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V3890 Sgr is a recurrent nova which has been seen in outburst three times sofar, with the most recent eruption occurring on 2019 August 27 UT. This latestoutburst was followed in detail by the Neil Gehrels Swift Observatory, fromless than a day after the eruption until the nova entered the Sun observingconstraint, with a small number of additional observations after the constraintended. The X-ray light-curve shows initial hard shock emission, followed by anearly start of the super-soft source phase around day 8.5, with the softemission ceasing by day 26. Together with the peak blackbody temperature of thesuper-soft spectrum being ~100 eV, these timings suggest the white dwarf massto be high, ~1.3 M_sun. The UV photometric light-curve decays monotonically,with the decay rate changing a number of times, approximately simultaneouslywith variations in the X-ray emission. The UV grism spectra show both line andcontinuum emission, with emission lines of N, C, Mg and O being notable. TheseUV spectra are best dereddened using an SMC extinction law. Optical spectrafrom SMARTS show evidence of interaction between the nova ejecta and wind fromthe donor star, as well as the extended atmosphere of the red giant beingflash-ionized by the super-soft X-ray photons. Data from NICER reveal atransient 83 s quasi-periodic oscillation, with a modulation amplitude of 5 percent, adding to the sample of novae which show such short variabilities duringtheir super-soft phase.

The 2019 eruption of recurrent nova V3890 Sgr: observations by Swift, NICER and SMARTS