The born-again planetary nebula A78: an X-ray twin of A30

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We present the XMM-Newton discovery of X-ray emission from the planetary nebula (PN) A78, the second born-again PN detected in X-rays apart from A30. These two PNe share similar spectral and morphological characteristics: They harbor diffuse soft X-ray emission associated with the interaction between the H-poor ejecta and the current fast stellar wind, and a point-like source at the position of the central star (CSPN). We present the spectral analysis of the CSPN, using for the first time a NLTE code for expanding atmospheres which takes line blanketing into account for the UV and optical spectra. The wind abundances are used for the X-ray spectral analysis of the CSPN and the diffuse emission. The X-ray emission from the CSPN in A78 can be modeled by a single C VI emission line, while the X-ray emission from its diffuse component is better described by an optically thin plasma emission model with temperature kT=0.088 keV (T≈1.0×10⁶ K). We estimate X-ray luminosities in the 0.2--2.0 keV energy band of LX,CSPN=(1.2±0.3)×10³¹ erg s⁻¹ and LX,DIFF=(9.2±2.3)×10³⁰ erg s⁻¹ for the CSPN and diffuse components, respectively.