Highly Polarized Intrinsic Emission and its Orthogonal Counterpart in Vela X-1

TL;DR

This study analyzes IXPE observations of the wind-fed XRP Vela X-1 to dissect its X-ray polarization with a three-power-law spectral model, revealing two orthogonal polarized components. Component 1, arising from the accretion-mound surface and viewed through a tenuous wind, displays a high intrinsic polarization ($PD\approx30$–$50\%$) and remains nearly orthogonal in PA to the second group (Components 2,3), which are more wind-absorption dominated. Phase-resolved analysis shows $PD_1$ peaking at $PD_1=50.9\%$ around phase 0.2–0.4, and Rotating Vector Model fits (with a 90° PA rotation for Component 1) yield a geometry with $i_p \approx 89^{\circ}$ and $\theta \approx 12^{\circ}$, consistent across separate and joint fits. The results imply vacuum-resonance effects and complex mound-related emission, marking the first high-significance detection of highly polarized intrinsic emission in an XRP and highlighting the potential of future missions like eXTP to refine these insights.

Abstract

Vela X-1 is one of the most archetypal wind-fed X-ray pulsars (XRPs), and the emergence of its orthogonal polarization states reveals distinctive polarimetric properties. Using data from Imaging X-ray Polarimetry Explorer (IXPE) observations of Vela X-1, we perform a polarization analysis of Vela X-1 using a triple power-law spectral model absorbed by varying column densities, successfully isolating two physically distinct orthogonal polarized components. The first polarized component corresponds to emission from the accretion mound surface that is not obscured by the wind clumps, with its polarization degree (PD) exceeding 30\%. In specific phase intervals, the PD reaches \(50.9 \pm 10.7\%\). This marks the first detection of such highly polarized neutron star emission in an XRP. The second polarized component likely originates from complex physical processes within or near the accretion mound, with its PD showing a potential negative correlation with column density. Furthermore, by rotating the predicted polarization angle (PA) of the first polarized component by 90$^\circ$, we successfully achieve separate fitting and simultaneous fitting of the two orthogonal polarization states using the rotating vector model (RVM).

Figures (8)

• Figure 1: Light curves from the two IXPE observations of Vela X-1. Time intervals (a)–(e) are used for time-resolved polarization analysis, respectively corresponding to the MJD time intervals: (a) 59684.8–-59687, (b) 59689.5--59690.5, (c) 59914–-59915, (d) 59915–-59917.2, and (e) 59917.2--59919.4.
• Figure 2: Top panel: the spectra of the combined dataset of two IXPE observations. The red, black and cyan colors represent the data from three DUs, respectively. The black solid line, dotted line, dashed line and dot–dashed line represent Component 1, Component 2, Component 3 and the Gaussian component of our fitting model, respectively. The fitting residuals are shown in the bottom panel.
• Figure 3: The polarization vectors of the combined dataset of two IXPE observations. The blue contours show the polarization vectors of Component 1, while the red shows the joint polarization vectors of Component 2,3. For each set of vectors, the three contours from outward to inward correspond to the 68.30%, 95.45%, and 99.73% confidence levels, respectively. Due to the narrow uncertainty ranges, the three red contours are visually indistinguishable at this scale.
• Figure 4: Correlation of parameters in Table \ref{['d_parameters']}. The left, middle and right panel respectively shows $\mathrm{PD_{2,3}}$ versus $\mathrm{Luminosity_{2,3}}$, $N_\mathrm{{H,{2}}}$ and $N_\mathrm{{H,{3}}}$.
• Figure 5: Results of the phase-resolved spectro-polarimetric analysis, using the combined dataset of two IXPE observations in 2--8 keV energy band. (A): The overall pulse profiles. (B): The phase-resolved $\mathrm{PD_{1}}$ and $\mathrm{PD_{2,3}}$. The purple line shows Component 1, and the light coral line shows Component 2,3. (C): Same colors indicate the variation of $\mathrm{Luminosity_{1}}$ and $\mathrm{Luminosity_{2,3}}$. (D): Same colors indicate the phase-resolved $\mathrm{PA_{1}}$ and $\mathrm{PA_{2,3}}$. The black solid curve shows the best-fit RVM for simultaneous fitting of $\mathrm{PA_1}$ and $\mathrm{PA_{2,3}}$. The black dashed curve represents the best-fit RVM for separate fitting of $\mathrm{PA_1}$ and $\mathrm{PA_{2,3}}$.