Eruptive mass-loss less than a year before the explosion of superluminous supernovae: I. The cases of SN 2020xga and SN 2022xgc

We present photometric and spectroscopic observations of SN2020xga and SN2022xgc, two hydrogen-poor superluminous supernovae (SLSNe-I) at z = 0.4296 and z = 0.3103 respectively, that show an additional set of broad Mg II absorption lines, blueshifted by a few thousand km s^{-1} with respect to the host galaxy absorption system.... Show more