Detection of Nitrogen and Neon in the X-ray Spectrum of GP Com with   XMM/Newton
  We report on X-ray spectroscopic observations with XMM/Newton of the ultracompact, double white dwarf binary, GP Com. With the Reflection Grating Spectrometers (RGS) we detect the Lyman alpha and beta lines of hydrogen-like nitrogen (N VII) and neon (Ne X), as well as the helium-like triplets (N VI and Ne IX) of these same elements. All the emission lines are unresolved. These are the first detections of X-ray emission lines from a double-degenerate, AM CVn system. We detect the resonance (r) and intercombination (i) lines of the N VI triplet, but not the forbidden (f) line. The implied line ratios for N VI, R = f/i < 0.3, and G = (f + i)/r ~1, combined with the strong resonance line are consistent with formation in a dense, collision-dominated plasma. Both the RGS and EPIC/MOS spectra are well fit by emission from an optically thin thermal plasma (model cevmkl in XSPEC). Helium, nitrogen, oxygen and neon are required to adequately model the spectrum, however, the inclusion of sulphur and iron further improves the fit, suggesting these elements may also be present at low abundance. We confirm in the X-rays the underabundance of both carbon and oxygen relative to nitrogen, first deduced from optical spectroscopy by Marsh et al. The average X-ray luminosity of ~3e30 ergs/s implies a mass accretion rate of ~9e-13 solar masses per year. The implied temperature and density of the emitting plasma, combined with the presence of narrow emission lines are consistent with production of the X-ray emission in an optically thin boundary layer just above the surface of the white dwarf. 
Tod E. Strohmayer