First measurement of polarization in X-rays for 4U 1820-303 with IXPE

4U 1820–303 is the first identified source of type I X-ray bursts. This is an ultracompact Neutron Star (NS), hosted in a Low Mass X-ray Binary system, accreting matter via Roche lobe overflow from a He white dwarf companion. This system is located near the center of the globular cluster NGC 6624. 4U 1820–303 has an orbital period of 685 s, discovered through flux variations in the X-rays, with about 2% peak-to-peak modulation amplitude. A peculiar behavior in 4U 1820–303 is the intrinsic luminosity variation by a factor of at least 2 along a “superorbital” period of about 170 d. IXPE observed this source in two different pointings, here we present the results of this first measurement of polarization in the X-ray band for this soft atoll source.