Sky and Telescope - July 2016 - 27

DISPERSION
MEASURE
The speed of light may
be a constant 300,000
kilometers per second
in a vacuum, but space
isn't quite as empty as
it appears. When radio
waves pass through
sparse clouds of ionized
gas, they slow down -
the longer their wavelength and the more
stuﬀ they're passing
through, the slower the
radio waves go. Astronomers measure the delay
in arrival times between
diﬀerent frequencies
(the so-called dispersion measure) to tally
up how much plasma
the radio waves have
traveled through and,
by proxy, the distance to
the source.

S&T: LEAH TISCIONE

Radio wave frequency

astronomers the number of electrons along the line
of sight, it doesn't tell them where those electrons are
located. So what if most of the electrons are in thick
clouds around the source itself?
At ﬁrst, Loeb thought that FRBs could be ﬂares emitted from stars within our own galaxy. These stars' thick
coronas could pack electrons tightly enough to have the
same eﬀect on passing radio waves as billions of lightyears of the mostly empty intergalactic medium.
But few agree with Loeb's initial speculation - and
even Loeb admits he enjoys going against the grain.
Burke-Spolaor says, "It's important to have the contrarian in the mix to make everyone double-think their
statistics." Nevertheless, she and many others are convinced that FRBs are extragalactic.
Here's why: although measuring dispersion can't tell
astronomers exactly where the electrons are located, it
can provide a pretty important clue. If those electrons
are part of cold and sparse plasma, such as the stuﬀ
between stars, the delay decreases with frequency in
a particular way (proportional to frequency squared).
If, however, those electrons are part of the hotter and
denser plasma found in a star's atmosphere, then the
delay wouldn't follow that rule. But the smear of all FRB
signals to date suggests that the pulses have mostly traveled through the sparser intergalactic medium.
The FRB detected at the Green Bank Telescope closed
the distance debate. Not only did Masui and colleagues
see that their FRB followed the dispersion measure
expected for cold plasma, they also saw a subtle stretching out of the pulse's shape. This asymmetry implies the
radio waves scattered oﬀ a thick envelope of plasma right
after they were emitted. Whatever this envelope is, it's
too big to support Loeb's idea of ﬂare-prone stars - and
it must lie in a distant galaxy.
Although Loeb agrees that his initial idea of ﬂaring
stars is out, he's still not sure these stars need to be
cosmological. "The one thing to keep in mind . . . is that
we should be agnostic," he maintains. "The mistake that
many people make is they jump into conclusions when
the data are very scarce."

Time

Placing FRBs on the Cosmic Map

AUSTRALIA TELESCOPE COMPACT ARRAY This collection of six
22-meter radio dishes resolves ﬁner details than Parkes but covers a smaller
area of the sky, so it's best suited to follow-up on FRB discoveries.

Sk yandTelescope.com July 2016

ALEX CHERNEY

To truly get a ﬁ x on an FRB's distance, astronomers need
to pin it down to a host galaxy. It's a surprisingly hard
task given that the Parkes receiver only sees details down
to 14.4 arcminutes across - about half the angular size
of the full Moon. Thousands of galaxies could ﬁt in such
a vast region. But if groups of radio telescopes working
together could spot the burst, or its afterglow, they could
narrow down where it came from. For example, the
Australia Telescope Compact Array (ATCA), a collection
of six 22-meter dishes, resolves regions as small as 1 arcsecond across. Astronomers could easily pinpoint a host
galaxy within that smaller area on the sky.

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Sky and Telescope - July 2016

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