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

Two XMM-Newton observations of the fast classical nova V2491Cyg 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 V4743Sgr and RSOph, 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 Oi and Ni 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 3400kms 1 , which does not change significantly during the epochs of observation. Comparisons with the slower nova V4743Sgr and the symbiotic recurrent nova RSOph are presented. The SSS spectrum of V4743Sgr 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 RSOph are as narrow as in V2491Cyg, but they are less blue shifted. A remarkable similarity in the continua of V2491Cyg and RSOph is found. The only differences are smaller line shifts and additional emission lines in RSOph 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 26.05u 29.45u and 30.0u 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. Subject headings: novae, cataclysmic variables - stars: individual (V2491 Cyg) - stars: individual (RS Oph) - stars: individual (V4743 Sgr) - AAVSO

[1]  M. Bode,et al.  Shaping of nova remnants by binary motion , 1997 .

[2]  B. Savage,et al.  A survey of interstellar H I from L-alpha absorption measurements. II , 1978 .

[3]  J. Ness,et al.  Variability and multiperiodic oscillations in the X-ray light curve of the classical nova V4743 Sgr , 2006, astro-ph/0607157.

[4]  N. Grevesse,et al.  Standard Solar Composition , 1998 .

[5]  F. A. Ringwald,et al.  Early post-maximum spectral evolution of the fast novae V4742 Sagittarii and V4743 Sagittarii , 2003 .

[6]  S. Naik,et al.  Near‐infrared observations of the novae V2491 Cygni and V597 Puppis , 2008, 0812.4096.

[7]  M. Bode,et al.  Book-Review - Rs-Ophiuchi 1985 and the Recurrent Nova Phenomenon , 1987 .

[8]  Alon Retter,et al.  Nova V4743 Sagittarii 2002: An Intermediate Polar Candidate , 2006, astro-ph/0604438.

[9]  P. H. Hauschildt,et al.  Numerical solution of the expanding stellar atmosphere problem , 1998 .

[10]  G. Ferland,et al.  CLOUDY 90: Numerical Simulation of Plasmas and Their Spectra , 1998 .

[11]  M. F. Bode,et al.  EXQUISITE NOVA LIGHT CURVES FROM THE SOLAR MASS EJECTION IMAGER (SMEI) , 2010, 1009.1737.

[12]  M. Irwin,et al.  A Planetary Nebula around Nova V458 Vulpeculae Undergoing Flash Ionization , 2008, 0810.0401.

[13]  Mario Livio,et al.  The Calibration of Novae as Distance Indicators , 1995 .

[14]  J. Ness,et al.  Multifrequency nature of the 0.75 mHz feature in the X-ray light curves of the nova V4743 Sgr , 2010, 1003.2336.

[15]  J. P. Osborne,et al.  The SSS Phase of RS Ophiuchi Observed with Chandra and XMM-Newton. I. Data and Preliminary Modeling , 2007, 0705.1206.

[16]  J. Scargle.  Studies in astronomical time series analysis. II - Statistical aspects of spectral analysis of unevenly spaced data , 1982 .

[17]  Sumner Starrfield,et al.  ROSAT X-ray observations of nova V1974 Cygni: The rise and fall of the brightest supersoft X-ray source , 1996 .

[18]  J. Krautter,et al.  A Chandra Low Energy Transmission Grating Spectrometer Observation of V4743 Sagittarii: A Supersoft X-Ray Source and a Violently Variable Light Curve , 2003 .

[19]  I. Hachisu,et al.  OPTICAL AND SUPERSOFT X-RAY LIGHT-CURVE MODELS OF CLASSICAL NOVA V2491 CYGNI: A NEW CLUE TO THE SECONDARY MAXIMUM , 2009, 0902.2424.

[20]  P. Hauschildt,et al.  The early spectral evolution of Nova Cassiopeiae 1993 , 1994 .

[21]  Manabu Ishida,et al.  The Early X-Ray Emission from V382 Velorum (Nova Velorum 1999): An Internal Shock Model , 2001 .

[22]  Sumner Starrfield,et al.  Theory and Observations of Classical Novae , 1978 .

[23]  Bradley E. Schaefer,et al.  Discovery of a Second Nova Eruption of V2487 Ophiuchi , 2009, 0908.2143.

[24]  G. Shaviv,et al.  Spherically symmetric, expanding, non-LTE model atmospheres for novae during their early stages , 1992 .

[25]  J. Wilms,et al.  Absorption Of X-rays In The Interstellar Medium , 2000, astro-ph/0008425.

[26]  M. F. Bode,et al.  X-ray detection of Nova Herculis 1991 five days after optical outburst , 1992, Nature.

[27]  J. P. Osborne,et al.  THE SUPERSOFT X-RAY PHASE OF NOVA RS OPHIUCHI 2006 , 2010, 1011.5327.

[28]  M. Bode,et al.  On the asphericity of nova remnants caused by rotating white dwarf envelopes , 1998 .

[29]  Brian Warner.  Classical Novae: Properties of novae: an overview , 2008 .

[30]  S. Baliunas,et al.  A Prescription for period analysis of unevenly sampled time series , 1986 .

[31]  J. Dickey,et al.  H I in the Galaxy , 1990 .

[32]  Steven N. Shore,et al.  The Physics of Early Nova Spectra , 1995 .

[33]  A. Talavera,et al.  Optical and UV monitor (OM) on-board XMM-Newton , 2009 .

[34]  S. Vaughan.  A simple test for periodic signals in red noise , 2005 .

[35]  J. Ness.  Observational evidence for expansion in the SSS spectra of novae , 2009, 0908.4549.

[36]  P. Kahabka,et al.  Luminous Supersoft X-Ray Sources , 1997 .

[37]  S. Vaughan,et al.  A Bayesian test for periodic signals in red noise , 2009, 0910.2706.

[38]  G. Sala,et al.  A classical nova, V2487 Oph 1998, seen in x-rays before and after its explosion. , 2002, Science.

[39]  Y. Avni,et al.  Energy spectra of X-ray clusters of galaxies , 1976 .

[40]  H. Duerbeck,et al.  Optical Imaging of Nova Shells and the Maximum Magnitude-Rate of Decline Relationship , 2000, astro-ph/0006458.

[41]  P. Hauschildt,et al.  Non-LTE model atmosphere analysis of Nova Cygni 1992 , 1994 .

[42]  Elena P. Pavlenko,et al.  Discovery of the optical variability of Nova V2491 Cyg , 2008 .

[43]  J. Ness,et al.  EVOLUTION OF X-RAY SPECTRA AND LIGHT CURVES OF V1494 AQUILAE , 2009, 0903.5386.

[44]  Eric D. Ragan,et al.  V2491 Cyg - a possible recurrent nova ? , 2008 .

[45]  M. Della Valle,et al.  The Extraordinary X-ray Light Curve of the Classical Nova V1494 Aquilae (1999 No. 2) in Outburst: The Discovery of Pulsations and a “Burst” , 2002, astro-ph/0210072.

[46]  D. R. van Rossum,et al.  Expanding atmosphere models for SSS spectra of novae , 2010, 1001.1462.

[47]  U. Munari,et al.  An extensive optical study of V2491 Cyg (Nova Cyg 2008 N.2), from maximum brightness to return to quiescence , 2010, 1009.0822.

[48]  Baltimore,et al.  NON-LOCAL THERMAL EQUILIBRIUM MODEL ATMOSPHERES FOR THE HOTTEST WHITE DWARFS: SPECTRAL ANALYSIS OF THE COMPACT COMPONENT IN NOVA V4743 Sgr , 2010, 1006.2918.

[49]  I. Steele,et al.  The morphology of the expanding ejecta of V2491 Cygni (2008 N.2) , 2010, 1011.2045.

[50]  Marina Orio,et al.  X-Ray Spectroscopy of the 2006 Outburst of RS Ophiuchi , 2007, 0709.4512.

[51]  J. Krautter,et al.  Swift Observations of the 2006 Outburst of the Recurrent Nova RS Ophiuchi. I. Early X-Ray Emission from the Shocked Ejecta and Red Giant Wind , 2006, astro-ph/0604618.

[52]  N. Gehrels,et al.  XMM-Newton RGS observation of V2491 Cyg , 2008 .

[53]  L. Amati,et al.  The X-ray emission from Nova V382 Velorum - I. The hard component observed with BeppoSAX , 2001, astro-ph/0102437.

[54]  J. Drake,et al.  SUZAKU DETECTION OF SUPERHARD X-RAY EMISSION FROM THE CLASSICAL NOVA V2491 CYGNI , 2009, 0904.1693.

[55]  Peter R. Young,et al.  CHIANTI—An Atomic Database for Emission Lines. VII. New Data for X-Rays and Other Improvements , 2006 .

[56]  J. Herder,et al.  The Reflection Grating Spectrometer on-board XMM-Newton: Status of the Calibrations , 2002 .

[57]  A. R. Taylor,et al.  Radio observations of the 1985 outburst of RS Ophiuchi , 1986 .

[58]  J. Osborne,et al.  A Chandra High-Resolution X-ray spectrum of KT Eri , 2010 .