Population Of Supersoft X-ray Sources Research Articles

Populations of super-soft X-ray sources in galaxies of different morphological types

Aims. We study populations of soft and super-soft X-ray sources (SSSs) in nearby galaxies of various morphological types with particular emphasis on characterizing populations of stable nuclear burning, accreting white dwarfs (WDs). Methods. Analyzing the content of the Chandra archive, we assembled a sample of nearby galaxies suitable for studying populations of SSSs. Our sample includes four spiral galaxies, two lenticular galaxies, and three ellipticals with stellar mass exceeding 1010 M⊙ and X-ray sensitivity of the order of a few × 1036 erg s−1. We used a combination of hardness ratio and median energy to pre-select X-ray sources with soft spectra, and used the temperature–X-ray luminosity diagram to identify SSSs, likely nuclear-burning, accreting WDs. Results. For spiral galaxies, there is a distinct and rare population of super-soft sources that are largely detached from the rest of the sources on the kTbb − LX plane. The boundary between these sources and the much more numerous population of harder (but still soft) sources is consistent with the boundary of stable hydrogen burning on the surface of WDs. The combined spectrum of soft sources located outside this boundary shows clear emission lines of Mg and S, the equivalent width of which is similar to that in the combined spectrum of a large number of confirmed supernova remnants in M 83. This supports earlier suggestions that the vast majority of the so-called quasi-soft sources are supernova remnants. In early-type galaxies, populations of super-soft sources are about a factor of eight less abundant, in broad agreement with the population synthesis calculations. Specific frequencies of super-soft sources are (2.08 ± 0.46) × 10−10 M⊙−1 in spiral galaxies and (2.47 ± 1.34) × 10−11 M⊙−1 in lenticular and elliptical galaxies, with the ratio of the latter to the former being 0.12 ± 0.05.

SINGLE-DEGENERATE TYPE Ia SUPERNOVAE ARE PREFERENTIALLY OVERLUMINOUS

Recent observational and theoretical progress has favored merging and helium-accreting sub-Chandrasekhar mass white dwarfs in the double-degenerate and the double-detonation channels, respectively, as the most promising progenitors of normal Type Ia supernovae (SNe Ia). Thus the fate of rapidly-accreting Chandrasekhar mass white dwarfs in the single-degenerate channel remains more mysterious then ever. In this paper, we clarify the nature of ignition in Chandrasekhar-mass single-degenerate SNe Ia by analytically deriving the existence of a characteristic length scale which establishes a transition from central ignitions to buoyancy-driven ignitions. Using this criterion, combined with data from three-dimensional simulations of convection and ignition, we demonstrate that the overwhelming majority of ignition events within Chandrasekhar-mass white dwarfs in the single-degenerate channel are buoyancy-driven, and consequently lack a vigorous deflagration phase. We thus infer that single-degenerate SNe Ia are generally expected to lead to overluminous 1991T-like SNe Ia events. We establish that the rates predicted from both the population of supersoft X-ray sources and binary population synthesis models of the single-degenerate channel are broadly consistent with the observed rates of overluminous SNe Ia, and suggest that the population of supersoft X-ray sources are the dominant stellar progenitors of SNe 1991T-like events. We further demonstrate that the single-degenerate channel contribution to the normal and failed 2002cx-like rates is not likely to exceed 1% of the total SNe Ia rate. We conclude with a range of observational tests of overluminous SNe Ia which will either support or strongly constrain the single-degenerate scenario.

Soft-band X/K luminosity ratios in late-type galaxies and constraints on the population of supersoft X-ray sources

We study X-ray to K-band luminosity ratios (L_X/L_K) of late-type galaxies in the 0.3-0.7 keV energy range. From the Chandra archive, we selected nine spiral and three irregular galaxies with point source detection sensitivity better than 5 x 10^36 erg/s in order to minimize the contribution of unresolved X-ray binaries. In late-type galaxies cold gas and dust may cause significant interstellar absorption, therefore we also demanded the existence of publicly available HI maps. The obtained L_X/L_K ratios vary between (5.4-68) x 10^27 erg/s/L_K,sun exceeding by factor of 2-20 the values obtained for gas-poor early-type galaxies. Based on these results we constrain the role of supersoft X-ray sources as progenitors of type Ia supernovae (SNe Ia). For majority of galaxies the upper limits range from ~3% to ~15% of the SN Ia frequency inferred from K-band luminosity, but for a few of them no meaningful constraints can be placed. On a more detailed level, we study individual structural components of spiral galaxies: bulge and disk, and, for grand design spiral galaxies, arm and interarm regions.

A Model for the Galactic Population of Supersoft X-Ray Sources

AbstractThree major sub-populations of Galactic supersoft X-ray sources may exist: semi-detached binaries with main sequence or subgiant donors, and symbiotic binaries (~550, ~460, and ~600 objects, respectively). Each group contains both permanent and transient sources. The intrinsic and interstellar absorptions reduce the number of observable sources to ~25. We derive the distributions of the sources over orbital periods, masses of the components, and ‘on’-times. The rate at which white dwarfs in Galactic binaries reach MCh is ~ 3 10−5 yr−1. The rate of He-shell detonations which may lead to supernovae may be up to 3 10−4 yr−1.

The derived population of luminous supersoft X-ray sources

The existence of a new class of astrophysical object, luminous supersoft X-ray sources, has been established through ROSAT satellite observations and analysis during the past approximately 3 yr. Because most of the radiation emitted by supersoft sources spans a range of wavelengths readily absorbed by interstellar gas, a substantial fraction of these sources may not be detectable with present satellite instrumentation. It is therefore important to derive a reliable estimate of the underlying population, based on the approximately 30 sources that have been observed to date. The work reported here combines the observational results with a theoretical analysis, to obtain an estimate of the total number of sources likely to be present in M31, the Magellanic Clouds, and in our own Galaxy. We find that in M31, where approximately 15 supersoft sources have been identified and roughly an equal number of sources are being investigated as supersoft candidates, there are likely to be approximately 2500 active supersoft sources at the present time. In our own Galaxy, where about four supersoft sources have been detected in the Galactic plane, there are likely to be approximately 1000 active sources. Similarly, with about six and about four (nonforeground) sources observed in the Large (LMC) and Small Magellanic Clouds (SMC), respectively, there should be approximately 30 supersoft sources in the LMC, and approximately 20 in the SMC. The likely uncertainties in the numbers quoted above, and the properties of observable sources relative to those of the total underlying population, are also derived in detail. These results can be scaled to estimate the numbers of supersoft sources likely to be present in other galaxies. The results reported here on the underlying population of supersoft X-ray sources are in good agreement with the results of a prior population synthesis study of the white dwarf accretor model for luminous supersoft X-ray sources. It should be emphasized, however, that the questions asked in these two investigations are distinct, that the approaches taken to answer these questions are largely independent and that the findings of these two studies could in principle have been quite different.