marisa: Yeah. So MeerKAT has its own sort of fast radio burst finding program. It’s already observed The Repeater. We’ve published on that. We did follow up on this Galactic magnetar. Did not see any bursts. So it was just that short window of activity. And then that source was gone. But the main program that’s looking for fast radio bursts is called MeerTRAP.

And the logic here, and maybe you would have picked this up from what I said earlier, is that basically you just want to spend as much time as you can scanning the skies. So if like, for example, the CHIME example where we know they are quite successful at finding FRBs. If you can equally spend as much time as you can on-sky and just from that data then try and find these bright pulses, then you’re most likely to actually catch an FRB.

So MeerTRAP is a commensal program. So basically it means when any other PI is looking at their favourite galaxy or doing whatever research they’ve applied to do on MeerKAT, then MeerTRAP is commensally also recording that data and in real time searching for pulses.

So it’s a real-time pipeline. Which means you try and find a bright pulse. You save what you think is a candidate and you delete everything else. Just because the data rates are too high to store everything.

And the program is also quite clever in how they record data. So you can think of using an interferometer in kind of two ways. So you can either do beam forming. So what I also use when I look at a pulsar. So you make sure you’ve got sensitivity to a very small part of the sky, because it’s a point source and you’re not trying to image. So that gives you a very high accuracy or spatial accuracy. And then you can simultaneously also record the data as if you’ve just incoherently added all the dishes. Which is the same to say, you’re pretending to use the telescope as if it just has one massive dish.

So MeerTRAP does both of that simultaneously. So you expect to find the bright pulses coming into your large field of view, the incoherent beam. And then hopefully you can, after the fact, use the interferometric data to really pinpoint with high spatial accuracy where the FRBs coming from.

So in this 2020 conference, I think we showed already 10 FRBs discovered by this MeerTRAP program. They’re not published yet so keep an eye out. There’s actually a couple of very exciting things that will, I think, soon be up. But I’ll let them reveal the news themselves.

And I think just to kind of summarize that important idea of being able to localize FRBs well. That’s kind of the bit that we’re only starting to do well for FRBs lately. So you want to be able to say, oh, this FRB comes from that galaxy or from that supernova remnant or from that globular cluster or some particular area in the galaxy so that you can learn more about the environment it’s coming from.

Yeah, at the moment that’s still a bit of a riddle, from the set of host galaxies that we have, there’s about 14, it’s a mixed bag. So some are coming from dwarf galaxies. Some are coming from spiral galaxies. Some are coming from old globular clusters, which doesn’t quite make sense. If you want them to be magnetars then you want them to be coming from sort of, you know, young stellar populations and yeah, so this mystery is far from, I think far from being over. A lot of good work for MeerKAT to do still.

[00:45:05] Dan: Oh, that’s great. we love a good mystery. And as you, as you started with, it’s an enigma, which is very appealing to an astronomer.

[00:45:13] Marisa: Absolutely. Yeah.

[00:45:15] Dan: Well, Marisa, thanks so much for joining us on The Cosmic Savannah. Is there anything else you’d like to share with our listeners before we wrap up?

[00:45:23] Marisa: Maybe what would be quite nice in this context is just to say that time domain astronomy in South Africa and across Africa is still very much in its sort of development stage. We don’t have a long history of doing time domain astronomy. And so with that, I mean, we don’t have a long history of studying pulsars and FRBs.

So if students are quite keen on this type of work, I’m sure you’ll put my details up somewhere where the podcast is at.

[00:45:50] Dan: We absolutely will.

[00:45:51] Marisa: Great. Please tell them to get in touch. Because that’s really something we’re hoping to develop. There will be a workshop on this type of work towards the end of September so I can also share the details for you for the website for that. And yeah, I think that’s about it.

[00:46:11] Dan: Great. Yeah. We’ll definitely share those details and good luck with the workshop. I hope you get a lot of attendees.

[00:46:18] Marisa: Thankyou, yeah.

[00:46:19] Dan: And yeah. Thanks again for joining us. Take care.

[00:46:23] Marisa: Thanks Daniel. Cheers.

[00:46:26] Dan: Cheers.

[00:46:35] Jacinta: Thanks for that. Dan, that was absolutely fascinating. I didn’t know anything about FRBs before this. I mean, I had heard, I knew that they were something that had been detected, but I had no idea how many had been found. The last I heard there was like a few and they had no idea what they were, but now this Magnetar that was in our own galaxy, the Milky Way. 10 million times brighter than anything we’ve seen it emit before at a mega-jansky! I’ve never even heard that unit used before, it’s so big. Wow. That

was so awesome.

[00:47:07] Dan: Yeah. Very cool. I mean, wonderful to think that we are reaching the point where we can understand what these things are. You know, going from 2007 and in less than 15 years we built detectors which can now detect them.

And we’re detecting more and more and more and better understanding them. And it’s wonderful the rate of progress. I wanted to ask you too, though. Didn’t you work at Parkes?

[00:47:30] Jacinta: I did. Yes, I did.

[00:47:31] Dan: Did you open the microwave?

[00:47:33] Jacinta: Look, look, it might’ve been me. I don’t know. I don’t know what day it was that they found this object. But, okay. So this situation is this. The microwave if you are using it and then you open the door without pressing stop first, you know how like you can just open the door and it will stop itself. A burst of microwaves comes out of the oven at that moment. And that’s what was being picked up as the FRB. But if you press stop first and then open the microwave, that’s okay.

So you are still allowed to have lunch in a microwave at Parkes, but you just have to press stop first. So the last time I went to Parkes, there was a notice, there was a note on the microwave, like saying you must press stop first.

[00:48:13] Dan: That’s brilliant. So the other thing which Marisa mentioned was when we were talking about pulsars was the Crab Nebula.

That’s a very, very interesting object.

[00:48:23] Jacinta: Yeah. That’s a pulsar, which is a remnant from the very first supernova that was ever observed or recorded to be observed from the Earth.

[00:48:31] Dan: Yeah. So it was recorded by Chinese astronomers in 1054, the supernova explosion. And now we can look at the Crab Nebula, which is a supernova remnant. It’s a very, very, pretty one. And we, we know exactly when it exploded, so how long that cloud of gas has been expanding and we can identify the pulsar in the center of it

[00:48:55] Jacinta: Yeah, I would love to see a supernova go off, like not too close to us, so that we’re all good. No, no radiation, dangerous radiation coming to us. But it would be awesome to see it in the night sky. But yeah, kind of speaking of the pulsars, and then this Magnetar, which is a very highly magnetized pulsar.

Just really exciting that this is one candidate for what could be causing either all FRBs or a particular type of FRB. And then MeerTRAP, Marisa mentioned MeerTRAP, which is the FRB survey that’s being conducted with the MeerKAT telescope. And she got into quite a few technical details there, but basically what she was saying is that MeerKAT is good for both just noticing that there’s an FRB in the sky, so like spotting it. But then also pinpointing its location. So finding out exactly where it is. So it’s actually quite hard to do both of those things simultaneously. So it was actually really cool that MeerKAT can do both of those things.

[00:49:47] Dan: Yeah it’s an incredible telescope, MeerKAT.

And I think that, you know, coming into the SKA, the things which are going to come out of this.

[00:49:54] Jacinta: We say that every episode.

[00:50:00] Dan: We’re too excited.

[00:50:01] Jacinta: Actually I had a question for your Dan. I didn’t realize that you previously did theoretical physics and that you only started astronomy in your PhD.

[00:50:08] Dan: Maybe it’s something I intentionally keep quiet. No, it’s not. I think it was yeah. Getting into physics and studying. I was mostly interested in the computational aspect actually.

So, in those days. Going back how many years now?

[00:50:33] Jacinta: Oh you’re not that old.

[00:50:34] Dan: Almost 20 years.

[00:50:35] Jacinta: It was not 20 years ago! Oh, it was almost 20 years ago.

[00:50:37] Dan: Right. As you were folks. So, yeah, when I was still starting to study the idea of using computers to solve physical problems was relatively new and novel. Obviously it had been done, but it wasn’t easy. Nowadays all astronomy is pretty much done on computers and the data reduction. So that was kind of the grab for me.

And I dabbled in a couple of different types of physics before settling on astronomy.

[00:51:04] Jacinta: Oh, cool. I did a physics degree as well, and I was just like Marisa. Really not interested in the experimental part. I have very little skill with practical things with my hands. I wasn’t particularly interested in the computational or theoretical side of it.

I was interested in the concepts, but not actually in doing, you know, the calculations and that sort of thing. And so observational astronomy was perfect for me because it was just exactly what I wanted to do, which was study astronomy, get some physics in there, get some maths in there, get a little bit of programming in there and then kind of bring it all together with telescopes.

So, yeah, it’s interesting how we each have our different kind of preferences within what seems to be a relatively small area or field, but it’s actually quite broad.

[00:51:50] Dan: Yeah. And now that we’ve gone down this avenue, it’s probably worth mentioning to younger astronomers or students who are listening to this and who are interested in studying astronomy, that astronomy is a broad field.

There isn’t a single path into it. And there’s a lot of different roles that need to be played and a lot of different interests, which are met in the field of astronomy. So don’t think that there’s only one way in, and that you must do X, Y, and Z if you want to be an astronomer. But you must do algebra, which involves XYZ. Alright, I think that’s it for today.

[00:52:33] Jacinta: Okay.

[00:52:34] Dan: Thanks very much for listening and for putting up with my jokes. We hope you’ll join us again for the next episode of The Cosmic Savannah.

[00:52:41] Jacinta: You can visit our website, thecosmicsavannah.com, where we’ll have the transcript, links and other stuff related to today’s episode.

[00:52:49] Dan: You can follow us on Twitter, Facebook, and Instagram @cosmicsavannah.

That’s Savannah spelled S A V A N N A H. And special thanks today to Dr. Marisa Geyer for speaking with us.

[00:53:01] Jacinta: Thanks to our social media manager, Sumari Hattingh. Also to Mark Allnut for music production. Jacob Fine for sound editing, Michal Lyzcek for photography, Karl Jones for astrophotography and Suzie Caras for graphic design.

[00:53:14] Dan: We gratefully acknowledge support from the South African National Research Foundation, the South African Astronomical Observatory and the University of Cape Town Astronomy Department.

[00:53:23] Jacinta: You can subscribe on Apple Podcasts, Spotify, or wherever you get your podcasts. And we’d really appreciate it if you could rate and review us or recommend us to a friend.

[00:53:32] Dan: And we’ll speak next to time on The Cosmic Savannah.

Episode 40: The mystery of the Fast Radio Burst

with Dr Marisa Geyer

This week’s guest

Related Links

Acknowledgements

Transcript