An X-ray emission-line spectrum of Nova V382Velorum 1999

Abstract

We report on the analysis of an X-ray grating spectrum of the Classical Nova V382Vel (1999), obtained with the Low Energy Transmission Grating (LETG)+HRC-S instrument onboard Chandra, which shows emission lines dominating over any continuum. Lines of Si, Mg, Ne, O, N and C are identified, but no Fe lines are detected. The total luminosity in the lines is ~4 × 10²⁷ergs^-1 (corrected for N_H= 1.2 × 10²¹cm^-2). The lines have broad profiles with full width at half-maximum corresponding to a velocity ~2900 +/-200kms^-1. Some structure is identified in the profiles, but for different elements we find different profile structures. While lines of O show a broadened Gaussian profile, those of Ne are double-peaked, suggesting a fragmented emitting plasma. Using the emission measure distribution, we derive relative element abundances and find abundances of Ne and N that are significantly enhanced relative to that of O, while Fe is not overabundant. The lack of any source emission longwards of 50Åand the OVIII Ly_α/Ly_β line ratio supports previous values of the hydrogen column density. We find weak continuum emission from the white dwarf, consistent with a blackbody spectrum with an upper limit to the temperature of T= 3 × 10⁵K, assuming a source radius of 6000km. The upper limit for the integrated blackbody luminosity is 2 × 10³⁶ergs^-1. The BeppoSAX and Chandra ACIS observations of V382Vel show that the nova was bright and in the Super-Soft phase as late as 1999 December 30. Our LETG observation obtained 6 weeks later, as well as all subsequent X-ray observations, showed a remarkable fading to a nearly pure emission line phase which suggests that nuclear burning on the white dwarf had turned off by February. In the absence of a photoionizing source, the emission lines were formed in a collisionally ionized and excited expanding shell.