Supersoft X-ray Phase of Single Degenerate Type Ia Supernova Progenitors in Early Type Galaxies

In the single degenerate (SD) scenario for Type Ia supernova (SN Ia) progenitors, an accreting white dwarf (WD) is expected to undergo a supersoft X-ray source (SSS) phase. Recently, Gilfanov & Bogdan (2010, hereafter GB10) claimed that observed X-ray fluxes of early type galaxies would be too low to be consistent with the prediction of the SD scenario based on rather simple assumptions. We present realistic evolutionary models of SD systems and calculate durations of SSS phases. In most cases, accreting WDs spend a large fraction of time in the optically thick wind phase and the recurrent nova phase rather than the SSS phase. Thus the SSS phase lasts only for a few hundred thousand years. This is by a factor of ~10 shorter than those adopted by GB10 where the SN~Ia progenitor WD was assumed to spend most of its life as a SSS. The theoretical X-ray luminosity of the SSS has a large uncertainty because of the uncertain atmospheric model of mass-accreting WDs and absorption of soft X-rays by the companion star's cool wind material. We thus adopt an average of the observed fluxes of existing symbiotic SSSs, i.e., ~0.4 x 10^{36} erg s^{-1} for 0.3--0.7 keV. Using these SSS duration and soft X-ray luminosity, we show that the observed X-ray flux obtained by GB10 is rather consistent with our estimated flux in early type galaxies based on the SD scenario. This is a strong support for the SD scenario as a main-contributor of SNe Ia in early type galaxies.Comment: 5 pages, 3 figures, to appear in ApJ

Accreting White Dwarfs as Supersoft X-ray Sources

I review various phenomena associated with mass-accreting white dwarfs (WDs) in the view of supersoft X-ray sources. When the mass-accretion rate is low (\dot M_{acc} < a few \times 10^{-7} M_\sun yr^{-1}), hydrogen nuclear burning is unstable and nova outbursts occur. A nova is a transient supersoft X-ray source (SSS) in its later phase which timescale depends strongly on the WD mass. The X-ray turn on/off time is a good indicator of the WD mass. At an intermediate mass-accretion rate an accreting WD becomes a persistent SSS with steady hydrogen burning. For a higher mass-accretion rate, the WD undergoes "accretion wind evolution" in which the WD accretes matter from the equatorial plane and loses mass by optically thick winds from the other directions. Two SSS, namely RX J 0513-69 and V Sge, are corresponding objects to this accretion wind evolution. We can specify mass increasing WDs from light-curve analysis based on the optically thick wind theory using multiwavelength observational data including optical, IR, and supersoft X-rays. Mass estimates of individual objects give important information for the binary evolution scenario of type Ia supernovae.Comment: 6 pages including 10 figures, conference proceedings of "Supersoft X-ray Sources - New Developments" (18th - 20th May 2009, European Space Astronomy Centre (ESAC), Villafranca del Castillo, Madrid, Spain

The Delay Time Distribution of Type Ia Supernovae and the Single Degenerate Model

We present a theoretical delay time distribution (DTD) of Type Ia supernovae on the basis of our new evolutionary models of single degenerate (SD) progenitor systems. Our model DTD has almost a featureless power law shape (\propto t^{-n} with n \approx 1) for the delay time from t \sim 0.1 to 10 Gyr. This is in good agreement with the recent direct measurement of DTD. The observed featureless property of the DTD has been suggested to be favorable for the double degenerate (DD) scenario but not for the SD scenario. If the mass range of the companion star to the white dwarf (WD) were too narrow in the SD model, its DTD would be too limited around the companion's main-sequence lifetime to be consistent with the observed DTD. However, this is not the case in our SD model that consists of the two channels of WD + RG (red giant) and WD + MS (main-sequence star). In these channels, the companion stars have a mass range of \sim 0.9-3 M_\sun (WD+RG) and \sim 2- 6 M_\sun (WD+MS). The combined mass range is wide enough to yield the featureless DTD. We emphasize that the SD scenario should include two important processes: the optically thick winds from the mass-accreting WD and the mass-stripping from the companion star by the WD wind.Comment: 5 pages including 2 figures, to appear in ApJL (accepted version

Binary population synthesis study on supersoft X-ray phase of single degenerate type Ia supernova progenitors

In the single degenerate (SD) scenario for type Ia supernovae (SNe Ia), a mass-accreting white dwarf is expected to experience a supersoft X-ray source (SSS) phase. However, some recent observations showed that the expected number of the mass-accreting WD is much lower than that predicted from theory, whatever in spiral or elliptical galaxies. In this paper, we did a binary population synthesis study on the relative duration of the SSS phase to their whole mass-increasing phase of WDs leading to SNe Ia. We found that for about 40% progenitor systems, the relative duration is shorter than 2% and the evolution of the mean relative duration shows that it is always smaller than 5%, whatever for young or old SNe Ia. In addition, before SNe Ia explosion, more than 55% progenitor systems are experiencing a dwarf novae phase, and only no more than 10% is staying SSS phase. These results are consistent with the recent observations, and imply that both in early- and late-type galaxies, only a small fraction of mass-accreting WD resulting in SNe Ia contribute to the supersoft X-ray flux. So, although our results are not directly related to the X-ray output of SN Ia progenitor, the low supersoft X-ray luminosity observed in early type galaxies may have no ability to exclude the validity of SD model. On the contrary, it is evidence to support the SD scenario.Comment: 9 pages, 5 figures, accepted for publication in RA

Catalog of 93 Nova Light Curves: Classification and Properties

We present a catalog of 93 very-well-observed nova light curves. The light curves were constructed from 229,796 individual measured magnitudes, with the median coverage extending to 8.0 mag below peak and 26% of the light curves following the eruption all the way to quiescence. Our time-binned light curves are presented in figures and as complete tabulations. We also calculate and tabulate many properties about the light curves, including peak magnitudes and dates, times to decline by 2, 3, 6, and 9 magnitudes from maximum, the time until the brightness returns to quiescence, the quiescent magnitude, power law indices of the decline rates throughout the eruption, the break times in this decline, plus many more properties specific to each nova class. We present a classification system for nova light curves based on the shape and the time to decline by 3 magnitudes from peak (t3). The designations are S for smooth light curves (38% of the novae), P for plateaus (21%), D for dust dips (18%), C for cusp-shaped secondary maxima (1%), O for quasi-sinusoidal oscillations superposed on an otherwise smooth decline (4%), F for flat-topped light curves (2%), and J for jitters or flares superposed on the decline (16%). Our classification consists of this single letter followed by the t3 value in parentheses; so for example V1500 Cyg is S(4), GK Per is O(13), DQ Her is D(100), and U Sco is P(3).Comment: Astronomical Journal, in press, 19 figures, 73 page

Final Fates of Rotating White Dwarfs and Their Companions in the Single Degenerate Model of Type Ia Supernovae

Taking into account the rotation of mass-accreting white dwarfs (WDs) whose masses exceed the Chandrasekhar mass, we extend our new single degenerate model for the progenitors of Type Ia supernovae (SNe Ia), accounting for two types of binary systems, those with a main sequence companion and those with a red-giant (RG) companion. We present a mass distribution of WDs exploding as SNe Ia, where the WD mass ranges from 1.38 to 2.3 Msun. These progenitor models are assigned to various types of SNe Ia. A lower mass range of WDs (1.38 Msun < M_WD <~ 1.5 Msun), which are supported by rigid rotation, correspond to normal SNe Ia. A variety of spin-down time may lead to a variation of brightness. A higher mass range of WDs (M_WD >~ 1.5 Msun), which are supported by differential rotation, correspond to brighter SNe Ia such as SN 1991T. In this case, a variety of the WD mass may lead to a variation of brightness. We also show the evolutionary states of the companion stars at SN Ia explosions and pose constraints on the unseen companions. In the WD+RG systems, in particular, most of the RG companions have evolved to helium/carbon-oxygen WDs in the spin-down phase before the SN Ia explosions. In such a case, we do not expect any prominent signature of the companion immediately before and after the explosion. We also compare our new models with the recent stringent constraints on the unseen progenitors of SNe Ia such as SN 2011fe.Comment: 6 pages including 3 figures, to appear in ApJ