A fast radio burst (FRB) is a short burst of intense radio energy originating from outside our galaxy. We aren’t sure what causes FRBs, though the likely candidate is a white dwarf or neutron star falling into a black hole. They only last a few milliseconds, which makes them a challenge to study, but their brief duration may also allow us to test the limits of general relativity.

If relativity is wrong, then different wavelengths from an FRB should arrive at different times. Credit: Purple Mountain Observatory, Chinese Academy of Sciences

The foundational idea of general relativity is known as the principle of equivalence. On a basic level it states that two objects of different masses should fall at the same rate under the influence of gravity. The principle is necessary to equate the apparent force of gravity with a curvature of spacetime. So far all tests of the equivalence principle have confirmed it to the limits of observation, but there’s an interesting catch. Since relativity also states that there is a connection between mass and energy, the equivalence principle should also hold for two objects of different energy. Specifically, two beams of light with different wavelengths (and therefore different energies) should be affected by gravity in the same way.

We know that the path of light is changed by the curvature of space (an effect known as gravitational lensing), but the curvature also affects the travel time of light from its source to us (known as the Shapiro time delay). According to relativity, the amount of curvature and the time delay shouldn’t depend upon the wavelength of light. This means we can in principle use FRBs to test this idea.

Since FRBs only last milliseconds, they provide a sharp pulse of light at a range of frequencies. If relativity is correct, then the pulse we observe won’t be affected by gravity. If the equivalence principle is wrong, then shorter wavelengths of radio waves from the burst could arrive at a different time than longer wavelengths. We already see different wavelengths arrive at different times due to the interaction between the radio waves and the interstellar plasma in our galaxy, but we know from other observations how much that shift should be. The key is to test whether there is an additional shift not accounted for by standard physics.

Relativity is an extremely well-tested scientific theory, so I wouldn’t count on FRBs showing an energy-based effect, but it’s great that we could have yet another way to test our model. It’s a win-win, since we’ll either confirm our theory yet again, or we’ll discover something new to explore.

Paper: Y. F. Huang & J. J. Geng. Collision between Neutron Stars and Asteroids as a Mechanism for Fast Radio Bursts. arXiv:1512.06519 [astro-ph.HE] arxiv.org/abs/1512.06519

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One of the challenges is radio astronomy is keeping your data free from extraneous signals. Humans use radio waves and microwaves for everything from transmitting music and mobile phone data to cooking a quick dinner. For this reason radio telescopes are often in fairly isolated places, such as the radio quiet zone in West Virginia or the ones in Australia. These limit the amount of contamination from stray signals. But astronomers who work at these remote telescope sites also have to live there, and they utilize things like microwaves in their daily lives. Sometimes that leads to some interesting science.

You might remember a while back I wrote about some strange signals being detected by the Parkes radio telescope in Australia. They were short, seemingly intense bursts of radio energy that would appear in the data from time to time. They seemed to fall into two groups. The first, known as perytons, had all the markings of being terrestrial. They were detected by a range of detectors rather than specific ones, and they didn’t show evidence of frequency dispersion, which is seen in deep space radio signals. The second type is known as fast radio bursts, or FRBs. These are also short lived, but have frequency dispersion and seem to come from specific directions in the sky.

The general consensus has been that FRBs are astrophysical in nature, while perytons are terrestrial. But it wasn’t entirely clear what perytons were. Now a paper published on the arxiv seems to have found the cause, and it’s hungry astronomers. The team began to wonder how a burst of radio energy might be produced locally. Since Parkes is in a radio quiet zone, that would seem to make the most sense. One obvious possibility is a microwave oven, but these operate at about 2.5 GHz, while the peryton signals were around 1.4 GHz.

The Cosmic Microwave Oven Background

But when the team looked through the peryton data, they found that each peryton was accompanied by a 2.5 GHz signal. They also found other 2.5 GHz signals that weren’t accompanied by perytons. The team speculated that perytons could be due to a microwave oven when the door is opened while still running. When you open the door of an active microwave oven, there is a short burst of radio waves as the oven is still powering down. This burst of energy isn’t harmful, but it could easily be picked up by a radio telescope. To test this idea, the team looked at where the telescope was aimed when each peryton was detected. Sure enough, a microwave was in the line of sight each time.

Just showing a correlation between perytons and an astronomer’s desire for hot pockets isn’t proof, but it seems reasonable given the evidence. The authors point out that if a peryton is detected without a corresponding 2.5 GHz signal, then that would disprove this hypothesis.

It’s important to point out that this does not demonstrate that FRBs are caused by microwave ovens. Perytons and FRBs are distinctly different in many ways. In fact, demonstrating a terrestrial origin to perytons helps us narrow down the astrophysical causes of FRBs, since it provides a simple way to distinguish them.

Paper: E. Petroff, et al. Identifying the source of perytons at the Parkes radio telescope. arXiv:1504.02165 [astro-ph.IM] (2015)

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There’s something strange about fast radio bursts, but they are not a message from aliens.

A fast radio burst (FRB) is a short, intense pulses of radio energy that have all the hallmarks of being astronomical in origin. Initially they were only detected at the Parkes radio telescope in Australia, which would make some kind of terrestrial origin likely. Later an FRB was detected at the Arecibo observatory in Puerto Rico, which made in more likely to originate from space. Earlier this year an FRB was detected in real time by multiple telescopes across the globe, which pretty much confirmed the origin as beyond Earth.

The FRBs seem to have discrete dispersion measures

One of the hallmarks of their astronomical origin their spectrum is dispersed. That is, instead of being a simple burst with all different frequencies happening at once, the frequencies were spread out, with higher frequencies 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 time between the arrival of the high and low frequencies can be used to calculate the dispersion measure. Since the greater the dispersion measure, the more gas and dust the signal has travelled through, it is a good way to estimate the distance of the source.

This is where FRBs start to get really weird. It turns out the dispersion measure of these fast radio bursts seem to occur in multiples of 187.5 pc/cm³. Since the dispersion measures put the distances to these FRBs as billions of light years away, that would imply that their distances are evenly spaced across the universe, which isn’t likely for a natural phenomenon. A more likely solution is that they are much closer and originate within the Milky Way. The dispersion we observe would then be due to some unknown process rather than interstellar medium. This new process would need to have some mechanism to account for the discrete dispersions. That by itself would be extremely interesting, since it would demonstrate that dispersion can occur at the source as well as through interstellar interactions.

There is another aspect of these FRBs that’s interesting, and would imply an even closer origin. It turns out that their timing is very suspicious. They always occur within a tenth of a second of an official integer second in UTC (coordinated universal time). That’s really suspicious for something supposedly interstellar in origin, and would point toward something like stray signals from mobile phone towers and the like. Were I a betting man, I’d place my money on the terrestrial horse.

It should be emphasized that while this is interesting, it is an analysis of only 11 data points. We’ve only detected a handful of FRBs, and until we detect many more any speculation on patterns and origins should be made cautiously. Unfortunately most of the headlines want to spin the idea that since they appear both cosmic in origin and discretely spaced FRBs could be caused by some extraterrestrial intelligence. But going directly from “we don’t know” to “therefore aliens” is the realm of science fiction and hack journalism, not science.

Paper: Michael Hippke, et al. Discrete steps in dispersion measures of Fast Radio Bursts. arXiv:1503.05245 [astro-ph.HE] (2015)

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A few days ago I wrote about an interesting type radio signal known as a fast radio burst. These are short, intense pulses of radio energy that have all the hallmarks of being astronomical in origin.  One possible source of FRBs could be a neutron star collapsing to a black hole. But there is still some discussion that such bursts could be terrestrial in origin because of another type of radio burst known as a peryton.

The first FRB is known as the Lorimer burst, named after Duncan Lorimer, whose pulsar research group discovered the burst. The Lorimer burst had a couple of distinguishing features that point to it being astronomical.  The first is that its spectrum is dispersed. That is, instead of being a simple burst with all different frequencies happening at once, the frequencies were spread out, with higher frequencies 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.

Another feature of the Lorimer burst is that it was localized within the detectors.  The Lorimer burst was observed at the Parkes radio telescope (seen in the figure), which has a 13 beam receiver. Each beam is the radio equivalent of a pixel in a digital camera.  The burst maxed out (saturated) one of the beams, but the others were largely unaffected.  This indicates that it was from a single astronomical direction, and not a local electromagnetic burst.

Since the Lorimer burst, there have been detections of at least four similar bursts, which is why there is an effort to determine what their astronomical origin might be.  But this is where it gets interesting, because in addition to the Lorimer-type bursts (FRBs) there is another type known as perytons.  Perytons have a similar intensity, and are also dispersed, but they max out all 13 beam receivers.  This means they are not from a directed astronomical source, and are therefore terrestrial.  These perytons only appear at Parkes, which is also the only place to have observed the FRBs.

Since both FRBs and perytons have only been detected at one radio telescope, you might thing that points pretty strongly to both of them being some kind of local interference or glitch unique to Parkes, but that isn’t the case.  For one, the Parkes radio telescope is particularly suited to detecting this type of signal, so it isn’t too surprising that only Parkes has detected FRBs. For another, perytons are distinctly different from FRBs. While both are dispersed, they are dispersed in different ways, and the peryton dispersion doesn’t match that of an astronomical source.

For now, it seems like Lorimer-type FRBs are astronomical in origin and perytons are terrestrial in origin. Beyond that they are a mystery.

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