Two symbiotic stars in the Small Magellanic Cloud (SMC), Lin 358 and SMC 3, have been supersoft X-ray sources (SSSs) for more than 10 years. We fit atmospheric and nebular models to their X-ray, optical, and UV spectra obtained at different epochs. The X-ray spectra are extremely soft and appear to be emitted by the white dwarf atmosphere, not by the nebula, as in some other symbiotics. We find that the white dwarf of SMC 3, the hottest of the two sources, had an approximately constant effective temperature T eA ' 500; 000 K in 1993Y1994, 2003, and 2006, without indications of a decrease in 12 years. The bolometric luminosity of this system in 2003 March was more than an order of magnitude lower than 3 years later; however, the time of the observation is consistent with a partial eclipse of the white dwarf, previously found in ROSAT and optical observations. The red giant wind must be asymmetric or very clumpy in SMC 3, because the filling factor of the nebula around the source is not higher than 0.1. The compact object in Lin 358 has been at T eA ! 180;000 K since 1993, and there is some evidence of a moderate increase. Atmospheric fits for both objects are obtained with log g ¼ 9, which is appropriate for white dwarf masses >1.18 M . No nova-like outbursts of these systems have been recorded in the last 50 years, despite continuous optical monitoring of the SMC, and there are no indications of cooling of the white dwarf, expected after a thermonuclear flash. We suggest therefore that in both systems hydrogen is burning steadily in a shell on the WD at the rate '10 À7 M yr À1 , sufficiently high to inhibit novatype mass loss as required for Type Ia supernovae progenitors.
The classical nova V4743 Sgr was observed with XMM–Newton for about 10 h on 2003 April 4, 6.5 months after optical maximum. At this time, this nova had become the brightest supersoft X‐ray source ever observed. In this paper, we present the results of a time‐series analysis performed on the X‐ray light curve (LC) obtained in this observation, and in a previous shorter observation done with Chandra 16 d earlier. Intense variability, with amplitude as large as 40 per cent of the total flux, was observed both times. Similarities can be found between the two observations in the structure of the variations. Most of the variability is well represented as a combination of oscillations at a set of discrete frequencies lower than 1.7 mHz. At least five frequencies are constant over the 16 d time interval between the two observations. We suggest that a period in the power spectrum of both LCs at the frequency of 0.75 mHz and its first harmonic are related to the spin period of the white dwarf (WD) in the system, and that other observed frequencies are signatures of non‐radial WD pulsations. A possible signal with a 24 000 s period is also found in the XMM–Newton LC: a cycle and a half are clearly identified. This period is consistent with the 24 278 s periodicity discovered in the optical LC of the source and thought to be the orbital period of the nova binary stellar system.
Abstract. We present XMM-Newton observations of the B giant β Centauri (B1 III). The spectra are rich of spectral lines from a wide range of ionization stages which indicate temperatures in the range ∼0.1−0.6 keV. Simultaneous fits to the , -, and -pn spectra yield three plasma temperatures (0.1, 0.2, and 0.6 keV), emission measures, and elemental abundances which are quite close to solar values. These temperatures are confirmed by DEM modeling. According to the derived models the intrinsic source X-ray luminosity in the energy range 0.3−10 keV is L x = 10 × 10 30 erg s −1 at a distance of 161 pc. An analysis of the X-ray light curve suggests that the photospheric variability does not have much of an effect on the properties of the X-ray luminosity. The sensitivity of the He-like forbidden and intercombination lines to a strong ultraviolet stellar radiation field is used to constrain the radial distances at which the lines of Ne , O , and N originate.
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