High-resolution X-ray spectroscopy of Cen X-3 with XMM-Newton

The spectral analysis of two XMM-Newton observations of the high-mass X-ray binary system Cen X-3 is presented. In particular, it is focused on the eclipse and out-of-eclipse spectra in order to compare the properties of the environment around the compact object. The high-resolution spectra obtained from the reflection grating spectrometer on board XMM-Newton was analysed focusing on studying eclipse and out-of-eclipse spectra separately. Several continuum models were explored in SPEX for which we studied the properties of emitting and absorbing matter depending on the emission and absorption lines identified in the spectra. It was found that the X-ray continuum is heavily absorbed by a neutral gas and photoionised matter. Emission lines from Si v, Mg xii, Mg xi, and Ne x were detected in the eclipse spectrum. In particular, H-like lines of Mg and Ne with a significance greater than 5 sigma in the eclipse spectrum and 3 sigma in the out-of-eclipse spectrum. But in the out-of-eclipse spectrum any absorption lines, if any, were detected with a significance less than 2 sigma. RGS light curve showed dips in the out-of-eclipse spectrum which are not due to an increase in the column absorption but may be produced by instabilities in the accretion stream. On the other hand, the level of counts above 20 was compatible with the X-ray background. A simple local continuum model was used to describe the He-like triplet of Ne and the derived values of R and G ratio parameters pointed out that the UV photospheric field should be important at the line production site and an electron density greater than 10(12) cm-3. As a consequence, a hybrid plasma may be present in the binary system.

💡 Research Summary

This paper presents a detailed high‑resolution X‑ray spectroscopic study of the high‑mass X‑ray binary Cen X‑3 using the Reflection Grating Spectrometer (RGS) aboard XMM‑Newton. Two archival observations (January 2001 and June 2006) were processed with SAS 16.0.0, filtered for background flares, and combined to produce flux‑calibrated spectra covering 7–38 Å. The data were split into eclipse (orbital phase ≈0.00–0.37) and out‑of‑eclipse (≈0.35–0.80) intervals, allowing a direct comparison of the circumstellar environment when the neutron star is hidden versus when it is directly visible.

Continuum modeling required a neutral absorption component (N_H≈1.2×10²⁶ m⁻²) plus an additional photo‑ionised absorber (N_H≈3.6×10²⁶ m⁻², log ξ≈1.58). The intrinsic emission was described by a power‑law (Γ≈1.66) and a black‑body (kT≈127 eV). Both a photo‑ionisation emission model (pion) and a collisional‑ionisation model (cie) were tested; the pion model gave a slightly better fit (χ²/dof≈5.47/391) but both reproduced the main spectral features.

Emission lines were identified with high significance in the eclipse spectrum: Si V, Mg XI (He‑like triplet), Mg XII (Ly α), and Ne X (Ly α). The H‑like Mg and Ne lines reach >5σ, while the same lines are detected at ≈3σ in the out‑of‑eclipse spectrum. No absorption lines are statistically significant (≤2σ) outside eclipse. The He‑like triplet of Ne was used to derive the R (=f/i) and G =