The Recurrent Nova U Scorpii from the 2010.1 to 2022.4 Eruptions; the Missed Eruption of 2016.78 +- 0.10 and the Critical Complex Period Changes

Abstract

U Sco is a recurrent nova with eleven observed eruptions, most recently in 2010.1 and 2022.4. I report on my program (running since 1989) of measuring eclipse times and brightnesses of U Sco in quiescence, from 2010 to 2022. The orbital period suddenly increased by +22.4 +- 1.0 parts-per-million across the 2010.1 eruption. This period change is greater than the near-zero period change (+3.9 +- 6.1 parts-per-million) across the 1999.2 eruption. This period change cannot come from any of the usual mechanisms, whereas the one remaining possibility is that the period changes are dominated by the little-known mechanism of the nova ejecting asymmetric shells. From 2010.1 to 2016.78, the O-C curve showed a steady period change that was large, with P-dot equal (-21.0 +- 3.2)X 10^-9. This is greatly higher than the steady period changes in the two previous inter-eruption intervals (-3.2 +- 1.9 and -1.1 +- 1.1 X 10^-9). This large, variable, and negative P-dot apparently comes from magnetic braking of the companion star's rotation. Starting in 2016.9 +- 0.6, the O-C curve showed a strong kink that is a unique characteristic of the sudden period change (+35.4 +- 7.1 parts-per-million) across a nova event. The brightness in quiescence after 2010.4 shows that the white dwarf accreted the trigger mass for the next nova event in the year 2017.1 +- 0.6. Photometric records show the only possible time for the eruption to peak (such that its total duration of 60 days was undetectable by any observation) is during a 75 day interval inside the 2016 solar gap, thus constraining the missed eruption to 2016.78 +- 0.10.