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Hard X-Ray Emission from White Dwarfs Reveals Their Hidden Companions

Using NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton satellite, astronomers have investigated unusual X-ray activity in three white dwarfs: KPD 0005+5106, PG 1159-035 and WD 0121-756.

Most stars, including the Sun, will become white dwarfs after they begin to run out of fuel, expand and cool into a red giant, and then lose their outer layers. This evolution leaves behind a stellar nub that slowly fades for billions of years.

Typically, white dwarfs give off low-energy X-rays. However, KPD 0005+5106, PG 1159-035 and WD 0121-756 also had surprisingly bright X-ray emission at higher energies.

KPD 0005+5106 stood out among this group. It had high-energy X-ray emission that was regularly increasing and decreasing in brightness every 4.7 hours.

Jupiter-like planet is lossing mass to its neighboring white dwarf.This recurring ebb and flow of X-rays indicates that KPD 0005+5106 has an object in orbit around it — either a very low-mass star or a Jupiter-like planet.

Material from the low-mass star or planet could be slamming into the north and south poles of the white dwarf, creating a bright spot of high-energy X-ray emission.

As the white dwarf and its companion orbit around each other this hot spot would go in and out of view, causing the high-energy X-rays to regularly increase and decrease.

Jupiter-like planet is lossing mass to its neighboring white dwarf.

“We didn’t know this white dwarf had a companion before we saw the X-ray data,” said Dr. You-Hua Chu, an astronomer in the Institute of Astronomy and Astrophysics at Academia Sinica.

“We’ve looked for the companion with optical light telescopes but haven’t seen anything, which means it is a very dim star, a brown dwarf, or a planet.”

KPD 0005+5106 is located about 1,300 light-years in the constellation of Cassiopeia.

It is one of the hottest known white dwarf stars, with a surface temperature of about 200,000 K.

“The companion object is almost 805,000 km (500,000 miles) away from the white dwarf, only about one thirtieth of the distance from Mercury to the Sun. Whatever this object is, it’s getting blasted with heat,” said Dr. Jesús Toala, an astronomer at the National Autonomous University of Mexico.

The astronomers looked at what would happen if this object was a Jupiter-mass planet — a possibility that agrees with the data more readily than a dim star or a brown dwarf.

In their models, the white dwarf would pull material from the planet onto the white dwarf, a process that the planet could only survive for a few hundred million years before eventually being destroyed.

This stolen material swirls around the white dwarf, which glows in X-rays that Chandra can detect.

“This is a slow demise for this object that’s basically being ripped apart by constant gravitational forces. It would be a very unpleasant place to be,” said Dr. Martín A. Guerrero, an astronomer at the Institute of Astrophysics of Andalusia.

The two other white dwarfs — PG 1159-035 and WD 0121-756 — were also thought to be solitary objects, but they show similar energetic X-ray emission to KPD 0005+5106. By analogy, this suggests they may also have faint companions, possibly planets.

“The hard X-ray emission from apparently single white dwarfs is powered by accretion from sub-stellar companions or giant planets, and is modulated by the orbital motion with a period of 4.7 hr,” the authors concluded.