Properties of the ionised plasma in the vicinity of the neutron-star X-ray binary EXO 0748-676

We present the spectral analysis of a large set of XMM-Newton observations of EXO 0748-676, a bright dipping LMXB. In particular, we focus on the dipping phenomenon as a result of changes in the properties of the ionised gas close to the source. Using the high-resolution spectra collected with the RGS, 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 T70 eV) and photoionised (ionisation parameter log\xi2.5) 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>10^14 cm^-3) filaments, located at a distance r>10^11 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. We find that the distance from the neutron star to the impact region (~ 5x10^10 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.

💡 Research Summary

The authors present a comprehensive spectroscopic study of the dipping low‑mass X‑ray binary EXO 0748‑676 using a large set of XMM‑Newton observations, with a particular focus on the changes in the ionised plasma that accompany the dipping phenomenon. High‑resolution spectra obtained with the Reflection Grating Spectrometer (RGS) were analysed for both the persistent (non‑dip) and dipping intervals. The continuum is found to be absorbed not only by neutral gas but also by two distinct ionised absorbers: a collisionally ionised component with a temperature of roughly 70 eV (≈ 8 × 10⁵ K) and a photo‑ionised component characterised by an ionisation parameter log ξ ≈ 2.5. Emission lines from O VII and O VIII are also detected, confirming the presence of multi‑phase plasma.

A key result is the first detection of a collisionally ionised absorber in a low‑mass X‑ray binary. Modelling indicates that this gas must be extremely dense (n > 10¹⁴ cm⁻³) and located at a distance r > 10¹¹ cm from the neutron star. The authors interpret the absorber as dense filaments or clumps, possibly formed where the accretion stream impacts the outer disc. During dips, the column density of the photo‑ionised absorber increases by a factor of 2–4 while its ionisation parameter drops, suggesting that cooler, denser material from the stream moves into the line of sight and partially shields the X‑ray source. The inferred impact region lies at ≈ 5 × 10¹⁰ cm, comparable to the size of the neutron star’s Roche lobe, reinforcing the picture of stream‑disc interaction as the driver of the dipping behaviour.

Two simple geometrical scenarios were explored. In the first, the persistent‑state absorber is assumed to have a flattened, disc‑like geometry, which can naturally explain the modest variations in absorption when the viewing angle changes. In the second, the authors consider the possibility that the collisionally ionised gas represents the hot inner edge of a circumbinary disc, an idea that would link the observed plasma to a larger‑scale structure surrounding the binary. While the current data cannot definitively discriminate between these configurations, both are consistent with the measured densities, sizes, and masses.

The paper provides quantitative estimates for the physical properties of the absorbers: the neutral column is N_H ≈ 2 × 10²¹ cm⁻², the photo‑ionised column in the persistent state is N_H ≈ 5 × 10²¹ cm⁻², and the collisionally ionised column is of comparable magnitude. The temperature of the collisionally ionised plasma (≈ 70 eV) and its high density imply a short cooling timescale, supporting the notion that the gas is continuously replenished by the impact of the accretion stream. The authors also discuss the implications for mass‑loss and angular‑momentum transport, noting that the dense filaments could carry a non‑negligible fraction of the accretion flow.

In summary, this work expands the phenomenology of LMXB absorption by demonstrating that, in addition to neutral and photo‑ionised material, a collisionally ionised plasma can coexist close to the neutron star. The observed changes during dips provide direct evidence for the dynamic nature of the stream‑disc interaction region, and the inferred geometry suggests either a flattened disc‑corona or the inner rim of a circumbinary disc. The study sets the stage for future high‑resolution X‑ray missions (e.g., XRISM, Athena) to map the spatial distribution of multi‑phase plasma in accreting binaries and to test the proposed geometrical models with greater precision.