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The Super-Soft Source Phase of the recurrent nova V3890 Sgr

The 30-year recurrent symbiotic nova V3890 Sgr exploded 2019 August 28 and was observed with multiple X-ray telescopes. An XMM-Newton observation during the SSS phase captured a high degree of X-ray variability including a deep dip in the middle of the observation, an initial rise of similar depth and shape and, after the deep dip, smaller dips of 10% amplitude, which might be periodic over 18.1-minutes. An eclipse model of the dips yields clump sizes and orbital radii of 0.5-8 and 5-150 white dwarf radii, respectively. The simultaneous UV light curve shows no significant variations beyond slow fading. The RGS spectrum contains both residual shock emission at short wavelengths and the SSS emission at longer wavelengths. The shock temperature has clearly decreased compared to an earlier Chandra observation (day 6). The dip spectrum is dominated by emission lines like in U Sco. The intensity of underlying blackbody-like emission is much lower with the blackbody normalisation yielding a similar radius as during the brighter phases, while the lower bolometric luminosity is ascribed to lower T_eff. This would be inconsistent with clump occultations unless Compton scattering of the continuum emission reduces the photon energies to mimic a lower effective temperature. However, systematic uncertainties are high. The absorption lines in the bright SSS spectrum are blue-shifted by 870+/-10 km/s before the dip and 900+/-10 km/s, after the dip. The reproduction of the observed spectrum is astonishing, especially that only a single absorbing layer is necessary while three such layers are needed to reproduce the RGS spectrum of V2491 Cyg. The ejecta of V3890 Sgr are thus more homogeneous than many other SSS spectra indicate. Abundance determination is in principle possible but highly uncertain. Generally, solar abundances are found except for N and possibly O higher by an order of magnitude.