2004ApJ...608L..53S -
Astrophys. J., 608, L53-L56 (2004/June-2)
Detection of nitrogen and neon in the X-ray spectrum of GP Comae Berenices with XMM/Newton.
STROHMAYER T.E.
Abstract (from CDS):
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 ~3x1030 ergs/s implies a mass accretion rate m{dot}~9x10–13 M☉/yr. The implied temperature and density of the emitting plasma, combined with the presence of narrow emission lines and the low-m{dot} value, are consistent with production of the X-ray emission in an optically thin boundary layer just above the surface of the white dwarf.
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Journal keyword(s):
Stars: Binaries: General - Stars: Novae, Cataclysmic Variables - Stars: Individual: Constellation Name: GP Comae Berenices - Stars: White Dwarfs - X-Rays: Binaries - X-Rays: Stars
Simbad objects:
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