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

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.

Submitted to AAS journals. We are grateful to some experts for useful comments

References in corpus (11)

• Analyzing the Data from X-ray Polarimeters with Stokes Parameters
• Design, Construction, and Test of the Gas Pixel Detectors for the IXPE Mission
• Re-detection and a Possible Time Variation of Soft X-ray Polarisation from the Crab
• Distances to Galactic X-ray Binaries with Gaia DR2
• PolarLight: a CubeSat X-ray Polarimeter based on the Gas Pixel Detector
• The X-ray Polarization of the Accretion Disk Coronae of Active Galactic Nuclei
• Broad-band Spectral Evolution of Scorpius X-1 along its Color-Color Diagram
• X-ray spectral and timing behavior of Scorpius X-1. Spectral hardening during the flaring branch
• X-ray polarimetry of the Crab nebula with PolarLight: polarization recovery after the glitch and a secular position angle variation
• In-orbit Operation and Performance of the CubeSat Soft X-ray Polarimeter PolarLight
• Discrimination of background events in the PolarLight X-ray polarimeter