Sky and Telescope - July 2016 - 26

Faraway Flashes

1300

D. LORIMER & M. KRAMER

CSIRO (2)

Frequency (MHz)

1350

Time

WIDE FIELD OF VIEW Left: The Parkes radio telescope is equipped with a multibeam receiver (inset) that, like a CCD camera
with multiple pixels, images 13 adjacent areas on the sky, each area 14 arcminutes across. Radiation from the Lorimer burst was
detected in three of these areas, while the perytons were found in all 13 areas. Right: A classic "waterfall" plot shows the smear,
or dispersion, of a pulsar's regular signal across frequency: its pulse appears ﬁrst at higher frequencies (A), then at lower frequencies (B). Signals from both perytons and FRBs show similar dispersions.

Flaring Stars Nearby, or Faraway Exotics?

7
6

Fast Radio Bursts:
Discoveries & Publications

30
25

5
20
4
15
3
10

0
2008

2010

2012

Year

July 2016 sky & telescope

2014

2016

Number of publications

Number of reported discoveries

Before astronomers could determine just what might
spark such a short and violent burst, they had to determine where these bursts originated. If they called the
Milky Way home, they might be run-of-the-mill sources.
But if their light crossed vast cosmological distances,
then they would require unknown phenomena, perhaps
pushing the boundaries of physics.
The answer lay in the Lorimer burst's smeared appearance in frequency and time: its higher-frequency radio
waves arrived roughly 300 milliseconds before their
lower-frequency cousins. This so-called dispersion occurs
when radio waves travel through clouds of plasma, such
as those found between stars and between galaxies.

The speed of light may be a constant 300,000 kilometers per second (671 million mph) in a vacuum, but
it's slightly slower in plasma. How slow depends on
the light's frequency: lower-frequency photons interact
more with free electrons, which slows their passage. By
comparing the arrival of the same radio signal at low and
high frequencies, astronomers can measure how many
electrons lie between Earth and the source. Assuming
those electrons are spread out randomly along our line
of sight, a higher measure of dispersion means more
electrons and therefore a more distant source.
Because the Lorimer burst and the following FRBs
had dispersion measures that couldn't be explained by
the electrons in our galaxy alone, most astronomers
thought they had to originate at cosmological distances.
But Abraham Loeb (Harvard University and HarvardSmithsonian Center for Astrophysics) wholeheartedly
disagrees. Loeb argues that, while dispersion tells

A GROWING FIELD
This graph plots the
number of fast radio
burst discoveries
reported in a given year,
as well as the number of
FRB-related papers published that year. Though
the ﬁrst fast radio burst
was spotted in 2007, it
took several years before
another was found. It
wasn't until discoveries
began to pile up that the
ﬁeld really took oﬀ. (The
2016 data cover January
through March.)

S&T: LEAH TISCIONE; SOURCE: NASA ASTROPHYSICS DATA SYSTEM /
PETROFF ET AL. / ARXIV 2016

terrestrial source wasn't to blame for the Lorimer burst.
By early 2015 it became clear that FRBs were the latest
astrophysical mystery waiting to unfold.

Sky and Telescope - July 2016

Table of Contents for the Digital Edition of Sky and Telescope - July 2016