Astronomy and Astrophysics, volume 497, 805-813 (2009/4-3)
Properties of the ionised plasma in the vicinity of the neutron-star X-ray binary EXO0748-676.
VAN PEET J.C.A., COSTANTINI E., MENDEZ M., PAERELS F.B.S. and COTTAM J.
Abstract (from CDS):
We present the spectral analysis of a large set of XMM-Newton observations of EXO0748-676, 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. 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. We find that the continuum is absorbed by a neutral gas, and by both a collisionally (temperature T∼70eV) and photoionised (ionisation parameter logξ∼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>1014cm–3) filaments, located at a distance r>1011cm. 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 (∼5x1010cm) 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.