The Swift GRB 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 XRT (0.3-10 keV) X-ray instrument count rates and the UVOT (1700-8000 Angstroms) 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 3 years after the outburst begins. Previous relationships, such as the nuclear burning duration vs. t_2 or the expansion velocity of the eject and nuclear burning duration vs. 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.
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 XRT (0.3-10 keV) X-ray instrument count rates and the UVOT (1700-8000 Å) 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 3 years after the outburst begins. Previous relationships, such as the nuclear burning duration vs. t 2 or the expansion velocity of the eject and nuclear burning duration vs. 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.
Subject headings: novae, cataclysmic variables -X-rays: stars -ultraviolet: stars
Novae occur in binary systems in which a Roche lobe filling secondary is losing hydrogen-rich material through the inner Lagrangian point onto a white dwarf (WD) primary. Mass transfer can also occur in long period systems if the secondary has a significant wind, e.g. the giant secondary in RS Oph or V407 Cyg. Core material is mixed into the accreted material and is violently ejected into space when the pressure at the WD-accretion interface becomes great enough to initiate a thermonuclear runaway (TNR). Novae eject, into the interstellar medium (ISM), a mixture of material accreted from the companion star, highly processed material from the underlying WD, and products of nucleosynthesis occurring during the TNR. As a result of the TNR, up to 10 -4 M ⊙ of material can be ejected from the WD enriched in C, N, O, Ne, Mg, Al and other species (José et al. 2006) at v ∼ 10 2 -10 4 km s -1 . Any remaining hydrogen still bound to the WD continues to burn in hydrostatic equilibrium until it is consumed or ejected via a wind.
Initially, the radiative output of a nova occurs in the optical but as the photosphere of the WD recedes, the spectral energy distribution shifts to higher energies (Gallagher & Starrfield 1978). The rate of the optical decline defines a nova’s primary characteristics (e.g., Warner 2008, and references therein), namely the time to decline 2 magnitudes from visual maximum, t 2 . The decline rate depends on the amount of mass ejected, its velocity, composition, and if it runs into circumbinary material. The bolometric luminosity during the outburst is high, near or exceeding the Eddington limit (for the fastest novae), and thus additional material is ejected via a strong stellar wind (Schwarz et al. 1998(Schwarz et al. , 2001)). In some novae the collision between this fast wind and the initial exploded mass or any pre-existing circumbinary material can produce X-ray emission from shocks. The emission from this early X-ray phase is hard, has a low luminosity, of order 10 33-35 erg s -1 , and declines relatively rapidly (Balman et al. 1998;Orio et al. 2001b). As fuel continues to burn, mass loss causes the photosphere of the WD to shrink (MacDonald et al. 1985). The effective temperature increases, peaking in the soft X-rays, at (2-8)×10 5 K (Krautter et al. 1996;Shore et al. 1996;Rauch et al. 2010). Once the ejecta have cleared sufficiently, and if the line of sight extinction is not severe, some novae exhibit characteristics similar to the Super Soft X-ray binary sources (SSSs: Kahabka & van den Heuvel 1997) with strong and soft, E peak < 1 keV, Xray emission. This point in novae evolution is called the SSS phase. At low spectral resolution, the UV/X-ray spectral energy distributions (SED) resembles blackbodies, but higher resolution Chandra or XMM grating observations reveal a significantly more complex picture. The spectra frequently have P-Cygni profiles or emission lines superimposed on a line blanketed atmosphere. Models sophisticated enough to interpret the high resolution data are only now becoming available (van Rossum & Ness 2010). Once nuclear burning ends, the X-ray light curve rapidly declines as the WD cools marking the end of the SSS phase and the outburst. At some point mass transfer resumes and eventually another eruption occurs. These are called classical novae (CNe) until a second outburst is observed the
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