The 2010 outburst and pre-outburst optical spectrum of the recurrent nova U Scorpii

Abstract

Aims. We study the evolution of the recurrent nova U Scorpii during its outburst in 2010. Methods. Optical spectroscopic observations of the nova were obtained during 0.83–162.5 days after outburst maximum. Optical linear polarisation observations were made before the onset of the super soft X-ray source (SSS) phase and during the SSS phase. Radio continuum observations were made in the 1280 MHz band during the early decline phase, and in the 610 MHz band at the onset and end of the SSS phase. We also present optical spectra obtained in the pre-outburst quiescence phase. Results. The overall spectral evolution during the 2010 outburst is similar to the previous outbursts. However, the dense temporal coverage, especially during the early phases, reveals several short term variations not reported previously. The early phase emission line widths indicate extremely high velocities for the nova ejecta, ~10 000 km s^-1. The line profiles are broad, boxy and structured. Narrow P-Cygni absorptions, at ~500 km s^-1 are seen associated with the He I lines, and also the Ca II, N I and hydrogen lines (at wavelengths >8000 Å) in the spectrum of day 6.83, obtained immediately after an optical flare. This absorption component could be arising in the plasma emitting the Nitrogen lines seen in X-rays, that probably lies outside the binary orbit. Rapid variations are detected in the strength of the 4660 Å N III feature, which shows a significant increase in the line strength on days 8.83 and 13.82, at orbital phases of 0.75 and 0.25, respectively. Instrinsic polarisation is detected just before the onset, and during the SSS phase, with pv ~ 1.4% during the SSS phase. The flux variations of the N III feature and the intrinsic polarisation are most likely associated with the reforming accretion disc/stream. We estimate the mass of the ejected (hydrogen) shell to be ~4.6 × 10^-6 M☉, for a spherical shell geometry. The non-detection of U Sco in the 1280 MHz and 610 MHz radio bands is consistent with the low mass of the nova ejecta, subgiant nature of the secondary, and the distance to the nova.