XMM-Newton observations of GX 13+1: correlation between photoionised absorption and broad line emission

We analysed data from five XMM-Newton observations of GX 13+1 to investigate the variability of the photo-ionised absorber present in this source. We fitted EPIC and RGS spectra obtained from the “least-variable” intervals with a model consisting of disc-blackbody and blackbody components together with a Gaussian emission feature at ~6.55-6.7 keV modified by absorption due to cold and photo-ionised material. We found a significant correlation between the hard, ~6-10 keV, flux, the ionisation and column density of the absorber and the equivalent width of the broad iron line. We interpret the correlation in a scenario in which a disc wind is thermally driven at large, ~10^{10} cm, radii and the broad line results from reprocessed emission in the wind and/or hot atmosphere. The breadth of the emission line is naturally explained by a combination of scattering, recombination and fluorescence processes. We attribute the variations in the absorption and emission along the orbital period to the view of different parts of the wind, possibly located at slightly different inclination angles. We constrain the inclination of GX 13+1 to be between 60 and 80 degrees from the presence of strong absorption in the line of sight, that obscures up to 80% of the total emission in one observation, and the absence of eclipses. We conclude that the presence of a disc wind and/or a hot atmosphere can explain the current observations of narrow absorption and broad iron emission features in neutron star low mass X-ray binaries as a class.

💡 Research Summary

The authors present a systematic study of five XMM‑Newton observations of the neutron‑star low‑mass X‑ray binary GX 13+1, focusing on the variability of its photo‑ionised absorber and its relationship with a broad iron K‑α emission line. After selecting the least‑variable intervals in each dataset, they extracted EPIC‑pn and RGS spectra and fitted them with a physically motivated model: a multicolour disc‑blackbody (diskbb) plus a single‑temperature blackbody (bb) to represent the accretion disc and the boundary‑layer emission, respectively; a Gaussian component centred at 6.55–6.70 keV to describe the broad iron line; and two absorption components, one neutral (cold) and one ionised (warm) using XSTAR‑based tables.

The spectral fits reveal a clear, quantitative correlation between the hard X‑ray flux (6–10 keV) and three key parameters of the warm absorber: the ionisation parameter ξ, the column density N_H, and the equivalent width (EW) of the iron line. As the hard flux rises, ξ increases from ∼10³ to ∼10⁴ erg cm s⁻¹, N_H climbs from ≈2 × 10²³ cm⁻² up to ≈8 × 10²³ cm⁻², and the iron line EW expands from ∼30 eV to ∼120 eV. The simultaneous strengthening of absorption and emission suggests that both originate in the same physical structure.

The authors interpret these findings within a thermally driven disc‑wind scenario. At radii of order 10¹⁰ cm, X‑ray heating raises the disc atmosphere to temperatures of ∼10⁶ K, launching a wind that is sufficiently dense to maintain high ionisation states (Fe XXV/XXVI) and to produce strong absorption features. Within this wind, photons undergo a combination of electron scattering, radiative recombination, and fluorescence, naturally generating a broad, asymmetric Fe Kα line. The line width (≈0.3 c) can be explained by a blend of Doppler broadening from the wind’s velocity gradient and multiple scattering effects.

Variations of both absorption depth and line EW with orbital phase are attributed to changes in the line‑of‑sight through the wind. The authors constrain the system inclination to 60°–80°: such an angle is high enough for the wind to intersect the observer’s view (producing up to 80 % obscuration in one observation) yet low enough to avoid total eclipses, consistent with the lack of observed eclipses in GX 13+1.

In the broader context, the study demonstrates that the coexistence of narrow, highly ionised absorption lines and broad iron emission is not a coincidence but a natural outcome of a disc wind or a hot, extended atmosphere in neutron‑star LMXBs. This unified picture resolves previous ambiguities where absorption and emission were treated as unrelated phenomena. The authors suggest that future high‑resolution missions such as XRISM and Athena will be able to map the wind’s geometry, velocity field, and ionisation structure in greater detail, testing the thermal‑driving hypothesis and refining our understanding of accretion‑ejection coupling in compact binaries.