Some of the brightest and strangest objects to grace the skies in recent years are members of a new class of supernovae. Just how they are formed remains a mystery, but their brilliance should make it easier to observe their dim host galaxies.
Supernovae come in different varieties. Type Ia blasts, for example, show no hydrogen in their spectra, and occur when the ember of a dead sun sucks in too much material from a companion. Type II explosions, which do have hydrogen, form when the core of a massive star collapses.
Now Robert Quimby of the California Institute of Technology in Pasadena and his colleagues report on six supernovae that do not fit the mould of any known type. These misfits contain oxygen but no hydrogen, and outshine type Ia blasts by a factor of 10. They also stay hot for weeks or months – longer than other supernovae.
“The peak brightness and total amount of energy released is extraordinary,” says Quimby. “Change the light bulbs in your house from 100 watt to 1000 watt. Live like that for a month and your electric bill – and tan – will show the difference.”
The creation of the new class was prompted by the discovery of four unusual supernovae in 2009 and 2010. “We knew they were weird, but I had the feeling that I had seen this somewhere before,” says Quimby. He looked back at the spectra of two objects that had previously stumped astronomers: a supernova that smashed brightness records after it was observed in 2005 and a bizarre object that brightened over a leisurely three monthsbefore fading in 2006. “I was utterly ecstatic when I saw the match,” he says.
How do the rare blasts arise? One possibility is that they originate in a heavyweight star, weighing up to 130 suns. Such stars undergo violent pulsations late in life, expelling shells of material periodically, like smoke rings, for months or even decades before their cores explode. The hydrogen shell is the first to be sloughed off, so it would be the farthest away by the time the core exploded.
Debris from the explosion would initially slam into shells rich in other elements, such as oxygen, heating them up and causing them to glow. If the hydrogen shell had expanded outwards far enough, it might evade detection, says Stan Woosley, a supernova theorist at the University of California, Santa Cruz.
Another scenario begins with a normal, hydrogen-poor supernova. Instead of leaving behind a typical fast-spinning neutron star in a cloud of expanding debris, as such supernovae normally do, it gives birth to a highly magnetised neutron star called a magnetar. The star’s intense magnetic field acts as a brake that slows its spin over a period of months. The energy this liberates heats up the surrounding supernova debris, making it shine. “The magnetar releases a huge amount of energy as it slows down,” says Quimby.
Woosley favours the pulsing shell explanation but says both models can explain the objects’ intensity and duration: “They’re the brightest supernovae in the universe and they stay bright for months instead of weeks.”
Both models rely on rare sources – either very massive stars or powerful magnetars, explaining why so few of the new class have been spotted so far. Quimby estimates that in our cosmic neighbourhood there may be 1000 to 10,000 normal supernovae for every superluminous one. “But we can see these to much larger distances,” he says.
That could make them useful beacons for studying their surroundings. All of the bright new supernovae have been found in dim dwarf galaxies, which are usually hard to study. “When a superluminous supernova goes off in one, we can use it as a backlight to study the gas in the host galaxy,” says Quimby. “That can tell us about how galaxies form and evolve.”