A significant detection of X-ray Polarization in Sco X-1 with PolarLight and constraints on the corona geometry

TL;DR

This paper reports the first significant detection of X-ray polarization in Sco X-1 using PolarLight, revealing energy and flux dependence, and constraining the corona geometry around the neutron star.

Contribution

It provides the first polarization measurement of Sco X-1 in the keV band and offers new constraints on the corona structure based on polarization data.

Findings

Detected polarization in 4-8 keV band with 4σ significance during high flux states.

Polarization angle aligns with the jet orientation at approximately 54°.

Results suggest an optically thin corona, challenging extended corona models.

Abstract

We report the detection of X-ray polarization in the neutron star low mass X-ray binary Scorpius (Sco) X-1 with PolarLight. The result is energy dependent, with a non-detection in 3-4 keV but a 4$\sigma$ detection in 4-8 keV; it is also flux dependent in the 4-8 keV band, with a non-detection when the source displays low fluxes but a 5$\sigma$ detection during high fluxes, in which case we obtain a polarization fraction of $0.043 \pm 0.008$ and a polarization angle of $52.6^\circ \pm 5.4^\circ$. This confirms a previous marginal detection with OSO-8 in the 1970s, and marks Sco X-1 the second astrophysical source with a significant polarization measurement in the keV band. The measured polarization angle is in line with the jet orientation of the source on the sky plane ($54^\circ$), which is supposedly the symmetric axis of the system. Combining previous spectral analysis, our measurements suggest that an optically thin corona is located in the transition layer under the highest accretion rates, and disfavor the extended accretion disk corona model.