Australian Sky & Telescope - April 2018

8 AUSTRALIAN SKY & TELESCOPE April 2018

NEWS NOTES

BRIAN P. IRWIN / DENNIS VAN DE WATER / SHUTTERSTOCK.COM

NEW OBSERVATIONS SHOW that a

source of repeating radio-wave flashes,

first detected in 2012, is embedded in a

potent magnetic field, perhaps that of a

massive black hole.

So-called fast radio bursts can

generate as much energy as 500 million

Suns in only a few milliseconds.

Astronomers have detected about 30 of

them so far, but they think as many as

10,000 sources might flash each day.

In 2016 astronomers announced that

one of these bursts, FRB 121102, had

flared several times, making it the only

fast radio burst known to repeat. In the

years since, it has erupted more than

200 times, enabling astronomers to

finally tie the source to a star-forming

found that the radio waves were almost

entirely linearly polarised, which means

the waves were almost all oscillating

in the same plane. Moreover, this

polarisation had been strongly twisted

in a signature known as Faraday

rotation, which occurs when light passes

through a magnetic field. In this case,

the waves’ polarisation is so twisted, it

suggests the magnetic field around the

source is at least 100 times stronger

than the average magnetic field in the

Milky Way. The only known source with

such a strong polarisation signature

is a pulsar near Sagittarius A* — the

supermassive black hole at the centre of

our galaxy.

In addition, the shortest burst lasts

less than 30 microseconds, suggesting

that the source is as small as 10 km

across, the typical size of a neutron star.

So the fast radio burst could be a

neutron star near a massive black hole.

But this answer poses another question:

Why would the two objects be so close

together? “Presumably that would

not just be a coincidence,” says Jason

Hessels (University of Amsterdam).

Michilli and his team members agree

that other scenarios could give rise to

this odd fast radio burst.

“It’s likely that it would be very

young and if it’s very young, it’s quite

possible that it could still be in some

kind of cocoon, [like] a supernova

remnant,” Hessels says. But that

hypothesis isn’t perfect either. The

remnant would have to be a million

times brighter than the Crab Nebula,

the brightest remnant in the Milky

Way. Another idea involves outflows

from black holes.

With a number of wide-field radio

telescopes coming online this year

and next, astronomers will soon be

detecting more fast radio bursts — and

perhaps more repeating ones — which

will help them get a better handle on

these enigmas.

■ SHANNON HALL

Mystery bursts might

originate near black hole

region in a dwarf galaxy 2.4 billion

light-years away.

New observations of polarised radio

emission, reported January 11 in the

journal Nature, suggest the repeated

bursts come from an environment with

an unusually strong magnetic field.

As such, the source might be a young

neutron star in the vicinity of a massive

black hole, whose strong field could

alter the neutron star’s emissions.

Daniele Michilli (University of

Amsterdam) and colleagues used the

Arecibo Observatory in Puerto Rico

and the Green Bank Telescope in West

Virginia to detect the bursts at higher

frequencies than before, measuring the

polarisation of the radio waves. They

W This illustration shows the 305-metre

Arecibo telescope and its suspended support

platform of radio receivers. The starburst at

upper left marks FRB 121102’s location.