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 Lyalpha and Lybeta 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 \&AP; 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 \&AP;3 x 10(30) ergs s(-1) implies a mass accretion rate m \&AP; 9 x 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-m value, are consistent with production of the X-ray emission in an optically thin boundary layer just above the. m surface of the white dwarf.