Nature Podcast：来自银河系的快速射电暴

Host: Benjamin Thompson

On 28 April this year, a powerful burst of radio waves lasting only a millisecond was detected here on Earth. This event, known as a fast radio burst or FRB, is one of the strangest phenomena we've observed in the Universe. FRBs have been known about for over a decade, but scientists haven't been able to explain what causes them until now. This week in Nature, three separate papers describe the first FRB that's been found within our own Galaxy, the Milky Way. Detecting an FRB so close to us has finally allowed researchers to pin down a source. Reporter Anand Jagatia spoke to one of the scientists who spotted the striking signal, graduate student Chris Bochenek from the California Institute of Technology in the US, who explained the long-standing mystery of FRBs.

Interviewee: Chris Bochenek

The first fast radio burst discovered output about as much energy in five milliseconds as the Sun outputs in about a day. All of them until this most recent burst have come from faraway galaxies. And because we can detect them from distant galaxies, that means that whatever is producing it, it must be extremely energetic.

Interviewee: Anand Jagatia

So, these fast radio bursts have puzzled scientists for quite a long time. What are some of the theories out there for what could produce them?

Interviewee: Chris Bochenek

For a long period of time, there were more fast radio burst theories, then there were fast radio bursts. But some of them include blitzars, which is what happens when a neutron star that is too massive to support itself collapses into a black hole. Merging neutron stars could produce fast radio bursts. But I think for a long time, the most popular theory has been magnetars, which are neutron stars which weigh a little bit more than the Sun but are comparable in size to Manhattan. What separates out magnetars from other neutron stars is their incredibly strong magnetic fields.

Interviewee: Anand Jagatia

Okay, how strong exactly is incredibly strong?

Interviewee: Chris Bochenek

So, a magnetar has magnetic fields 100 to 1,000 times as strong as a typical neutron star. And these magnetic fields are so strong that the structure of atoms themselves changes. So, you might think of like an electron cloud around an atomic nucleus as being somewhat spherical. But around a neutron star, the magnetic fields are so strong that this picture of an atom turns into more of like a pencil shape.

Interviewee: Anand Jagatia

So, powerful enough to squash the shape of atoms. Why has it been so hard to pin down fast radio bursts to magnetars if that's the most popular idea for their source?

Interviewee: Chris Bochenek

Because we've been seeing them from so far away. Even if you see fast radio bursts and other galaxies, you're not going to be able to pinpoint it to a specific star. So, the really great thing about this fast radio burst is that it happened so close by that we got to look at it right in the face. And so, this is the first time a fast radio burst has been identified as coming from a specific source.

Interviewee: Anand Jagatia

And that source, as you report in the paper, is a magnetar. So, how do you know that the fast radio bursts actually came from one?

Interviewee: Chris Bochenek

Yeah, so there are a couple ways that we know that this fast radio burst came from the magnetar. One is that this radio burst happened at the same time from the same relative sky location as an X-ray burst. So, we believe that the radio burst and the X-ray burst are related. The X-ray detectors can put the burst within about five arcminutes of the known position of this magnetar, which is pretty close.

Interviewee: Anand Jagatia

Right okay, and this fast radio burst was detected by, among others, a telescope that you were involved in building called STARE2. Can you tell us a bit more about how it works?

Interviewee: Chris Bochenek

Yeah, so STARE2 is a network of three fairly bare-bones radio antennas. What we use to actually detect the radio waves is basically just a six-inch pipe with two cake pans attached around it. STARE2 is a bit of an interesting radio telescope in that we made a very specific choice in its design. We knew that a fast radio burst in the Milky Way would be incredibly bright, so we don't need a very sensitive detector. But we also knew that a fast radio burst in the Milky Way would be a rare event, so we've got to be looking at the right place at the right time. So, we sacrificed orders of magnitude in sensitivity in order to be able to look at most of the sky at any given time.

Interviewee: Anand Jagatia

So, what was it actually like then when you opened the data and you saw there was a fast radio burst there? How did you react?

Interviewee: Chris Bochenek

I can't say we ever really expected that we would find a fast radio burst in the Milky Way. It just seemed like a bit of a wild idea. So, when I looked at my data to see the burst for the first time, I was actually like paralysed, like I couldn't do anything, like I just sat there for a couple minutes thinking like, is this it?

Interviewee: Anand Jagatia

What does this observation actually mean, and does it mean that we now know for sure what causes fast radio bursts? I mean, do all of them come from magnetars?

Interviewee: Chris Bochenek

I think that it's safe to say that some fast radio bursts come from magnetars. One outstanding puzzle is that some fast radio bursts are known to repeat and others are not, and we don't really know why, although magnetars are one of the more popular theories for repeating fast radio bursts as well.

Interviewee: Anand Jagatia

So, what's next now in the field? This is the first time a fast radio burst has been observed kind of directly within our Galaxy. What else do people want to know about them and about where they come from?

Interviewee: Chris Bochenek

I think one of the most important experiments that people are doing with fast radio bursts is trying to detect them with instruments that are capable of determining where exactly within a galaxy they came from so we can do detailed studies of those environments. I also want to do surveys of some nearby more star-forming galaxies that probably do have a lot of fast radio bursts because they have more young stars, which means they probably have more magnetars, which means you might not have to wait as long as we did for a detection.