Thursday, November 17, 2016

Action! New Insight on Mysterious Radio Signals

If the story of fast radio bursts inspired a movie, you might find it in the mystery category. Or science fiction. Maybe comedy. Action would probably work too. I don’t know about gangster or western, but the right director could probably make it work. It’s the story of fleeting, mysterious, space-based signals reaching the Earth from unknown objects in unknown locations. You can see the broad appeal.

Clearly, there is a lot that we don’t know yet. Here is what we do know: Fast radio bursts, or FRBs, are short but very bright flashes of radio waves. Most astronomers think that they come from far outside of our galaxy. Fewer than 20 have been detected so far, but it’s likely that if we had the right equipment we could detect thousands of them every day. Today in the AAAS journal Science, an international team of researchers highlight what they’ve learned from the brightest FRB on record, and what it may tell us about the universe.

The 64 meter Parkes Radio Telescope
Image Credit: David McClenaghan, CSIRO (CC BY 3.0)

The first FRB signal was discovered in 2006 by astronomer Duncan Lorimer and his student David Narkovic while analyzing five-year-old data from the Parkes radio telescope in Australia. The astronomy community didn’t immediately embrace their finding; no one had seen anything like it before. Plus, just a few years later, a different researcher saw a similar pattern in data also from the Parkes telescope that ended up being the “exotic” signal of microwave ovens in the lunch room. But FRBs prevailed.

Eventually, signals were also detected at Arecibo Observatory in Puerto Rico and others, and they’ve become an intriguing—and confusing—area of study. There have been conflicting interpretations of some recent data, and opinions abound on what the sources might be. However, there seems to be a growing consensus that they are produced by very energetic processes far from our galaxy.

Although we don’t know yet what their sources are, the signals contain a lot of information about what is between their sources and us. This information could help us better map out how the universe is structured. If you look at a toddler before bath time, you can likely map out much of the day. Noodles in the hair? Spaghetti for dinner. Colorful stained fingers? Got into the markers. Sticky shirt? Bribed with a sucker while doing errands.

Similarly, the signal of an FRB tells us about what it went through to get here. For example, FRB signals contain a range of radio frequencies, but things don’t line up quite right. As you go from higher to lower frequencies, the packets of waves become more stretched out and take longer to arrive. This is characteristic of a signal that has interacted with intergalactic clouds. The time delays indicate that they may have traveled for billions of light years before reaching our telescopes.

The subject of this latest paper is the brightest FRB we’ve ever seen, FRB 150807. Its brightness and means of detection provided the team with the opportunity to narrow down its source more precisely than scientists have been able to for any other FRB. The research was led by Vikram Ravi from Caltech and Ryan Shannon from CSIRO Astronomy and Space Science (CASS) and the International Centre for Radio Astronomy Research (ICRAR).

Most FRBs are detected by telescopes that see a large area of the sky, but don’t have very good resolution. That makes it hard for them to identify the source of a signal that only lasts for a few milliseconds. In this case, however, the FRB was observed in two Parkes detectors. Combined with the brightness, this enabled the researchers to narrow down the origin to a small patch of sky that contains a handful of possible sources. Most of them are galaxies more than 1.5 billion light years away.

From the signal, the astronomers were also able to tease out information on the magnetic field between the FRB source and the Earth. By comparing this with the magnetic field they measured between the Earth and a pulsar in a similar region of the sky but within our galaxy, the astronomers were able to put limits on the strength of the magnetic field in the intergalactic space between the pulsar and the FRB source.

Much remains unknown about FRBs, and even this latest research includes assumptions that the researchers note aren’t confirmed yet. Many expect 2017 to clear a few things up, with radio telescopes like CHIME (British Columbia, Canada), the Molonglo Observatory Synthesis Telescope (Canberra, Australia), and the Deep Synoptic Array prototype at Owens Valley Radio Observatory (California) being optimized to search for FRBs and pinpoint their locations. Whether the bursts are from young rotating neutron stars, magnetars, merging black holes, or other dramatic objects or events, it will no doubt make for a gripping story and help us better understand our place in all of this.

Kendra Redmond

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