XMM-NEWTONX-RAY AND ULTRAVIOLET OBSERVATIONS OF THE FAST NOVA V2491 Cyg DURING THE SUPERSOFT SOURCE PHASE

Abstract

Two XMM-Newton observations of the fast classical nova V2491 Cyg were carried out in short succession on days 39.93 and 49.62 after discovery, during the supersoft source (SSS) phase, yielding simultaneous X-ray and UV light curves and high-resolution X-ray spectra. The first X-ray light curve is highly variable, showing oscillations with a period of 37.2 minutes after an extended factor of three decline lasting ∼ 3 hours, while the second X-ray light curve is less variable. The cause of the dip is currently unexplained and has most likely the same origin as similar events in the early SSS light curves of the novae V4743 Sgr and RS Oph, as it occurred on the same time scale. The oscillations are not present during the dip minimum and also not in the second observation. The UV light curves are variable but contain no dips and no period. High-resolution X-ray spectra are presented for 4 intervals of differing intensity. All spectra are atmospheric continua with deep absorption lines and absorption edges. Two interstellar lines of O i and N i are clearly seen at their rest wavelengths, while a large number of high-ionization absorption lines are found at blue shifts indicating an expansion velocity of 3000−3400 km s −1 , which does not change significantly during the epochs of observation. Comparisons with the slower nova V4743 Sgr and the symbiotic recurrent nova RS Oph are presented. The SSS spectrum of V4743 Sgr is much softer with broader and more complex photospheric absorption lines. The ejecta are extended, allowing us to view a larger range of the radial velocity profile. Meanwhile, the absorption lines in RS Oph are as narrow as in V2491 Cyg, but they are less blue shifted. A remarkable similarity in the continua of V2491 Cyg and RS Oph is found. The only differences are smaller line shifts and additional emission lines in RS Oph that are related to the presence of a dense stellar wind from the evolved companion. Three unidentified absorption lines are present in the X-ray spectra of all three novae, with projected rest wavelengths 26.05Å, 29.45Å, and 30.0Å. No entirely satisfactory spectral model is currently available for the soft X-ray spectra of novae in outburst, and careful discussion of assumptions is required.

SUZAKUOBSERVATION OF THE CLASSICAL NOVA V2491 Cyg IN QUIESCENCE

“…Although these distances have been inferred with the maximum-magnitude rate-of-decay (MMRD) relationship and these values may thus be highly uncertain (the relationship does not hold for RNe and sometimes is not very precise for CNe, moreover the nova had a rare secondary optical maximum), the optical and especially the supersoft X-ray luminosity in the outburst indicate that the distance cannot be much smaller. Especially the large supersoft X-ray flux (Page et al 2010;Ness et al 2011) is evidence of a distance of at least 10 kpc, otherwise the WD would have been underluminous compared with the models of the same high effective temperature (from 9 × 10 5 to a 10 6 K, see Starrfield et al 2012;Wolf et al 2013) and also with previous observations of novae in the SSS phase (see Orio 2012, for a review). We analysed the same dataset with the PDM method and found that the 0.02885 days period is most probably a daily alias of a 0.02655 days (38.23 min) period, which is very close to the one we found in X-rays.…”

Multiwavelength modelling the SED of supersoft X-ray sources I. The method and examples

“…In general, the atmosphere models have a steeper Wien tail of the spectrum than the blackbody, because of the presence of deep absorption lines and absorption edges seen in the high-resolution X-ray spectra (e.g. Ness et al , 2011). Thus to match the specific profile of the supersoft fluxes, a model atmosphere is required to obtain more reliable parameters than with a simple blackbody.…”

Reanalysis of high-resolutionXMM-Newtondata of V2491 Cygni using models of collisionally ionized hot absorbers

“…(2010) have only assumed solar abundances for simplicity while working with the NLTE expanding atmosphere models and Ness et al (2011) have found consistent fits using static NLTE atmosphere models with fixed abundances where oxygen abundance was in a range O/O =10-30. This latter model has similar abundances to what we derived in this study.…”

“…The spectra have very broad lines with complex profiles and large expansion velocities (∼4000-6000 km s −1 ) (Lynch et al 2008;Tomov et al 2008). V2491 Cyg shows both the hard X-ray and the soft X-ray components during the outburst stage (analysis can be found in Page et al 2010;Ness et al 2011). Ibarra et al (2009) show that V2491 was a persistent X-ray source using archival ROS AT , XMMNewton, and Swift data during its quiescent phase before the optical outburst.…”