The Swift gamma-ray burst satellite is an excellent facility for studying novae. Its rapid response time and sensitive X-ray detector provides an unparalleled opportunity to investigate the previously poorly sampled evolution of novae in the X-ray regime. This paper presents Swift observations of 52 Galactic/Magellanic Cloud novae. We included the X-Ray Telescope (0.3-10?keV) instrument count rates and the UltraViolet and Optical Telescope (1700-8000??) filter photometry. Also included in the analysis are the publicly available pointed observations of 10 additional novae the X-ray archives. This is the largest X-ray sample of Galactic/Magellanic Cloud novae yet assembled and consists of 26 novae with Super Soft X-ray emission, 19 from Swift observations. The data set shows that the faster novae have an early hard X-ray phase that is usually missing in slower novae. The Super Soft X-ray phase occurs earlier and does not last as long in fast novae compared to slower novae. All the Swift novae with sufficient observations show that novae are highly variable with rapid variability and different periodicities. In the majority of cases, nuclear burning ceases less than three years after the outburst begins. Previous relationships, such as the nuclear burning duration versus t 2 or the expansion velocity of the eject and nuclear burning duration versus the orbital period, are shown to be poorly correlated with the full sample indicating that additional factors beyond the white dwarf mass and binary separation play important roles in the evolution of a nova outburst. Finally, we confirm two optical phenomena that are correlated with strong, soft X-ray emission which can be used to further increase the efficiency of X-ray campaigns.

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Swift X-Ray Observations of Classical Novae. II. The Super Soft Source Sample

Nova explosions occur on the white dwarf component of a Cataclysmic Variable binary stellar system that is accreting matter lost by its companion. When sufficient material has been accreted by the white dwarf, a thermonuclear runaway occurs and ejects material in what is observed as a Classical Nova explosion. We describe both the recent advances in our understanding of the progress of the outburst and outline some of the puzzles that are still outstanding. We report on the effects of improving both the nuclear reaction rate library and including a modern nuclear reaction network in our one-dimensional, fully implicit, hydrodynamic computer code. In addition, there has been progress in observational studies of Supernovae Ia with implications about the progenitors and we discuss that in this review.

The Thermonuclear Runaway and the Classical Nova Outburst

Swift X-ray observations of the ~60 day supersoft phase of the recurrent nova RS Ophiuchi (RS Oph) 2006 show the progress of nuclear burning on the white dwarf (WD) in exquisite detail. First seen 26 days after the optical outburst, this phase started with extreme variability likely due to variable absorption, although intrinsic WD variations are not excluded. About 32 days later, a steady decline in count rate set in. NLTE model atmosphere spectral fits during the supersoft phase show that the effective temperature of the WD increases from ~65 eV to ~90 eV during the extreme variability phase, falling slowly after about day 60 and more rapidly after day 80. The bolometric luminosity is seen to be approximately constant and close to Eddington from day 45 up to day 60, the subsequent decline possibly signaling the end of extensive nuclear burning. Before the decline, a multiply-periodic ~35 s modulation of the soft X-rays was present and may be the signature of a nuclear fusion driven instability. Our measurements are consistent with a WD mass near the Chandrasekhar limit; combined with a deduced accumulation of mass transferred from its binary companion, this leads us to suggest that RS Oph is a strong candidate for a future supernova explosion. The main uncertainty now is whether the WD is the CO type necessary for a Type Ia supernova. This may be confirmed by detailed abundance analyses of spectroscopic data from the outbursts.

https://iopscience.iop.org/article/10.1088/0004-637X/727/2/124/pdf

The supersoft x-ray phase of nova rs ophiuchi 2006

We present a spectral analysis of the archival X-ray data of classical Nova V1974 Cygni 1992 (Nova Cygni 1992) obtained by the ROSAT Position Sensitive Proportional Counter (PSPC). The X-ray spectrum is fitted with a two-component model. The first component is a white dwarf atmosphere emission model developed for the remnants of classical novae near the Eddington luminosity. The model is used to fit the soft X-ray data in the ~0.1-1.0 keV range, where the bulk of emission is below 0.7 keV. The second component is a Raymond-Smith model of thermal plasma applied to the hard X-ray emission above ~1.0 keV. The postoutburst X-ray spectrum of the remnant white dwarf is examined in the context of evolution on the Hertzsprung-Russell diagram using an O-Ne- and a C-O-enhanced atmosphere emission model. A constant bolometric luminosity evolution is detected with increasing effective temperature and decreasing photospheric radius using the O-Ne-enhanced model. The unabsorbed soft X-ray flux for the constant bolometric luminosity phase is found to be in the range (1.7-2.2) × 10-7 ergs s-1 cm-2. A peak effective temperature of 51 eV (5.9 ×105 K) is detected 511 days after outburst. We also present the spectral development of the hard X-ray component. It is found to evolve independently of the soft one. The maximum of the hard X-ray emission is reached at ~150 days after outburst with an unabsorbed flux of ~2.0 × 10-11 ergs s-1 cm-2 corresponding to a luminosity of (0.8-2.0) × 1034 ergs s-1 at a 2-3 kpc source distance. The time evolution of the hard X-ray flux and the plasma temperatures decreasing from 10 keV to 1 keV suggest emission from shock-heated gas as the origin of the hard X-ray component.

https://iopscience.iop.org/article/10.1086/305600/pdf

The X-Ray Spectral Evolution of Classical Nova V1974 Cygni 1992: A Reanalysis of the ROSAT Data

We searched for X-ray counterparts of optical novae detected in M 31 and M 33. We combined an optical nova catalogue from the WeCAPP survey with optical novae reported in the literature and correlated them with the most recent X-ray catalogues from ROSAT, XMM-Newton, and Chandra, and – in addition – searched for nova correlations in archival data. We report 21 X-ray counterparts for novae in M 31 – mostly identified as supersoft sources (SSS) by their hardness ratios – and two in M 33. Our sample more than triples the number of known optical novae with a supersoft X-ray phase. Most of the counterparts are covered in several observations allowing us to constrain their X-ray light curves. Selected brighter sources were classified by their XMM-Newton EPIC spectra. We use the well-determined start time of the SSS state ( �� ) ( �� ) ( �� )

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Optical novae: the major class of supersoft X-ray sources in M 31

We address questions concerning the evolution of classical nova systems through outburst and the time scale for the return to their preoutburst state. Observations indicate that evolution on a purely nuclear burning time scale is not sufficient. We call attention to the sequence of events involving common envelope evolution which naturally occurs following thermonuclear runaways in the accreted hydrogen shells on the white dwarf components of these close binary systems. We estimate that a significant fraction of the envelope may be lost as a result of dynamical friction during a relatively short-lived common envelope phase which is experienced by most classical nova systems. The residual hydrogen-rich envelope matter may be consumed by nuclear burning or wind-driven mass loss or both on time scales compatible with observations of nova systems. As mass is consumed, the photospheric radius decreases and consequently the photospheric temperature rises. The associated hardening of the radiation from novae causes them progressively to become UV, EUV, and ultimately soft X-ray sources. Estimates of the fluxes during these late stages are provided and shown to be compatible with their being detected with EXOSAT or equivalent X-ray facilities. Such detection would provide strong confirmation of theoretical models for the classicalmore » novae. We also point out that infrared emission from X-ray heated grains may be detectable during the soft X-ray phase.« less

M.Y. Fujimoto

Papers

The decline and fall of classical novae