Fast radio bursts (FRBs) are one
of our Universe's most confounding mysteries - but astronomers may have just
figured out the incredible environment where one of the most famous and
well-studied FRBs is coming from. The repeating radio signals from FRB 121102
are likely produced somewhere extreme, like the area around a massive black
hole.
FRB 121102 made its first
appearance in November 2012, but it took researchers a few years before pinning
down its unusual nature. Most fast radio bursts only occur once, which makes
them untraceable - but FRB 121102 would go on to repeat. This afforded a unique
opportunity. Fast radio bursts are extremely powerful radio signals, generating
as much energy as 500 million suns, but they're also extremely short, lasting
just milliseconds. Because most of them burst once and never return, they are
impossible to predict, and impossible to trace. This is one of the main reasons
why we don't know what causes them.But FRB 121102 has proved to be exceptional.
In March 2016, researchers announced they'd found 10 other bursts from the same
location in archival data. Then in December 2016, 6 bursts were detected from
FRB 121102; then 15 more in August 2017.
This allowed researchers to
locate the source of these signals - a star-forming region in a dwarf galaxy
more than 3 billion light-years from Earth. And now an international team of
researchers has narrowed it down further still, by studying data from radio
telescopes that collected the signal - and are more convinced than ever that
the source is a neutron star. But if it is a neutron star, it's in a crazy
environment - either very close to a black hole or in a very powerful nebula.
This is because of the way the radio signal is "twisted".The radio
signals of FRB 121102 are almost completely polarised. When these polarised
signals travel through a magnetic field, they become twisted - the stronger the
field, the greater the twist. This is called Faraday rotation, and it allows
researchers to learn more about the waves' origin.
In the case of FRB 121102, the
twisting of the signals' polarisation is some of the greatest ever observed,
which means they had to pass through a very intense magnetic field. "The
only known sources in our galaxy that are twisted as much as FRB 121102 are in
the Galactic Centre, which is a dynamic region near a massive black hole. Maybe
FRB 121102 is in a similar environment in its host galaxy," said PhD
candidate Daniele Michilli of the University of Amsterdam and ASTRON, the
Netherlands Institute for Radio Astronomy. "However, the twisting of the
radio bursts could also be explained if the source is located in a powerful
nebula or supernova remnant." This is consistent with the neutron star
hypothesis. Neutron stars are one result of massive star undergoing a
core-collapse supernova. (If it is greater than a certain mass, the star will
turn into a black hole instead.) They are very small, and very dense, and they
emit radio pulses as they spin. A type of neutron star called a magnetar has an
extremely strong magnetic field, and these can produce bursts - similar to how
the Sun produces solar flares.
This has been proposed as a
possible source of the fast radio bursts. However, the most powerful of these
flares ever observed has been four orders of magnitude below FRB 121102. The
researchers believe that the source is a regular neutron star, and are hoping
to find out more. "We are continuing to monitor how the properties of the
bursts change with time," said Jason Hessels from the University of
Amsterdam and ASTRON.With these observations we hope to distinguish between the
two competing hypotheses of a neutron star either near a black hole or embedded
in a powerful nebula." Meanwhile, we still don't have a lead on the dozens
of other fast radio bursts that have been observed. And, because they don't
repeat - FRB 121102 is the only repeater - it's very possible that FRB 121102
is unique in its source, and the others emanate from different sources. The
researchers presented their findings at the 231st meeting of the American
Astronomical Society, and their paper has been published in the journal Nature.
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