STURM R., HABERL F., GREINER J., PIETSCH W., LA PALOMBARA N., EHLE M., GILFANOV M., UDALSKI A., MEREGHETTI S. and FILIPOVIC M.
Abstract (from CDS):
The XMM-Newton survey of the Small Magellanic Cloud (SMC) was performed to study the population of X-ray sources in this neighbouring galaxy. During one of the observations, the symbiotic binary SMC3 was found at its highest X-ray luminosity as observed until now. In SMC3 wind accretion from a giant donor star onto a white dwarf is believed to cause steady hydrogen burning on the white dwarf surface, making such systems candidates for supernova type Ia progenitors. It was suggested that the X-ray source is eclipsed every ∼4.5 years by both the companion star and its stellar wind to explain the large X-ray variability seen in ROSAT data. We use the available X-ray data to test this scenario. We present the ∼20 year X-ray light curve of SMC3 and study the spectral evolution as seen with XMM-Newton/EPIC-pn to investigate possible scenarios which can reproduce the high X-ray variability. We did not find any significant variations in the photo-electric absorption, as would be expected during eclipse ingress and egress. Instead, the X-ray spectra from different intensity levels, when modelled by black-body emission, can be better explained by variations either in normalisation (by a factor of ∼50) or in temperature (kT between 24eV and 34eV). The light curve shows maxima and minima with slow transitions between them. To explain the gradual variations in the X-ray light curve and to avoid changes in absorption by neutral gas, a predominant part of the stellar wind must be ionised by the X-ray source. Compton scattering with variable electron column density (of the order of 5x1024cm–2) along the line of sight could then be responsible for the intensity changes. The X-ray variability of SMC3 could also be caused by temperature changes in the hydrogen-burning envelope of the white dwarf, an effect that could even dominate if the stellar wind density is not sufficiently high.
stars: individual: SMC3 - binaries: symbiotic - X-rays: binaries - galaxies: individual: Small Magellanic Cloud - white dwarfs