An XMM-Newton Observation of Nova LMC 1995: A Bright Supersoft X-Ray Source

Nova LMC 1995, previously detected during 1995-1998 with ROSAT, was observed again as a luminous supersoft X-ray source with XMM-Newton in 2000 December. This nova offers the possibility to observe the spectrum of a hot white dwarf, burning hydrogen in a shell and not obscured by a wind or nebular emission as in other supersoft X-ray sources. Notwithstanding uncertainties in the calibration of the European Photon Imaging Camera (EPIC) instruments at energy E<0.5 keV, using atmospheric models in nonlocal thermonuclear equilibrium we derived an effective temperature in the range 400,000-450,000 K, a bolometric luminosity L_bol~=2.3×10³⁷ ergs s^-1, and verified that the abundance of carbon is not significantly enhanced in the X-ray's emitting shell. The reflection grating spectrometer's grating spectra do not show emission lines (originated in a nebula or a wind) observed for some other supersoft X-ray sources. The crowded atmospheric absorption lines of the white dwarf cannot be resolved. There is no hard component (expected from a wind, a surrounding nebula, or an accretion disk), with no counts above the background at E>0.6 keV and an upper limit F_X,hard=10^-14 ergs s^-1 cm^-2 to the X-ray flux above this energy. The background-corrected count rate measured by the EPIC instruments was variable on timescales of minutes and hours but without the flares or sudden obscuration observed for other novae. The power spectrum shows a peak at 5.25 hr, possibly because of a modulation with the orbital period. We also briefly discuss the scenarios in which this nova may become a Type Ia supernova progenitor.