Astronomers using ESA’s Integral gamma-ray observatory have demonstrated beyond doubt that dead stars known as white dwarfs can reignite and explode as supernovae. The finding came after the unique signature of gamma rays from the radioactive elements created in one of these explosions was captured for the first time.
The explosions in question are known as Type Ia supernovae, long suspected to be the result of a white dwarf star blowing up because of a disruptive interaction with a companion star. However, astronomers have lacked definitive evidence that a white dwarf was involved until now. The ‘smoking gun’ in this case was evidence for radioactive nuclei being created by fusion during the thermonuclear explosion of the white dwarf star.
“Integral has all the capabilities to detect the signature of this fusion, but we had to wait for more than ten years for a once-in-a-lifetime opportunity to catch a nearby supernova,” says Eugene Churazov, from the Space Research Institute (IKI) in Moscow, Russia and the Max Planck Institute for Astrophysics,in Garching, Germany.
Although Type Ia supernovae are expected to occur frequently across the Universe they are rare occurrences in any one galaxy, with typical rates of one every few hundred years.
Integral’s chance came on 21 January 2014, when students at the University College London’s teaching observatory at Mill Hill, UK, detected a type Ia supernova, later named SN2014J, in the nearby galaxy M82.
According to the theory of such explosions, the carbon and oxygen found in a white dwarf should be fused into radioactive nickel during the explosion. This nickel should then quickly decay into radioactive cobalt, which would itself subsequently decay, on a somewhat longer timescale, into stable iron.
Because of its proximity – at a distance of about 11.5 million light-years from Earth, SN2014J is the closest of its type to be detected in decades – Integral stood a good chance of seeing the gamma rays produced by the decay. Within one week of the initial discovery, an observing plan to use Integral had been drawn-up and approved.
Using Integral to study the aftermath of the supernova explosion, scientists looked for the signature of cobalt decay – and they found it, in exactly the quantities that the models predicted.
“The consistency of the spectra, obtained by Integral 50 days after the explosion, with that expected from cobalt decay in the expanding debris of the white dwarf was excellent,” says Churazov, who is lead author of a paper describing this study and reported in the journal Nature.
With that confirmation in hand, other astronomers could begin to look into the details of the process. In particular, how the white dwarf is detonated in the first place.
In January 2014, a supernova was discovered in the nearby galaxy M82. At a distance of about 11.5 million light-years from Earth, SN2014J as it is known, is the closest of its type to be detected in decades.
This composite Hubble image shows the supernova in visible light, obtained on 31 January with Hubble’s Wide Field Camera 3, superimposed on a mosaic of the entire galaxy taken in 2006 with Hubble’s Advanced Camera for Surveys.