SGR 1806–2024 ASTRONOMY • FEBRUARY 2018
the Richter scale. (By comparison, the
2004 Boxing Day tsunami in Indonesia was
triggered by an earthquake registering 9.1
on this scale.) Even though SGR 1806–20
is 50,000 light-years away, the tremor
caused a 0.2-second f lare brighter than
the Full Moon. It knocked research and
communication satellites brief ly off line
and temporarily altered the shape of
Earth’s upper atmosphere. The burst
had enough energy in it to power the Sun
for 150,000 years and could have caused
billions of dollars in damage had the
magnetar been closer.
Some astronomers argue a similar mag-
netar f lare in a far-off galaxy could also cre-
ate FRBs, but the magnetar theory has its
own problems, one of which originates from
the SGR 1806–20 event. Parkes Observatory
happened to be observing at the time of
that f lare — the opposite part of the sky,
granted, but an FRB created within our own
galaxy would be bright enough to swamp
the telescope regardless of where it was
pointing. Nothing unusual was recorded.
Another popular theory argues that
FRBs could come from young neutron
stars, just a few years after their birth.A neutron star is created at the core of a
supernova as the star dies in a fiery explo-
sion, with a mass as large as two Suns
squeezed into a ball of neutrons just
10 miles (16 km) across. Pulsars, a subset of
neutron stars, give off a beam of radio radi-
ation, seen to regularly f licker from Earth
as the neutron star rapidly rotates (as fast
as a thousand times a second).
Supernovae are rare events. A Milky
Way-sized galaxy averages one such explo-
sion per century, and none has been
recorded in our galaxy since the invention
of the telescope. But one of the brightest
radio sources in the sky is the Crab Pulsar,
known to give off random giant pulses last-
ing a fraction of a nanosecond and sud-
denly exceeding the brightness of normal
pulses by a factor of several thousand. And
we happen to know its age: The Crab Pulsar
was created in a supernova explosion
recorded as a “guest star” in A.D. 1054 by
Chinese astronomers, so bright it was vis-
ible in daytime. Could FRBs be caused by
similar giant pulses from even younger pul-
sars, just a few decades old? No one knows.
Some astronomers have even speculated
that FRBs could originate from other intel-
ligent life in the universe. “Should you con-
sider it as a first explanation? No,”
emphasizes Joe Lazio of the California
Institute of Technology. “But I think we
can say, based on our own capabilities in
our own civilization, that we can’t rule it
out.” Lazio argues that FRBs could still
prove to be transmissions from alien radar
systems, developed by alien species in dis-
tant galaxies that we incidentally pick up.
It may sound far-fetched, but Lazio cites
how Arecibo Observatory often doubles as
the world’s most powerful radar transmit-
ter, bouncing radio beams off asteroids and
other solar system objects in order to map
them. Arecibo’s radar signals could con-
ceivably be detected, for a brief moment, by
any stars behind the body being mapped.
Even our own radio dishes could pick up
an Arecibo-like signal from within a few
light-years of Earth. So, based on our cur-
rent capabilities, who’s to rule out that we
aren’t incidentally picking up signals from
a sophisticated intergalactic radar system?
Finally, FRBs could come from more
than one source. The field of gamma-ray
bursts (GRBs) is an example of how this
scenario could play out. U.S. military satel-
lites designed to detect gamma radiation
from nuclear weapons tests first observed
GRBs in the 1960s; the existence of GRBs
from deep space was declassified in 1973.
By 1994, no fewer than 128 models of GRBsMagnetars are young neutron stars whose intense magnetic fields are a quadrillion times stronger
than Earth’s and a thousand times stronger than the average neutron star’s. They are believed to
periodically give off optical light and gamma rays as sources called soft gamma repeaters. ESO/L.CALÇADA
The starquake that shook the crust of SGR 1806–20, a magnetar in the Milky Way, launched a flare
that was recorded in December 2004. The gamma rays from this quake were incredibly powerful,
briefly lighting up Earth’s upper atmosphere. Although SGR 1806–20’s outburst did not cause an
associated FRB, similar bursts from extragalactic magnetars remain one theorized source of these
signals. UNIVERSITY OF HAWAII