An extinct, shrunken white dwarf star may have a brief recovery accompanied by an explosion of X-rays. Such an eruption has now been observed for the first time – by chance – by a group of astronomers.
One moment there was nothing special in the sky. Four hours later, a glowing X-ray flash appeared. When the telescope, which had seen the flash, recaptured the same area in the sky four hours later, the X-rays were already dead.
The flash was measured by eROSITA, a measuring instrument on a space telescope 1.5 million kilometers from Earth. “The X-rays were really very clear,” says astronomer Ole König from Friedrich-Alexander University in Germany. ‘Actually too bright. It has overexposed the central part of the detector. We had to find a smart way to extract the information from the overexposed data. ‘
Looking for the solution
The rapid X-ray eruption appears to be caused by a white dwarf star. Astronomers predicted the existence of these explosions years ago, but they had never been observed. Until now.
This discovery of König and his colleagues was to some extent fortunate. “Every year there are 40 to 50 explosions in the Milky Way among white dwarfs,” says König. “Only the heaviest of them cause bright X-rays.” And then you just have to direct your telescope to the right place in the sky at the right time. X-ray outbreaks are difficult to predict.
white dwarf star
Stars with a mass and composition like our sun end their lives as white dwarf stars. They have blown away some of their cause. All that remains is a collapsed nucleus, consisting mainly of carbon and oxygen. No nuclear fusion takes place inside. The fuel for this energy-generating nuclear reaction – hydrogen – has run out.
A white dwarf star is about the size of Earth and has a mass of 0.6 to 1.3 times that of the Sun. White dwarfs are therefore extremely compact. A teaspoon of it weighs as much as a truck.
These faded, shrunken stars can have an occasional resuscitation. For example, if they form a binary star system with a ‘living’ star. In that case, the white dwarf can draw hydrogen gas from its neighboring star. This will lie on the surface like a rug. Due to the enormous gravity of the white dwarf, this gas becomes increasingly compressed as more is added. This causes the temperature to rise. At some point, the temperature is so high that nuclear fusion starts in the blanket of hydrogen. This releases so much energy that an explosion occurs that blows the hydrogen gas away.
Here is what probably happened on the surface of a white dwarf star over 8,250 light-years away. The X-rays released by that explosion reached eROSITA on July 7, 2020. Astronomers spent more than a year analyzing the overexposed image.
Frederick Walter of Stony Brook University is impressed with the analysis from König and colleagues. In a commentary published at the same time, he writes: “The lesson from this observation is that even with a brief, unexpected discovery, a wise team can learn that a rare event has taken place.”
The analysis shows that it is a relatively large white dwarf with about the same mass as the sun. The glowing hydrogen gas blown off by the explosion reached a temperature of about 327,000 degrees.
‘We know almost nothing about the neighboring star,’ says König. “Just that it’s a faint star that we can not see yet.”
To understand if all white dwarf explosions are similar, researchers will search eROSITA’s data in search of more fluke hits.