Properties of the ionised plasma in the vicinity of the neutron-star X-ray binary EXO 0748–676
SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: firstname.lastname@example.org
2 Kapteyn Astronomical Institute University of Groningen Postbus 800 9700 AV Groningen, The Netherlands
3 Columbia Astrophysics Laboratory, Department of Astronomy, Columbia University, 550 West 120th Street, New York, NY 10027, USA
4 Exploration of the universe Division, Code 660, NASA/GSFC, Greenbelt, MD 20771, USA
Accepted: 17 February 2009
Aims. We present the spectral analysis of a large set of XMM-Newton observations of EXO 0748–676, a bright dipping low-mass X-ray binary. In particular, we focus on the dipping phenomenon as a result of changes in the properties of the ionised gas close to the source.
Methods. We analysed the high-resolution spectra collected with the reflection grating spectrometer on board XMM-Newton. We studied dipping and persistent spectra separately. We used the Epic data to constrain the broad-band continuum. We explored two simple geometrical scenarios for which we derived physical quantities of the absorbing material like the density, size, and mass.
Results. We find that the continuum is absorbed by a neutral gas, and by both a collisionally (temperature T ~ 70 eV) and photoionised (ionisation parameter log ξ ~ 2.5) absorbers. Emission lines from OVII and OVIII are also detected. This is the first time that evidence of a collisionally ionised absorber has been found in a low-mass X-ray binary. The collisionally ionised absorber may be in the form of dense (n > 1014 cm-3) filaments, located at a distance r 1011 cm. During dips, the photoionised absorber significantly increases its column density (factor 2–4) while becoming less ionised. This strengthens the idea that the colder material of the accretion stream impinging the disc is passing on our line of sight during dips. In this scenario, we find that the distance from the neutron star to the impact region (~ 51010 cm) is similar to the size of the neutron star's Roche lobe. The gas observed during the persistent state may have a flattened geometry. Finally, we explore the possibility of the existence of material forming an initial, hotter portion of a circumbinary disc.
Key words: atomic processes / stars: binaries: general / X-rays: binaries / X-rays: individuals: EXO 0748–676
© ESO, 2009