Two supernovae (circled) on either side of galaxy NGC 1316's bright core resemble eyes and a nose in this visible-light image from NASA's Swift satellite. Another bright spot at far left is a star in our own galaxy (Image: NASA/Swift/S Immler)

Supernovae explode in rare double-whammy

November 20, 2006

NewScientist.com news service

A portrait of two supernovae that exploded just five months apart in the same galaxy has been made by NASA's Swift telescope. The galaxy, called NGC 1316, has now produced four supernovae in 26 years, the highest rate ever measured.

All four supernovae were of type Ia, which are thought to occur when a stellar ember called a white dwarf collects too much matter from a companion star, igniting a runaway nuclear reaction that tears the white dwarf apart.

The supernovae were both initially detected from the ground by an amateur astronomer in South Africa named Berto Monard. Swift was then called upon to make observations at ultraviolet and X-ray wavelengths.

The first event was detected on 19 June 2006 and appears as a yellowish spot on the right. The second event was detected on 5 November, and appears as another spot at mid-left.

Delayed reaction

The white patch between the two supernovae is the galaxy's bright core. The spot at far left is a star in our own galaxy that happens to lie in the same direction as NGC 1316.

The two supernovae add to two previous ones in the same galaxy, also of type Ia, that appeared in 1980 and 1981. This is the highest rate of supernovae of any kind ever recorded for a single galaxy. A typical large galaxy has around three supernovae per century.

The host for these supernovae is a massive elliptical galaxy about 80 million light years away that appears to have merged with a spiral galaxy within the past 2 billion years.

It is possible that the high rate of supernovae is a sort of delayed reaction to the merger, says Peter Brown of Pennsylvania State University in State College, US, a member of the team that made the Swift observations.

Stellar baby boom

Mergers tend to trigger high rates of star formation, and after a couple of billion years of stellar evolution, this could lead to higher rates of type Ia supernovae, he says. "If you have more stars being born, then you will have more that will die," he told New Scientist.

But he says that it could also be a statistical fluke. A large number of galaxies are being monitored for supernovae, and every once in a while one would expect to observe a series like this in a single galaxy just by chance, he says.

In addition to monitoring the sky for gamma-ray bursts