An invisible companion could explain the strong X-ray emission of the Eskimo Nebula.

Planetary nebulae are one of the final stages in the life of low-mass and intermediate-mass stars, such as the Sun. After exhausting their fuel, these stars eject their outer layers, which form an envelope of ionized gas around a white dwarf-type star. Not only is the Eskimo Nebula (NGC 2392) one of the most iconic planetary nebulae, but also one of the most peculiar and enigmatic: apart from presenting a double envelope of gas, its central star is the most energetic of all the central stars of planetary nebulae with X-ray emission. This emission is very difficult to explain if its central star is not in a binary (or multiple) system. However, direct observations show a single star.

Indirect detection methods have found variations in the radial velocity of the central star of NGC 2392, indicating that there is an invisible companion orbiting around it. Two different orbital periods have been proposed, one of only three hours and the other of almost two days. "Faced with this scenario, we have analyzed the temporal variability of the X-ray emission of the central star of the Eskimo Nebula," says Martín Guerrero (IAA-CSIC), principal investigator of the study.

This X-ray emission presents a period of 0.253 days (about 6 hours) that does not coincide with the previously proposed periods. The origin of this modulation is not entirely clear, but it could indicate processes of material falling on an invisible companion star or an accretion disk in a binary system in which each period would be related to the rotation or orbit of different components. 

"It seems as if the central star of the Eskimo Nebula had a much warmer companion and a more powerful stellar wind," says Martin Guerrero (IAA-CSIC). "Processes of exchange of matter between one star and another could explain the gas jet that emanates from the central star at 200 kilometres per second". In this way, this work could suppose a confirmation of the hypothesis that in the center of NGC 2392 there is a multiple system.