A rotating neutron star with a powerful magnetic field whips up subatomic particles around it. Artwork credit: NASA / Swift / Aurore Simonnet, Sonoma State University
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A rotating neutron star with a powerful magnetic field whips up subatomic particles around it. Artwork credit: NASA / Swift / Aurore Simonnet, Sonoma State University

Astronomers trace a mysterious radio burst to its source… 3.6 billion light years away!

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Jul 1, 2019

Just last week, astronomers made a big step forward in figuring out what the heck fast radio bursts are: For the first time, a single one-off burst was traced back to its host galaxy, and it’s a whopping 3.6 billion light years away!

Fast radio bursts (or FRBs) are pretty much what the name says: Very brief pulses of radio waves coming from space. And by brief, I mean yikes: The quickest last for a fraction of a millisecond, with some stretching out to several milliseconds. So yeah, fast.

The first FRB was found in 2007, when astronomers were looking at archived data from 2001. Since then something less than 100 have been observed … but we don't know what they are. A big part of the problem is that they flash once and then disappear. That makes following up on them really hard; it's difficult to get a position on them when they're over so quickly.

We do know FRBs are far away, because radio waves traveling through space undergo a process called dispersion, where they interact with ionized gas between the stars and galaxies. This spreads the frequency of the radio waves out in space such that lower frequencies (or longer wavelengths, if you prefer) get delayed a bit more than higher frequencies (shorter wavelengths). This can be used to get a handle on the distance the radio waves have traveled, and the answer is always far. Well outside our galaxy.

That means the sources are powerful. Also, the fact that the burst is so short means something small is making it; big objects like galaxies can’t organize themselves in such a way to emit a sharp pulse — the general rule is that the size of the object has to be no larger than the duration of the pulse in terms of the speed of light. For example, something lasting one second would have to be less than one light-second across, equal to 300,000 km.

In this case, a millisecond pulse means the object generating an FRB has to be very small indeed, just a few dozen kilometers across. That pretty much limits it to neutron stars (which are maybe 20 km across) and stellar mass black holes (from 12 km across up to much larger). But how they might be generating these bursts is still a mystery.

A big break happened in 2012 when an FRB was seen to repeat. This allowed follow-up observations to pin down FRB 121112 (the number represents the date it was seen as YYMMDD) to a dwarf galaxy about 2.3 billion light years away. The galaxy has a high rate of star birth, and the radio observations of the burst indicated it was embedded in a dense nebula and had a strong magnetic field, which made it seem likely the burst source was a magnetar, a very powerful form of pulsar. Hooray!

A rotating neutron star with a powerful magnetic field whips up subatomic particles around it. Artwork credit: NASA / Swift / Aurore Simonnet, Sonoma State University

A rotating neutron star with a powerful magnetic field whips up subatomic particles around it. Artwork credit: NASA / Swift / Aurore Simonnet, Sonoma State University

Yeah, not so fast. This new burst, FRB 180924, throws a monkey in the wrench. It was discovered using the Australian Square Kilometre Array Pathfinder, a 36-antenna array. ASKAP generates a lot of data, which are hard to archive. So astronomers did a clever thing: They created an observation mode specifically for fast pulses from space, where the data are stored in a "ring buffer" for 3.1 seconds, meaning older data are continuously overwritten as new data come in. If the software detects a bright, short pulse it immediately downloads that bit of the datastream to permanent storage for astronomers to examine.

FRB 180924 was just such a burst. The software worked as promised, and for the very first time a single, isolated FRB could be examined in detail!

An observation of the fast radio burst FRB 180924 using the Very Large Telescope reveals it to be near the edge of a disk galaxy some 3.6 billion light years away. The scale bar represents about 16,000 light years. Two nearby galaxies appear to be unaffiliated with the FRB. Credit: Bannister et al.

An observation of the fast radio burst FRB 180924 using the Very Large Telescope reveals it to be near the edge of a disk galaxy some 3.6 billion light years away. The scale bar represents about 16,000 light years.

An observation of the fast radio burst FRB 180924 using the Very Large Telescope reveals it to be near the edge of a disk galaxy some 3.6 billion light years away. The scale bar represents about 16,000 light years. Two nearby galaxies appear to be unaffiliated with the FRB. Credit: Bannister et al.

 

Looking at the data from FRB 180924, they could see the signal arrived at each of the 36 antennas at a slightly different time, meaning they could combine all the data (using a technique called interferometry) to nail down the location of the burst on the sky. Once they did that they cross-referenced the position with an optical catalog and found a galaxy there, called DES J214425.25−405400.81 (from the Dark Energy Survey and the coordinates of the galaxy on the sky). Using the huge Gemini and Keck telescopes, they took spectra of the galaxy and found the distance of 3.6 billion light years, so it definitely wasn't local.

This is where things get fun. The galaxy appears to be what’s called a lenticular or early type spiral, which means it’s decently big (maybe half the size of the Milky Way), but it doesn’t have a lot of gas in it. It’s not making lots of stars, and that means the idea that this came from a magnetar is unlikely.

Magnetars are young pulsars, and pulsars are what’s left after a high-mass star explodes. Stars like that don't live long, so you need active star birth to get magnetars now. The average stellar population of this galaxy is older than 4 billion years, and that makes a magnetar pretty unlikely.

Well, maybe. It could be that the galaxy is making stars, just slowly, and by happenstance made a massive one. But from what we know of how stars are born this is very unlikely. It seems to me that something else created this burst, some other type of phenomenon. It may be related to pulsars, or it may be something else entirely. But given how different both host galaxies are, and both FRBs are, it seems like we're looking at different sources, or at least different events from similar sources.

So that’s difficult, but hey: That's what finding entirely new astronomical phenomena is all about! It reminds me of gamma-ray bursts, which have a fascinating history. We observed a lot of them before a series of breakthroughs allowed us to really zero in on what was causing them … and it turned out there are varying events that cause them (exploding stars, merging neutrons stars, and more).

Still, this is a substantial breakthrough! It means single FRBs can be analyzed, when before we had no idea where they were happening. And since they outnumber repeating FRBs by a factor of 30, this will allow us to get a lot more info on a lot more of them.

And bonus: The radio waves are affected by the gas in galaxies, between galaxies, and magnetic fields they encounter as well. This means that we may be able to use them as probes for material that is otherwise very difficult to study. Neat!

It took a while for us to start to figure out gamma-ray bursts, and to be honest we have a long way to go. But FRBs are now on that same threshold we stood on when we got our first good localizations of GRBs, and we've come a long way since then. I think it won't be too long before we finally start to get a grasp on these weird, enigmatic, and soul-chillingly distant phenomena as well.

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