When low-mass stars like the Sun use up all of their fuel in nuclear reactions, they turn into white dwarfs . Despite being like dead stars, white dwarfs can "come back to life" through intense explosions, which release powerful X-ray emissions; then, gradually, they return to the glow they had before they exploded.
It was one of these explosions that appeared in the eROSITA data. Positioned half a million kilometers from Earth, the instrument was scanning the sky for X-ray emissions. However, in 2020, it identified a strong emission coming from a region of the sky that was quiet a short time before. After a few hours, the radiation disappeared.
According to the results of the study, the white dwarf in question is relatively large, with a mass comparable to that of the Sun. The explosion generated a fireball with a temperature of over 300,000 °C, almost 60 times hotter than our star. Generally, white dwarfs quickly run out of fuel and cool, and the X-rays weaken until they become visible light.
When this happens, it is as if a bright star visible to the naked eye appears in the sky . This phenomenon has been observed before and was given the name “new star” due to its unexpected appearance. Detecting the phenomenon is not easy: "In addition to the challenge of the short duration of the flash, there is also the fact that the spectrum of the emitted X-rays is very smooth," said co-author Dr. Victor Doroshenko from the University of Tübingen.
The white dwarf explosion
Upon re-analyzing the location of the detections, the authors noticed that the radiation had disappeared. "These X-ray emissions last only a few hours and are almost invisible to predict, and the instrument observing them needs to be pointed directly at the explosion at the right time," said Ole König, the study's lead author. "In a way, it was really a happy coincidence."
X-ray bursts of the kind had been predicted more than 30 years ago, but have never been directly observed until now. "These ' novas ' happen all the time, but detecting them during the first few moments, when most X-ray emissions are produced, is very difficult," noted Dr. Doroshenko. In addition, white dwarfs produce weak radiation, which is difficult to detect from Earth.
This changes if there are any neighboring stars nearby; in this case, the white dwarf will “snap” hydrogen from its companion's envelope, forming a layer a few meters thick on its surface. There, gravity generates pressure so high that it brings the white dwarf back to life, releasing the layer in a big burst of X-rays like the one seen by eROSITA.
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