An X-ray emission-line spectrum of Nova V382 Velorum 1999

Mon. Not. R. Astron. Soc. 364, 1015–1024 (2005) doi:10.1111/j.1365-2966.2005.09664.x An X-ray emission-line spectrum of Nova V382 Velorum 1999 J.-U. Ness,1,2
S. Starrfield,3 C. Jordan,1 J. Krautter4 and J. H. M. M. Schmitt2 1 Department of Physics, Rudolf Peierls Centre for Theoretical Physics, University of Oxford, 1 Keble Road, Oxford OX1 3NP 2 Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany 3 Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504, USA 4 Landessternwarte Königstuhl, D-69117 Heidelberg, Germany Accepted 2005 September 21. Received 2005 September 13; in original form 2005 January 25 ABSTRACT Key words: stars: individual: V382 Vel – stars: novae, cataclysmic variables – stars: white dwarfs – X-rays: binaries – X-rays: individual: V382 Vel. 1 INTRODUCTION Classical novae (CN), a class of cataclysmic variables (CVs), are thought to be thermonuclear explosions induced on the surface of white dwarfs as a result of continuous accretion of material from a companion main-sequence star. A sufficient accumulation of hydrogen-rich fuel causes a thermonuclear runaway (TNR). Extensive modelling of the TNR has been carried out in the past (e.g. Starrfield 1989, and references therein). These models showed that only a part of the ejected envelope actually escapes, while the remaining material forms an envelope on the white dwarf with ongoing nuclear burning, radiation-driven winds and turbulent motions. These processes result in a shrinking of the nuclear burning white dwarf radius with increasing temperatures (Starrfield et al. 1991;
E-mail:  C 2005 The Authors. Journal compilation  C 2005 RAS Krautter 2002). During this phase of ‘constant bolometric luminosity’, the nova emits strong X-ray radiation with a soft spectral signature. CN have been observed with past X-ray missions, e.g. Einstein, ROSAT, ASCA and BeppoSAX. While X-ray lightcurve variations were studied, the X-ray spectra obtained had low dispersion and were quite limited. The transmission and reflection gratings aboard Chandra and XMM–Newton now provide significantly improved sensitivity and spectral resolution, and these gratings are capable of resolving individual emission or absorption lines. The Chandra Low Energy Transmission Grating (LETG) spectrum of the Classical Nova V4743 Sgr (Ness et al. 2003b) showed strong continuum emission with superimposed absorption lines, while V1494 Aql showed both absorption and emission lines (Drake et al. 2003). Essentially all X-ray spectra of CN differ from each other, so no classification scheme has so far been established. A review of X-ray observations of novae is given by Orio (2004). We report on the analysis of an X-ray grating spectrum of the Classical Nova V382 Vel (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 × 1027 erg s−1 (corrected for N H = 1.2 × 1021 cm−2 ). The lines have broad profiles with full width at half-maximum corresponding to a velocity ∼2900 ± 200 km s−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 50 Å and the O VIII 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 × 105 K, assuming a source radius of 6000 km. The upper limit for the integrated blackbody luminosity is 2 × 1036 erg s−1 . The BeppoSAX and Chandra ACIS observations of V382 Vel 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. 1016 J.-U. Ness et al. 2 T H E N OVA 2.1 V382 Velorum 2.2 Previous X-ray observations of V382 Vel Early X-ray observations of V382 Vel were carried out with the RXTE on day 5.7 by Mukai & Swank (1999), who did not detect Table 1. Summary of X-ray, UV and optical observations of V382 Vel. Date Day after outburst Mission Remarks 5–498 La Silla 1999 May 26 1999 June 7 5.7 15 RXTE BeppoSAX 1999 June 9/10 1999 June 20 1999 June 24 1999 July 9 1999 July 18 1999 May 31– 1999 Aug. 29 20.5 31 35 50 59 ASCA RXTE RXTE RXTE RXTE HST/STIS 1999 Nov. 23 1999 Dec. 30 2000 Feb. 6–Jul 3 2000 Feb. 14 2000 Apr. 21 2000 Aug. 14 185 223 BeppoSAX Chandra (ACIS) FUSE Chandra (LETG) Chandra (ACIS) Chandra (ACIS) V max = 2.3 (1999 23 May) fast Ne Nova; d = 1.7 kpc (±20 per cent) Faint in X-rays First X-ray detection no soft component Highly absorbed bremsstrahlung Decreasing plasma temperature and column density .. .. UV lines indicate fragmentation of ejecta; C and Si underabundant, O, N, Ne, Al over-abundant d = 2.5 kpc; N H = 1.2 × 1021 cm−2 Hard and soft component 268 335 450 O VI line profile Details in this paper Reference Della Valle et al. (2002) Mukai & Ishida (2001) Orio et al. (2001) Mukai & Ishida (2001) Mukai & Ishida (2001) Mukai & Ishida (2001) Mukai & Ishida (2001) Mukai & Ishida (2001) Shore et al. (2003) Orio et al. (2001, 2002) Burwitz et al. (2002) Shore et al. (2003) Burwitz et al. (2002) Burwitz et al. (2002) Burwitz et al. (2002)  C 2005 The Authors. Journal compilation  C 2005 RAS, MNRAS 364, 1015–1024 The outburst of the Classical Nova V382 Vel was discovered on 1999 May 22 (Gilmore 1999). V382 Vel reached a V max brighter than 3, making it the brightest nova since V1500 Cyg in 1975. Della Valle et al. (2002) described optical observations of V382 Vel and classified the nova as fast and belonging to the Fe II broad spectroscopic class. Its distance was estimated to be 1.7 ± 0.34 kpc. Infrared observations detected the [Ne II] emission line at 12.8 µm characteristic of the ‘neon nova’ group and, subsequently, V382 Vel was recognized as an ONeMg nova (Woodward et al. 1999). An extensive study of the ultraviolet (UV) spectrum was presented by Shore et al. (2003), who analysed spectra obtained with the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST) in 1999 August as well as spectra from the Far Ult (...truncated)