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

TL;DR

The paper analyzes two XMM-Newton/RGS observations of Cen X-3, focusing on eclipse and out-of-eclipse spectra to characterize the reprocessing environment around the neutron star. Using SPEX with photoionised and collisionally ionised plasma models, it finds that the eclipse spectrum is dominated by heavily absorbed emission with strong H-like Mg XII and Ne X lines, compatible with either photoionised or collisional plasmas, while the out-of-eclipse spectrum requires a more complex continuum and reveals high-ionisation emission lines. Phase-resolved analysis shows the absorbing column increases as the neutron star emerges from eclipse and that the emission-measure of plasma components evolves, suggesting a hybrid, density-rich reprocessor likely located near the neutron star or in the inner stellar wind. The results imply a hybrid plasma with UV-field influences on line production, and they demonstrate the value of high-resolution spectroscopy for probing the accretion/reprocessing environment in high-mass X-ray binaries, with XRISM and similar facilities expected to refine these diagnostics.

Abstract

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.

Figures (9)

• Figure 1: Light curves and hardness ratio of the first (0111010101) low state observation (left) and the second (0400550201) high state (right) observation with RGS. Intervals with high background have already been taken out.
• Figure 2: RGS data, photoionisation dominated model (green circle), collisional dominated model (red triangle), Gaussian emission lines (blue square) and residuals (expressed in terms of number of standard deviations) for the 0111010101 orbital phase-averaged spectrum.
• Figure 3: RGS data, model (two cie components) and relative residuals for the 0111010101 orbital phase-averaged spectrum.
• Figure 4: RGS data, continuum model and residuals (in terms of number of standard deviations) for the 0400550201 orbital phase-averaged spectrum. The dashed lines indicate possible emission/absorption lines detected at $3\, \sigma$ level.
• Figure 5: The Ne ix triplet with best fit model. Resonance (r), intercombination (i) and forbidden (f) components are indicated at observed wavelengths.