The Kepler telescope is designed to detect exoplanets by observing stars. When a planet passes in front of a star the light from the star gets dimmed because of the obstruction caused by the planet. This method can be used to detect astronomical objects other than the exoplanets too. Objects which brighten or dim can be studied to identify astronomical objects which pass the stars. A new search in Kepler’s archived data has revealed an unusual brightening of a dwarf nova. It brightened 1600 times in less than a day and slowly faded back.
Scientists say that the star system may consist of a white dwarf star and a brown dwarf companion about one tenth the mass of white dwarf.
A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth. (Source: Wikipedia)
A brown dwarf is a type of sub-stellar object occupying the mass range between the heaviest gas giant planets and the lightest stars, having a mass between approximately 13 and 75–80 times that of Jupiter or approximately 2.5×1028 kg to about 1.5×1029 kg. Below this range are the sub-brown dwarfs (sometimes referred to as rogue planets), and above it are the lightest red dwarfs. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.Unlike the stars in the main sequence, brown dwarfs are not massive enough to sustain nuclear fusion of ordinary hydrogen (1H) to helium in their cores. (Source: Wikipedia)
The brown dwarf orbits the white dwarf every 83 minutes and the distance between stars is about 250000 miles. That distance is nearly the distance between earth and moon. The white dwarfs strong gravity pulls material from brown dwarf like a vampire sucking blood from its victim. The material stripped from the brown dwarf forms a disk around the white dwarf.
It was a very rare chance that Kepler was pointed in the right direction when the system had a super outburst more than 1000 times brighter. Kepler’s rapid observations take data every 30 minutes. This data was very important to catch the details of this outburst.
The data was hidden in Kepler’s data archive until it was identified by Ryan Ridden Harper and his team of the Space Telescope Science Institute (STScl), Baltimore, Maryland and the Australian National University in Canberra, Australia.”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,” said Ridden-Harper.
Kepler recorded the entire event observing a slow rise in brightness followed by a rapid intensification. The sudden brightness can be explained by theories. But the cause of the slow start remains a mystery. Standard physics theories do not explain this phenomenon. This phenomenon has been observed in two other dwarf nova outbursts also.
“These dwarf nova systems have been studied for decades, so spotting something new is pretty tricky,” said Ridden-Harper. “We see accretion disks all over—from newly forming stars to supermassive black holes—so it’s important to understand them.”
Some theories suggest that a super outburst is triggered when the accretion disk reaches a tipping point. As it gathers material, it grows in size until the outer edge experiences gravitational resonance with the orbiting brown dwarf. As a result, the disk might get super heated due to the thermal instability triggered. Observations show that the temperatures rise from 5000 Fahrenheit to 10000 Fahrenheit in its normal state and 17000 to 21000 Fahrenheit at the peak state of the super outburst.
These types of dwarf nova systems are very rare. Only about 100 are known yet. The time between outbursts may be years or decades. So it’s a challenge to observe one.
“The detection of this object raises hopes for detecting even more rare events hidden in Kepler data,” said co-author Armin Rest of STScI.
Team expect to continue analyzing data of Kepler and data from another exoplanet hunter, Transiting Exoplanet Survey Satellite (TESS) mission to search if there were such undetected incidents.
“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 Peter Garnavich of the University of Notre Dame in Indiana.
The work was published on 21st October 2019 in the Royal Astronomical Society.
Source : Royal Astronomical Society