On 2022 January 1, Hua Feng and his collaborators reported in The Astrophysical Journal Letters a significant detection of X-ray polarization in Sco X-1 with the space mission PolarLight, which has been operating on-orbit for more than three years since its launch in 2018 October. The results help place stringent constraints on the geometry of the hot corona, and mark Sco X-1 as the second astrophysical source with a significant polarization measurement in the keV band.
Sco X-1 is the first discovered extrasolar X-ray source and the brightest persistent object in the keV sky besides the Sun. It is a low-mass X-ray binary containing a neutron star. X-ray spectroscopy suggests that Comptonization in a hot corona is responsible for the production of X-rays. However, the location and geometry of the corona is uncertain. It could be extended above the accretion disk, or located in the transition layer between the disk and surface of neutron star. The measured polarization angle is in line with the jet orientation of the source on the sky plane, which is supposedly the symmetry axis of the system. The 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.
The daily operation of PolarLight is conducted by Xiangyun Long, a graduate student in the Engineering Physics Department. The PolarLight experiment demonstrates that a nano-satellite is useful for both scientific exploration and student training. The collaboration of PolarLight also includes INFN-Pisa, INAF-Rome, IHEP, NBUT, North Night Vision Technology, and Spacety.
Link to the paper：https://iopscience.iop.org/article/10.3847/2041-8213/ac4673.