An XMM-Newton view of planetary nebulae in the Small Magellanic Cloud

The X-ray luminous central star of SMP SMC 22

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy e-mail: sandro@iasf-milano.inaf.it
² Dipartimento di Fisica, Università degli Studi di Milano, via G. Celoria 16, 20133 Milano, Italy
³ Institute for Astronomy and Astrophysics, Kepler Center for Astro and Particle Physics, Eberhard Karls University, Sand 1, 72076 Tübingen, Germany
⁴ Max Planck Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching bei München, Germany
⁵ University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW 1797, Australia

Received: 15 March 2010
Accepted: 4 May 2010

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 reproduced well 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  K is in close agreement with that found from the optical/UV data. The unabsorbed flux in the 0.1–0.5 keV range is ~3 × 10^-11 erg cm^-2 s^-1, corresponding to a luminosity of ~1.2 × 10³⁷ erg s^-1 at a 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) × 10³⁵ erg s^-1 (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 lower 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, which is rapidly evolving toward the white dwarf's cooling branch, and to a small amount of intrinsic absorption in the nebula itself.