The NASA Kepler space telescope was designed to hunt for exoplanets by looking for stars that dim as a planet transits a star’s face. Luckily, the same design makes Kepler able to also spot other astronomical transients.
Now, a new trawl through Kepler’s archive has uncovered an unusual super-outburst from a previously-unknown dwarf nova.
In a sense, we discovered this system accidentally
Mr Ryan Ridden-Harper
The star brightened by a factor of 1,600 over less than a day before slowly fading.
This star system consists of a white dwarf star with a brown dwarf companion approximately one-tenth the size of its neighbour.
White dwarfs are the remains of an ancient star containing about a Sun’s worth of material in an Earth-sized sphere.
Brown dwarfs boast a mass between 10 and 80 Jupiters and are too small to undergo nuclear fusion.
The brown dwarf circles the white dwarf star every 83 minutes at a distance of only 250,000 miles (400,000km) – the average distance between the Earth and Moon.
The pair are close enough for the white dwarf’s strong gravity to commune matter from the brown dwarf, devouring its essence away like a vampire.
The stripped material forms a disk as it spirals toward the white dwarf, known as an accretion disk.
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Kepler was the only instrument capable of witnessing the event, because the system was too close to the Sun from Earth’s perspective.
The NASA space probe’s rapid cadence of observations, taking data every 30 minutes, was critical for catching every detail of the rare outburst.
The event remained hidden in Kepler’s archive until identified by a team led by Ryan Ridden-Harper of the Space Telescope Science Institute (STScI), and the Australian National University.
Mr Ryan Ridden-Harper said: “In a sense, we discovered this system accidentally.
“We weren’t specifically looking for a super-outburst. We were looking for any sort of transient.”
Kepler captured the entire event, observing a slow rise in brightness followed by a rapid intensification.
While the reason for the sudden brightening is understood, the cause of the slow start remains a mystery.
Standard theories of accretion disk physics do not predict this phenomenon, which has only observed two other dwarf nova super-outbursts before.
Mr Ridden-Harper added: ”These dwarf nova systems have been studied for decades, so spotting something new is pretty tricky.
“We see accretion disks all over—from newly forming stars to supermassive black holes — so it’s important to understand them.”
One theory suggests a super-outburst is triggered when the accretion disk reaches a tipping point.
As the stripped material accumulates material, it grows until the outer edge experiences gravitational resonance with the orbiting brown dwarf.
This might trigger thermal instability, causing the disk to superheat.
Observations indicate the disk’s temperature rises from about 2,700 to 5,300C (5,000 to 10,000F) in its normal state to a high of 9,700–11,700C (17,000–21,000F) at the peak of the super-outburst.
Armin Rest of STScI, a co-author, said: “The detection of this object raises hopes for detecting even more rare events hidden in Kepler data.”
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