Abstract This paper reports the first detection of polarization in X-rays for atoll-source 4U 1820−303, obtained with Imaging X-ray Polarimetry Explorer (IXPE) at 99.999% confidence level (CL). Simultaneous polarimetric measurements were also performed radio Australia Telescope Compact Array. The IXPE observations 1820−303 coordinated Swift Telescope, Neutron Star Interior Composition Explorer,...