An elusive supernova
explosion, detectable only in radio wavelengths, was discovered last month in
the nearby galaxy M82.
The object,
dubbed SN 2008iz, is the closest supernova discovered by scientists in the past
five years. It would have been visible even to amateur telescopes, were it not
for the dense gas and dust surrounding the exploding star, which left it
invisible in every part of the spectrum except the radio wavelengths.
The
supernova's home galaxy, M82, is an irregular galaxy in a nearby
galaxy group located 12 million light-years from Earth.
Despite
being smaller than the Milky Way, it harbors a vigorous central starburst in
the inner few hundred light-years. In this stellar factory more stars are
presently born than in the entire Milky Way.
M82 is
often called an "exploding galaxy," because it looks as if being torn
apart in optical and infrared images as the result of numerous supernova
explosions from massive stars. Many remnants from previous supernovas are
seen in radio images of M82, and a new supernova explosion was long overdue.
Astronomers
have been waiting to catch the next big blast for more than 25 years and had
started to wonder why the galaxy has been so silent in recent years. In the
end, it took a little digging and looking in the right wavelengths.
The new
discovery was first made when Andreas Brunthaler of the Max-Planck-Institut fur
Radioastronomie in Bonn, Germany, examined data from April 8 with the Very
Large Array (VLA) of the National Radio Astronomy Observatory, an
interferometer of 27 identical 25-meter telescopes in New Mexico.
"I
then looked back into older data we had from March and May last year, and there
it was as well, outshining the entire galaxy!" Brunthaler said.
Radio
emission can be detected only from core collapse supernovas, where the core of
a massive star collapses and produces a black hole or a neutron star. It is
produced when the shock wave of the explosion propagates into dense material
surrounding the star, usually material that was shed from the massive
progenitor star before it exploded.
But
observations of M82 taken last year with optical telescopes to search for new
supernovas showed no signs of this explosion. The supernova is also hidden on
ultraviolet and X-ray images.
The
supernova exploded close to the center of the galaxy in a very dense
interstellar environment, which could explain why M82 has been silent for so
long: many of these events may actually be something like "underground
explosions," where the bright flash of light is covered under huge clouds
of gas and dust and only radio waves can penetrate this dense material.
"This
cosmic catastrophe shows that using our radio telescopes we have a front-row
seat to observe the otherwise hidden universe," said Heino Falcke of the
University of Nijmegen/ASTRON.
By
combining data from the 10 telescopes of the Very Long Baseline Array (VLBA),
the VLA, the Green Bank Telescope in the USA, and the Effelsberg 100-meter
telescope in Germany, using the technique of Very Long Baseline Interferometry
(VLBI), the team was able to produce images that show a ring-like structure
expanding at more than 40 million km/h or 4 percent of the speed of light,
typical for supernovae.
The team
estimates that the supernova exploded in late January or early February 2008.
Only three months after the explosion, the ring was already 650 times larger
than Earth's orbit around the sun.
The
discovery will be detailed an upcoming issue of the journal Astronomy &
Astrophysics Letters.
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