Optical spectroscopy of XMMU J115113.3-623730 = Nova Cen 2008

ATel #2771; J. P. Hughes (Rutgers Univ.), P. Slane, B. Posselt (Harvard-Smithsonian CfA), P. Charles, A. Rajoelimanana, R. Sefako (South African Astronomical Observatory), J. Halpern (Columbia Univ.), and D. Steeghs (Univ. of Warwick)
on 4 Aug 2010; 17:37 UT
Credential Certification: John P. Hughes (jph@physics.rutgers.edu)

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On 18 July 2010 at UT 17:22:56 and 17:45:27 we obtained two non-flux-calibrated optical spectra of the transient X-ray source XMMU J115113.3-623730 (which we also designate as Nova Cen 2008) reported by Greiner et al. (ATEL #2746) using the Grating Spectrograph on the 1.9-m telescope at the South African Astronomical Observatory. The 300 lines/mm grating with a 1.2 arcsec wide slit was used giving a wavelength coverage of 3800 Å to 7500 Å and a spectral resolution of 5 Å. The integration time of each exposure was 1200 s. Spectra of Cu/Ar arc lamps were taken to determine the wavelength calibration. Data reduction was carried out using standard IRAF tasks.

The combined spectrum shows a number of strong emission lines including H&alpha and the rest of the Balmer series; He II (4541, 4686, 5411); O VI 5291; C IV 5801, 5812; N V 4945 and 4604; [Fe VII] 5721; a strong broad line near 6086 Å (possibly [Ca V] or [Fe VII]); and another one near 6380 Å (possibly [Fe X]). He I emission is weak or absent. The hydrogen and metal lines are broadened by approximately 20 Å to 30 Å (FWHM), while the He II lines are much narrower: 10 Å or less. The width of H&alpha (and H&beta) corresponds to a velocity broadening of approximately 1400 km/s. The equivalent widths of H&alpha, O VI 5291, [Fe VII] 5721, and [O III] 5007 are 36 Å, 15 Å, 3 Å, and 2 Å, respectively. H&alpha and O VI 5291 show somewhat asymmetric line profiles.

Several narrow interstellar absorption features are present in the spectrum as well, two of which we identify as Na D and the 5778 diffuse interstellar band with equivalent widths of 1.4 Å and 0.5 Å. These indicate a modest amount of reddening, E(B-V) ~ 0.57, and an approximate distance of 1 kpc, in broad agreement with the X-ray-derived results of Greiner et al. (ATEL #2746).

XMMU J115113.3-623730 displays a high ratio of He II 4686 to H&beta, the presence of O VI and N V lines, and supersoft X-ray emission which are the characteristics of the class of V Sagittae stars (Steiner and Diaz 1998, PASP, 110, 276). In addition, such features can also be found in, for example, the recurrent nova U Sco (Barlow et al. 1981, MNRAS, 195, 61) and the classical nova GQ Mus (Pequignot et al. 1993, A&A, 271, 219; Diaz and Steiner 1994, ApJ, 425, 252). Optical spectra of the classical nova V598 Pup (which was suggested as a possible analog by Greiner et al.) do not show the high excitation lines that dominate our spectra.

The broad line widths and asymmetries suggest the presence of an outflow or jets; while the presence of forbidden line emission ([Fe VII] and possible [O III] 5007) indicates that the source is in or entering the nebular stage.

Our spectra thus corroborate the relatively-nearby nova classification for this object, but further spectroscopic monitoring of Nova Cen 2008 is warranted to study its evolution and look for dynamical signatures of binary motion.