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

TL;DR

This study reveals highly polarized emission components in Vela X-1, a wind-fed X-ray pulsar, using IXPE data, identifying two orthogonal polarization states linked to different physical regions near the neutron star.

Contribution

First detection of highly polarized neutron star emission in an XRP, with a novel analysis isolating two orthogonal polarized components and modeling their properties.

Findings

Polarization degree exceeds 30%, reaching over 50% in some phases.

Two orthogonal polarized components are identified, linked to different physical regions.

Successful modeling of polarization states using the rotating vector model.

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).