(Marcin) #1



Antimatter is matter’s opposite. For each par-
ticle of “normal” matter, there is an antimatter
particle with an opposite charge; for example,
positrons are the antimatter equivalent of elec-
trons. When matter and antimatter meet, they
annihilate each other, giving off gamma rays in
the process.
Astronomers have observed such gamma
rays in the center of our galaxy since the 1970s.
Despite extensive follow-up, researchers today
are still looking for the exact source (or sources)
of this emission. It could run the gamut from
the mundane — natural processes in a star’s life
— to the exotic, such as dark matter (see p. 12).
In a paper published May 22 in Nature
Astronomy, lead author Roland M. Crocker of
the Research School of Astronomy and
Astrophysics at Australian National University
and his co-authors suggest that positrons
resulting from white dwarf mergers could
contribute significantly to the observed
gamma-ray signal.
White dwarfs are left behind after a Sun-like
star runs out of fuel. If two low-mass stars
(about 1.4 to twice the mass of our Sun) circle
each other closely in a binary system, they can
interact via mass transfer, a process in which
gas from the stars is exchanged. The end result
is two white dwarfs that may eventually merge.
That merger can produce radioactive isotopes

that decay into positrons.
The current resolution of instruments used
to study this emission is not high enough to
see individual sources, such as single super-
nova remnants. Thus, more precise measure-

ments and computer simulations will be
needed to determine the positron production
rates from such events. Such information will
also shed light on the processes that shaped
the young Milky Way. — A. K.

White dwarf mergers seen as antimatter source

MOON RIVER. River channels on Saturn’s moon Titan resemble those seen on Mars rather than Earth. Titan and Mars
both show stream formation without influence from plate tectonics.

MAKING POSITRONS. The merger of two white dwarfs, depicted here as they spiral toward each other, could
provide a major source of positrons in the Milky Way’s center. NASA/TOD STROHMAYER (GSFC)/DANA BERRY (CHANDRA X-RAY OBSERVATORY)
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