Fast Radio Bursts (FRBs) are short, intense pulses of radio energy that originate billions of light years away. They have incredibly intense energies, but last for only milliseconds, so it isn’t clear what could possibly cause them. Ideas include a neutron star collapsing into a black hole, the collision of two neutron stars, and even an evaporating black hole. Another idea that makes the rounds is that they are produced by an advanced alien civilization. 

One idea is that perhaps FRBs are used as some kind of intergalactic navigation beacons, similar to the way we could use pulsars to navigate our galaxy. A more recent idea is that they could be created by aliens to send space probes to distant stars, similar to Breakthrough Starshot’s idea to use lasers to send a tiny probe to Proxima Centauri. Going directly from “we don’t know” to “therefore aliens” is the realm of science fiction not science, but team of astronomers recently did a bit more than wild speculation. They asked whether it was conceivably possible for such powerful signals to be created artificially.

In the recent paper, they noted that FRBs have characteristics similar to the types of energy beams that could be used to power large light sails. If FRBs are, in fact, being used to power starships, then they would likely be a long lasting beam of energy directed at the starship. We would see them as a short burst because beam would sweep past us as the transmitter and starship line up just the right way. Calculating the energy requirements for such a beam, the team found that a solar powered array about twice the diameter of Earth could collect enough power to create it, and a water-cooled system orbiting a star could transmit the beam without overheating. In principle, at least, alien FRBs appear to be simply a matter of powerful engineering and not exotic physics.

The team went further and estimated the size of a starship that such a beam could power. Rough calculations put the upper size at about a billion tons, or the mass of about 20 cruise ships. For humans that would mean about 40,000 passengers or so, which is plenty large enough to start a colony on another star system. Given that the alien civilization would be capable of making planet-sized power transmitters, you might figure they would have mastered other things like cryogenic freezing or the ability to clone new members of the species once their destination is reached.

This all sounds like wild science fiction, and it’s almost certainly not true. But the team does point some things worth exploring further. Given the number of FRBs we observe, they probably wouldn’t all be caused by alien civilizations. So there would likely be some key signature differences between natural and alien FRBs. In particular, we now know that some FRBs repeat, which means these particular ones can’t be caused by cataclysmic events such as neutron star mergers. Alien FRBs could repeat, since the orbit of the transmitter could bring it back into alignment with Earth periodically. By studying FRBs that repeat, we might be able to see some kind of pattern that points to an artificial source.

There’s a long history of strange astronomical phenomena that seem alien at first, but turn out to be natural after all. FRBs will likely turn out to be natural as well. But it can be worthwhile to cautiously speculate about alien signals. After all, there are a lot of planets out there, and the existence of alien civilizations isn’t beyond the realm of possibility.

Paper: Manasvi Lingam and Abraham Loeb. Fast Radio Bursts from Extragalactic Light Sails.  arXiv:1701.01109 [astro-ph.HE] (2017)

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Just when you think you’ve got a mystery solved, new data revives the mystery again. It’s a common story in science, and this time its about fast radio bursts. 

Fast radio bursts (FRBs), as you might recall are short, intense, bursts of radio waves. They have indications of being distant in origin, but similar bursts known as perytons were found to be due to local radio noise. Because of their short duration they are difficult to study, or even to verify their origin in space. Recently there was observation of a particularly strong FRB that seemed to be from a distant galaxy. The evidence to support this idea came from two radio telescopes. The first observed the FRB, while the second observed a radio afterglow in the same general region. From theses two observations the source could be triangulated as a distant galaxy.

But new evidence indicates that the radio afterglow isn’t from the FRB, but rather from the material surrounding a supermassive black hole in the distant galaxy, completely independent from the fast radio burst. So the origin of these radio bursts is still a mystery after all.

If that wasn’t bad enough, there’s also evidence that FRBs can repeat. So far these bursts seemed to be one-time events, which would imply they are caused by catastrophic events such as colliding neutron stars or a neutron star collapsing into a black hole. If they repeat that would indicate a transitory origin, such as flares from highly magnetized neutron stars. It’s also possible that FRBs can have multiple causes, with some repeating and some not.

At this point what’s clear is that we don’t have a good understanding of FRBs after all.

Paper: P. K. G. Williams and E. Berger. Cosmological Origin for FRB 150418? Not So Fast. arXiv:1602.08434 [astro-ph.CO] (2016)
Paper: L. G. Spitler, et al. A repeating fast radio burst. Nature doi:10.1038/nature17168 (2016)

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They aren’t local, they aren’t aliens, and they might help us understand dark matter. 

Fast radio bursts (FRBs) are blasts of radio energy that last for only a fraction of a second but are extraordinarily bright. Because of their short duration, and the fact that the same FRB never repeats, they’ve been very difficult to study. They were first discovered in 2007, and for a while they were observed only at the Parkes radio telescope in Australia. Because of this, and the fact that FRBs were so incredibly bright, there was a great deal of speculation that they could be due to local radio interference rather than some new astronomical event. In fact a similar short-burst phenomenon known as perytons were found to be due to stray signals from an unshielded microwave.

Fast radio bursts are somewhat different from perytons, and have two distinct features that imply they are distant in origin. The first is that rather than being a simple burst with a range of frequencies happening at once, the frequencies are spread out, with higher frequencies arriving first and lower ones later. This whistler effect is characteristic of a pulse that has traveled through the interstellar medium. It occurs because when an electromagnetic pulse interacts with charged ions, different frequencies are slowed by different amounts, with the lower frequencies slowed down more. So you get a dispersion effect. Stray bursts or chirps from terrestrial sources generally don’t have the same dispersion because they don’t travel through plasma and they don’t travel far. The second is that the bursts seen by a single radio detector rather than a range of nearby detectors. This implies it comes from a particular point in the sky rather than somewhere near the telescope.

Unfortunately radio telescopes are not good at determining an FRB’s location in the sky. This makes it difficult to determine their cause. This led to a great deal of speculation about their origin, including the idea that they might be due to some alien civilization. Combined with the fact that perytons were caused by a microwave, FRBs began to take on a fringe science status.

But recently the Parkes observatory detected a fast radio burst, then two hours later the Australia Telescope Compact Array in New South Wales saw a fading radio glow in the same region of the sky. Using the two observations to triangulate its position in the sky, a team of astronomers narrowed the source down to an elliptical galaxy 6 billion light years away. To verify this source the team compared the dispersion effect of the FRB signal with the amount of ionized material between us and this particular galaxy as estimated by the WMAP probe. The amount of frequency dispersion agreed with the estimated amount of ionized material, indicating that it did indeed originate from the distant galaxy.

Now that we know they are real astrophysical events, the next step is to confirm their cause. The leading idea is that they are due to neutron stars, either through neutron star collisions or perhaps when a neutron star collapses into a black hole. They might also help us solve the mystery of dark matter. In order to better understand dark matter we need to know exactly how much faint regular matter there is between galaxies. Since the dispersion measure of FRBs gives us an excellent measure of the amount of material between us and a particular distant galaxy, they can be used to help map the distribution of faint matter throughout the universe.

There’s still much to learn about fast radio bursts, but this recent work brings them clearly back out of the fringe.

Paper: E. F. Keane, et al. The host galaxy of a fast radio burst. Nature 530, 453–456 (25 February 2016).

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One of the stranger phenomena in astronomy are fast radio bursts, or FRBs. These are bursts of radio energy that last for a fraction of a second, but are extraordinarily bright. They don’t repeat like radio pulsars, but instead just occur as a single burst. They are so bright and short that for a while it was thought that they were terrestrial in origin rather than astronomical. As we gathered more data, it became clear that these strange radio bursts were really from space. Now it seems we’ve observed FRBs in real time.

The results are being published in MNRAS, and they show an FRB originating from 5.5 billion light years away. The signal was detected by multiple telescopes within hours of the event, so it was possible to see if there was any “afterglow” or secondary emissions after the burst. They didn’t find any, which eliminates some models such as strange supernovae or gamma ray bursts from the list of possible sources. Right now one of the main ideas is that they are caused by large, rapidly spinning neutron stars that then collapse into black holes. To know for sure, we’ll need to observe FRBs at a range of frequencies.

For now we know it is very real, and very strange. But this is what often happens in science. What begins as a mystery leads to answers that point toward an even larger mystery.

Paper: E. Petroff, et al. A real-time fast radio burst: polarization detection and multiwavelength follow-up. MNRAS 447 (1): 246-255 (2015)

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A while back I wrote about a phenomena known as fast radio bursts (FRBs). These short bursts of radio energy have been a bit of a puzzle. On the one hand they they have all the appearance of being astronomical in nature. For one thing, the frequencies of the signal are spread out so that higher frequencies arrive before lower ones. This is known as dispersion, and is an indicator having traveled through the interstellar medium. On the other hand, the signals are unusually strong, and their short duration is similar to radio interference from sources on Earth.  They’ve also only been detected at one radio telescope (the Parkes radio telescope in Australia). That is, until now.

It turns out another FRB was detected at the Arecibo observatory in Puerto Rico. It was actually detected in 2012, but an analysis of this event has recently been published in the Astrophysical Journal. Analysis of this radio burst shows it has the same overall properties as the ones observed at Parkes, which gives credence to the Parkes events being astronomical. But in this paper the authors take the analysis further.  They looked at the dispersion measure and found that the burst had a dispersion three times larger than the maximum dispersion found in objects within our galaxy in that region.  Since the amount of dispersion is a property of the amount of interstellar medium a signal passes through, this means the signal must come from well beyond our galaxy.  So the event is not only astronomical in nature, it is intergalactic.

So it seems that the mystery over the legitimacy of these FRBs is solved, but that leads us to the question about the cause of these bright radio bursts. One idea is that they could be caused by massive neutron stars as they collapse into black holes. Other possibilities could be neutron star mergers, or possibly even evaporating black holes (though this is a bit of a stretch). But now that we’ve identified FRBs, we can look for more of these events in radio observation data. By some estimates there could be 10,000 such events a day. Their short duration simply makes them easy to overlook.

It seems what was thought as merely distant noise is actually evidence of interesting new astrophysics.

Paper: L. G. Spitler et al., Fast Radio Burst Discovered in the Arecibo Pulsar ALFA Survey, The Astrophysical Journal (2014).

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Neutron stars typically form when a large star dies in a supernova explosion. The outer layers of the star are cast outward to form a supernova remnant, while the core of the star collapses into a dense neutron star.  What keeps a neutron star from collapsing under its own weight is the pressure of the neutrons pushing against each other, similar to the way electron pressure works in a white dwarf star. But there is a limit to how much weight the neutrons can counter, known as the Tolman-Oppenheimer-Volkoff (TOV) limit.  This limit means that a neutron star can’t be more massive than about three solar masses.  More than that, and it would collapse into a black hole.  

Because of this limit, it’s generally thought that the supernovae of most large stars produce neutron stars, but the supernovae of really large stars can produce black holes.  But a recent paper in Astronomy and Astrophysics proposes that some really large stars might produce a neutron star that is more massive than the TOV limit, which only collapses into a black hole later.

In the paper, the authors propose that if the progenitor star is rapidly rotating, the resulting neutron star it produces could also be spinning rapidly. Since the TOV limit is for a non-rotating (or slowly rotating) neutron star, a fast-rotating neutron star could be over the limit. Basically, the rapid rotation would cause the neutron star to bulge out a bit, preventing it from collapsing into a black hole.  Of course neutron stars have strong magnetic fields, and this means that they radiate electromagnetic energy as they rotate, which causes them slow down over time.  So eventually these supermassive neutron stars will slow down enough that they collapse into a black hole.

The Lorimer burst, first observed FRB.

What’s interesting about this idea is that it could explain a mysterious phenomena known as fast radio bursts, or FRBs. These are bursts of radio energy that last for a fraction of a second, but are extraordinarily bright.  They don’t repeat like radio pulsars, but instead just occur as a single burst. They appear similar to gamma ray bursts, but don’t seem to have a corresponding burst of gamma rays, or even x-rays.

Neutron stars have a strong magnetic field, but uncharged black holes don’t have magnetic fields (known as the no-hair theorem). So when a supermassive neutron star (with a strong magnetic field) slows enough to collapse into a black hole (which can’t have a magnetic field) the magnetic field must snap free.  This would produce a large burst of radio energy without the burst of x-rays and gamma rays that occur when a supernova-produced black hole forms.  The authors have named such objects blitzars.

So blitzars could explain these FRBs.  To be sure, we’ll need to analyze the spectra in more detail. Right now we only have a handful of FRB observations, which isn’t enough to confirm them as FRBs. There are also other possible solutions to the mystery, such as massive stellar flares, and binary white dwarf or neutron star mergers.

But it could be that these strange radio burst are simply the result a neutron star that has been over the limit.

Paper: Heino Falcke1 and Luciano Rezzolla. Fast radio bursts: the last sign of supramassive neutron stars. A&A 562, A137 (2014).

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