The XMM-Newton survey of the Small Magellanic Cloud: a new X-ray view of the symbiotic binary SMC 3
Max-Planck-Institut für extraterrestrische Physik,
2 INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica Milano, via E. Bassini 15, 20133 Milano, Italy
3 XMM-Newton Science Operations Centre, ESAC, ESA, PO Box 50727, 28080 Madrid, Spain
4 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Str.1, 85741 Garching, Germany
5 Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117997 Moscow, Russia
6 Warsaw University Observatory, Aleje Ujazdowskie 4, 00-478 Warsaw, Poland
7 University of Western Sydney, Locked Bag 1797, Penrith South DC, NSW1797, Australia
Accepted: 31 March 2011
Context. 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 SMC 3 was found at its highest X-ray luminosity as observed until now.
Aims. In SMC 3 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.
Methods. We present the ~20 year X-ray light curve of SMC 3 and study the spectral evolution as seen with XMM-Newton/EPIC-pn to investigate possible scenarios which can reproduce the high X-ray variability.
Results. 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 24 eV and 34 eV). The light curve shows maxima and minima with slow transitions between them.
Conclusions. 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 5 × 1024 cm-2) along the line of sight could then be responsible for the intensity changes. The X-ray variability of SMC 3 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.
Key words: stars: individual: SMC3 / binaries: symbiotic / X-rays: binaries / galaxies: individual: Small Magellanic Cloud / white dwarfs
© ESO, 2011