Pan-chromatic Observations of the Recurrent Nova LMC 2009a (LMC 1971b)

Abstract

Nova LMC 2009a is confirmed as a recurrent nova (RN) from positional coincidence with nova LMC 1971b. The observational data set is one of the most comprehensive for any Galactic or extragalactic RN: optical and near-IR photometry from outburst until over 6 years later; optical spectra for the first 6 months, and Swift satellite ultraviolet (UV) and X-ray observations from 9 days to almost 1 year post-outburst. We find M<SUB>V</SUB> = -8.4 ± 0.8<SUB>r</SUB> ± 0.7<SUB>s</SUB> and expansion velocities between 1000 and 4000 km s<SUP>-1</SUP>. Coronal line emission before day 9 indicates shocks in the ejecta. Strengthening of He IIλ4686 preceded the emergence of the super-soft source (SSS) in X-rays at ∼63-70 days, which was initially very variable. Periodic modulations, P = 1.2 days, most probably orbital in nature, were evident in the UV and optical from day 43. Subsequently, the SSS shows an oscillation with the same period but with a delay of 0.28P. The progenitor system has been identified; the secondary is most likely a sub-giant feeding a luminous accretion disk. Properties of the SSS infer a white dwarf (WD) mass 1.1 M<SUB>☉</SUB> ≲ M<SUB>WD</SUB> ≲ 1.3 M<SUB>☉</SUB>. If the accretion occurs at a constant rate, {\dot{M}}<SUB>{{acc</SUB>}}≃ {3.6}<SUB>-2.5</SUB><SUP>+4.7</SUP>× {10}<SUP>-7</SUP> {M}<SUB>☉ </SUB> yr<SUP>-1</SUP> is needed, consistent with nova models for an inter-eruption interval of 38 years, low outburst amplitude, progenitor position in the color-magnitude diagram, and spectral energy distribution at quiescence. We note striking similarities between LMC 2009a and the Galactic nova KT Eri, suggesting that KT Eri is a candidate RN.