Astronomers detect regular rhythm of radio waves, with origins unknown
Signal from 500 million light years away is the first periodic pattern of radio bursts detected

Date:
June 17, 2020
Source:
Massachusetts Institute of Technology
Summary:
A team of astronomers has picked up on a curious, repeating rhythm
of fast radio bursts emanating from an unknown source outside our
galaxy, 500 million light years away.



FULL STORY ========================================================================== [Starry sky (stock | Credit: (c) pixel / stock.adobe.com] Starry sky
(stock image).

Credit: (c) pixel / stock.adobe.com [Starry sky (stock | Credit: (c)
pixel / stock.adobe.com] Starry sky (stock image).

Credit: (c) pixel / stock.adobe.com Close A team of astronomers, including researchers at MIT, has picked up on a curious, repeating rhythm of
fast radio bursts emanating from an unknown source outside our galaxy,
500 million light years away.


==========================================================================
Fast radio bursts, or FRBs, are short, intense flashes of radio waves
that are thought to be the product of small, distant, extremely dense
objects, though exactly what those objects might be is a longstanding
mystery in astrophysics.

FRBs typically last a few milliseconds, during which time they can
outshine entire galaxies.

Since the first FRB was observed in 2007, astronomers have catalogued over
100 fast radio bursts from distant sources scattered across the universe, outside our own galaxy. For the most part, these detections were one-offs, flashing briefly before disappearing entirely. In a handful of instances, astronomers observed fast radio bursts multiple times from the same
source, though with no discernible pattern.

This new FRB source, which the team has catalogued as FRB 180916.J0158+65,
is the first to produce a periodic, or cyclical pattern of fast radio
bursts. The pattern begins with a noisy, four-day window, during which
the source emits random bursts of radio waves, followed by a 12-day
period of radio silence.

The astronomers observed that this 16-day pattern of fast radio bursts reoccurred consistently over 500 days of observations. "This FRB
we're reporting now is like clockwork," says Kiyoshi Masui, assistant
professor of physics in MIT's Kavli Institute for Astrophysics and
Space Research. "It's the most definitive pattern we've seen from one
of these sources. And it's a big clue that we can use to start hunting
down the physics of what's causing these bright flashes, which nobody
really understands." Masui is a member of the CHIME/FRB collaboration,
a group of more than 50 scientists led by the University of British
Columbia, McGill University, University of Toronto, and the National
Research Council of Canada, that operates and analyzes the data from
the Canadian Hydrogen Intensity Mapping Experiment, or CHIME, a radio
telescope in British Columbia that was the first to pick up signals of
the new periodic FRB source.



==========================================================================
The CHIME/FRB Collaboration has published the details of the new
observation today in the journal Nature.

A radio view In 2017, CHIME was erected at the Dominion Radio
Astrophysical Observatory in British Columbia, where it quickly began
detecting fast radio bursts from galaxies across the universe, billions
of light years from Earth.

CHIME consists of four large antennas, each about the size and shape of
a snowboarding half-pipe, and is designed with no moving parts. Rather
than swiveling to focus on different parts of the sky, CHIME stares
fixedly at the entire sky, using digital signal processing to pinpoint
the region of space where incoming radio waves are originating.

From September 2018 to February 2020, CHIME picked out 38 fast radio
bursts from a single source, FRB 180916.J0158+65, which the astronomers
traced to a star-churning region on the outskirts of a massive spiral
galaxy, 500 million light years from Earth. The source is the most active
FRB source that CHIME has yet detected, and until recently it was the
closest FRB source to Earth.



==========================================================================
As the researchers plotted each of the 38 bursts over time, a pattern
began to emerge: One or two bursts would occur over four days, followed
by a 12-day period without any bursts, after which the pattern would
repeat. This 16-day cycle occurred again and again over the 500 days
that they observed the source.

"These periodic bursts are something that we've never seen before,
and it's a new phenomenon in astrophysics," Masui says.

Circling scenarios Exactly what phenomenon is behind this new
extragalactic rhythm is a big unknown, although the team explores some
ideas in their new paper. One possibility is that the periodic bursts
may be coming from a single compact object, such as a neutron star,
that is both spinning and wobbling -- an astrophysical phenomenon known
as precession. Assuming that the radio waves are emanating from a fixed location on the object, if the object is spinning along an axis and that
axis is only pointed toward the direction of Earth every four out of 16
days, then we would observe the radio waves as periodic bursts.

Another possibility involves a binary system, such as a neutron star
orbiting another neutron star or black hole. If the first neutron star
emits radio waves, and is on an eccentric orbit that briefly brings it
close to the second object, the tides between the two objects could be
strong enough to cause the first neutron star to deform and burst briefly before it swings away. This pattern would repeat when the neutron star
swings back along its orbit.

The researchers considered a third scenario, involving a radio-emitting
source that circles a central star. If the star emits a wind, or cloud
of gas, then every time the source passes through the cloud, the gas
from the cloud could periodically magnify the source's radio emissions.

"Maybe the source is always giving off these bursts, but we only see them
when it's going through these clouds, because the clouds act as a lens,"
Masui says.

Perhaps the most exciting possibility is the idea that this new FRB, and
even those that are not periodic or even repeating, may originate from magnetars - - a type of neutron star that is thought to have an extremely powerful magnetic field. The particulars of magnetars are still a bit
of a mystery, but astronomers have observed that they do occasionally
release massive amounts of radiation across the electromagnetic spectrum, including energy in the radio band.

"People have been working on how to make these magnetars emit fast radio bursts, and this periodicity we've observed has since been worked into
these models to figure out how this all fits together," Masui says.

Very recently, the same group made a new observation that supports
the idea that magnetars may in fact be a viable source for fast radio
bursts. In late April, CHIME picked up a signal that looked like a fast
radio burst, coming from a flaring magnetar, some 30,000 light years
from Earth. If the signal is confirmed, this would be the first FRB
detected within our own galaxy, as well as the most compelling evidence
of magnetars as a source of these mysterious cosmic sparks.


========================================================================== Story Source: Materials provided by
Massachusetts_Institute_of_Technology. Original written by Jennifer
Chu. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Amiri, M., Andersen, B., Bandura, K. et al. Periodic activity from
a fast
radio burst source. Nature, 2020 DOI: 10.1038/s41586-020-2398-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200617150005.htm

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