Detection of Nitrogen and Neon in the X-Ray Spectrum of GP Comae Berenices with XMM/Newton

Abstract

We report on X-ray spectroscopic observations with XMM/Newton of the ultracompact, double white dwarf binary GP Com. With the Reflection Grating Spectrometers (RGSs), we detect the Lyα and Lyβ 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 fitted by emission from an optically thin thermal plasma with an emission measure proportional to (kT/6.5 keV)0.8 (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 that 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 ≈3 × 1030 ergs s-1 implies a mass accretion rate ≈ 9 × 10-13 M☉ yr-1. The implied temperature and density of the emitting plasma, combined with the presence of narrow emission lines and the low- value, are consistent with production of the X-ray emission in an optically thin boundary layer just above the surface of the white dwarf.