Using the archival data from NASA’s Kepler Space Telescope, astronomers have identified a previously unknown dwarf nova that underwent a ‘supermaxima’ outburst (super-outburst), brightening by a factor of 1,600 times in less than a day. While the outburst itself has a theoretical explanation, the slow rise in brightness that preceded it remains a mystery.
An artist’s impression of the KSN:BS-C11a system. Image credit: NASA / L. Hustak, STScI / R. Ridden-Harper, STScI & Australian National University.
Named KSN:BS-C11a, the dwarf nova system consists of a white dwarf star with a brown dwarf companion about one-tenth as massive as the white dwarf.
The brown dwarf circles the white dwarf every 83 minutes at a distance of only 400,000 km (250,000 miles) — about the distance from Earth to the Moon.
The white dwarf is stripping material from the brown dwarf, sucking its essence away like a vampire.
The material forms an accretion disk around the white dwarf, which is the source of the super-outburst.
Such systems are rare and may go for years or decades between outbursts, making it a challenge to catch one in the act.
It was sheer chance that Kepler was looking in the right direction when KSN:BS-C11a underwent a super-outburst.
In fact, Kepler was the only instrument that could have witnessed it, since the system was too close to the Sun from Earth’s point of view at the time.
The event remained hidden in Kepler’s archive until identified by Ryan Ridden-Harper, an astronomer from the Space Telescope Science Institute and the Australian National University, and his colleagues from Australia, Chile, and the United States.
“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,” Ridden-Harper said.
Kepler captured the entire event, observing a slow rise in brightness followed by a rapid intensification.
While the sudden brightening is predicted by theories, the cause of the slow start remains a mystery.
Standard theories of accretion disk physics don’t predict this phenomenon, which has subsequently been observed in two other dwarf nova super-outbursts.
“These dwarf nova systems have been studied for decades, so spotting something new is pretty tricky,” Ridden-Harper explained.
“We see accretion disks all over — from newly forming stars to supermassive black holes — so it’s important to understand them.”
Theories suggest that a super-outburst is triggered when the accretion disk reaches a tipping point. As it accumulates material, it grows in size until the outer edge experiences gravitational resonance with the orbiting brown dwarf. This might trigger a thermal instability, causing the disk to get superheated.
Indeed, observations show that the disk’s temperature rises from about 2,700-5,300 degrees Celsius (5,000-10,000 degrees Fahrenheit) in its normal state to a high of 9,700-11,700 degrees Celsius (17,000-21,000 degrees Fahrenheit) at the peak of the super-outburst.
This type of dwarf nova system — the WZ Sagittae type — is relatively rare, with only about 100 known.
An individual system may go for years or decades between outbursts, making it a challenge to catch one in the act.
“The detection of this object raises hopes for detecting even more rare events hidden in Kepler data,” said Dr. Armin Rest, also from the Space Telescope Science Institute.
“The continuous observations by Kepler/K2, and now TESS, of these dynamic stellar systems allows us to study the earliest hours of the outburst, a time domain that is nearly impossible to reach from ground-based observatories,” said Dr. Peter Garnavich, from the University of Notre Dame.
“We’ve used it to see stars as they explode, the secret lives of black holes and now things previously missed — this vampire star that had been lurking in the darkness of space,” said Dr. Brad Tucker, from the Research School of Astronomy and Astrophysics at the Australian National University.
The findings appear in the Monthly Notices of the Royal Astronomical Society.
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R. Ridden-Harper et al. 2019. Discovery of a new WZ Sagittae-type cataclysmic variable in the Kepler/K2 data. MNRAS 490 (4): 5551-5559; doi: 10.1093/mnras/stz2923
