An XMM-Newton view of Planetary Nebulae in the Small Magellanic Cloud. The X-ray luminous central star of SMP SMC 22

TL;DR

This study reports the detection of a highly luminous X-ray source from the central star of planetary nebula SMP SMC 22 in the Small Magellanic Cloud, providing insights into its temperature, composition, and evolution.

Contribution

First X-ray detection and spectral analysis of SMP SMC 22's central star, revealing its high temperature and luminosity consistent with a high-mass progenitor.

Findings

SMP SMC 22 has an effective temperature of 150,000 K.

Its X-ray luminosity is approximately 1.2x10^37 erg/s.

Other planetary nebulae in SMC show much lower or undetectable X-ray emission.

Abstract

During an X-ray survey of the Small Magellanic Cloud, carried out with the XMM-Newton satellite, we detected significant soft X-ray emission from the central star of the high-excitation planetary nebula SMP SMC 22. Its very soft spectrum is well fit with a non local thermodynamical equilibrium model atmosphere composed of H, He, C, N, and O, with abundances equal to those inferred from studies of its nebular lines. The derived effective temperature of 1.5x10^5 K is in good agreement with that found from the optical/UV data. The unabsorbed flux in the 0.1-0.5 keV range is about 3x10^{-11} erg cm^-2 s^-1, corresponding to a luminosity of 1.2x10^37 erg/s at the distance of 60 kpc. We also searched for X-ray emission from a large number of SMC planetary nebulae, confirming the previous detection of SMP SMC 25 with a luminosity of (0.2-6)x10^35 erg/s (0.1-1 keV). For the remaining objects that were not detected, we derived flux upper limits corresponding to luminosity values from several tens to hundreds times smaller than that of SMP SMC 22. The exceptionally high X-ray luminosity of SMP SMC 22 is probably due to the high mass of its central star, quickly evolving toward the white dwarf's cooling branch, and to a small intrinsic absorption in the nebula itself.