From Sky & Telescope: “Bevy of Mysterious Radio Bursts Finds Second Repeating Source”

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From Sky & Telescope

January 9, 2019
Shannon Hall

CHIME Canadian Hydrogen Intensity Mapping Experiment -A partnership between the University of British Columbia, the University of Toronto, McGill University, Yale and the National Research Council in British Columbia, at the Dominion Radio Astrophysical Observatory in Penticton,British Columbia

More than a decade ago, astronomers discovered that every day the sky sparkles with thousands of bursts of radio waves. These flashes are hundreds of millions of times more energetic than the sun but so fleeting that astronomers miss them time and again.

That has made it hard to pin down the origins of these so-called “fast radio bursts,” or FRBs for short. Yet there are tantalizing hints that they could represent an entirely new class of astrophysical objects. As such, they’re arguably one of the most intriguing mysteries in astrophysics, which makes their often-missed detection even more infuriating.

Luckily, the tides are turning.

A new telescope known as the Canadian Hydrogen Intensity Mapping Experiment (CHIME), nestled in the mountains of British Columbia, has already spotted 13 bursts. And of those bursts, reported January 9th in Nature and at a meeting of the American Astronomical Society, one appears to repeat — an advance that might help astronomers settle its exotic origin.

A Baker’s Dozen

The bursts were detected over a period of just three weeks last summer, while CHIME was running at only a fraction of its full capacity. “Immediately, it was clear that this is good news,” said Victoria Kaspi (McGill University) at the meeting.

First, it’s a resounding endorsement of the telescope’s capabilities. And while Kaspi was hesitant to say just how many bursts might become visible once the telescope is in full swing, early estimates suggest that CHIME might ultimately detect anywhere from 2 to 50 bursts per day — a feat that would truly revolutionize the field.

Second, Kaspi noted that the radio waves from many of these bursts appear to have been scattered on their journey to Earth. That means that the FRBs likely originated in special environments that contain a lot of turbulent gas, such as near a supermassive black hole, a young supernova remnant, or a star-forming region, she said.

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A composite image of the field around the first repeating fast radio burst, FRB 121102 (indicated), showed that the burst came from a dwarf galaxy.
Gemini Observatory / AURA / NSF / NRC

The Gift That Keeps on Giving

The highlight of the bounty is the single burst that flared time and again. First detected on August 14th, CHIME saw it pop up five additional times. The only other known repeating FRB was detected in 2012 and has reappeared hundreds of times since. So, a second “suggests that these repeaters are not as rare as we might have thought previously,” Kaspi said.

What’s more: Both repeaters give important clues about their origins. The sheer fact that the bursts repeat, for example, suggest that they cannot be produced by some one-off cataclysmic event, like a core-collapse supernova or a merger of neutron stars. Both events would only occur once and a second burst would be impossible.

But that’s not all. Both FRBs have another intriguing characteristic: Their frequencies drift downward over time. That means that the first few bursts arrived at the telescope with much higher frequencies than the final few bursts. “This is quite bizarre,” says Jason Hessels (Netherlands Institute for Radio Astronomy) who was not involved in the recent study. “But it’s also exciting because it’s a clue to determining what kind of physics creates this burst.”

So what might cause such a downward drift? Late last year, Hessels attempted to answer that very question with regards to the first repeating radio burst. He argued that the drift could be intrinsic to the burst, meaning the burst starts very close to an energetic source (say, a supermassive black hole) and then moves farther away over time. Such a pattern has been seen before. As solar flares propagate outward, for example, the Sun’s magnetic field strength drops — an effect that causes the flare’s radio emission to similarly drop.

Alternatively, the drift could come from something around the burst. A cloud of extremely hot and electrically charged gas, or plasma, for example, might act as a lens, which would bend the radio waves in much the same way that water bends rays of light.

The fact that the two events look so similar is what most excites Hessels about the newest repeater. “It really suggests they’re of the same ilk,” he says. And while Kaspi agrees that the similarity is “striking,” she notes that we can’t draw any firm conclusions yet.

Astronomers are keeping their eyes on the mysterious burst with the hope that they will be able to tie it to the galaxy it lives in, enabling them to better understand its environment. And of course, they’re also eagerly awaiting the scores of radio bursts that CHIME will soon detect.

References:

CHIME/FRB Collaboration “A second source of repeating fast radio bursts.” Nature, available online on 9 January 2019.

CHIME/FRB Collaboration “Observations of fast radio bursts at frequencies down to 400 megahertz.” Nature, available online on 9 January 2019.

J.W.T. Hessels et al. “FRB 121102 Bursts Show Complex Time-Frequency Structure.” Submitted to The Astrophysical Journal.

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Sky & Telescope magazine, founded in 1941 by Charles A. Federer Jr. and Helen Spence Federer, has the largest, most experienced staff of any astronomy magazine in the world. Its editors are virtually all amateur or professional astronomers, and every one has built a telescope, written a book, done original research, developed a new product, or otherwise distinguished him or herself.

Sky & Telescope magazine, now in its eighth decade, came about because of some happy accidents. Its earliest known ancestor was a four-page bulletin called The Amateur Astronomer, which was begun in 1929 by the Amateur Astronomers Association in New York City. Then, in 1935, the American Museum of Natural History opened its Hayden Planetarium and began to issue a monthly bulletin that became a full-size magazine called The Sky within a year. Under the editorship of Hans Christian Adamson, The Sky featured large illustrations and articles from astronomers all over the globe. It immediately absorbed The Amateur Astronomer.

Despite initial success, by 1939 the planetarium found itself unable to continue financial support of The Sky. Charles A. Federer, who would become the dominant force behind Sky & Telescope, was then working as a lecturer at the planetarium. He was asked to take over publishing The Sky. Federer agreed and started an independent publishing corporation in New York.

“Our first issue came out in January 1940,” he noted. “We dropped from 32 to 24 pages, used cheaper quality paper…but editorially we further defined the departments and tried to squeeze as much information as possible between the covers.” Federer was The Sky’s editor, and his wife, Helen, served as managing editor. In that January 1940 issue, they stated their goal: “We shall try to make the magazine meet the needs of amateur astronomy, so that amateur astronomers will come to regard it as essential to their pursuit, and professionals to consider it a worthwhile medium in which to bring their work before the public.”

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