Hidden Radio Supernova Finally Found
The left image, taken with the Hubble Space Telescope, shows the body of M82 in blue and hydrogen gas breaking out from the central starburst in red. The VLA image (top left) clearly shows the supernova (SN 2008iz), taken in May 2008. The high-resolution VLBI images (lower right) shows an expanding shell at the scale of a few light days and proves the transient source as the result of a supernova explosion in M82.
Credit: Milde Science Communication, NASA, ESA, and The Hubble Heritage Team (STScI/AURA), A. Brunthaler, MPIfR

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.