U Sco 2010 outburst: a new understanding of the binary accretion disk and the secondary star
We present optical and NIR spectroscopic observations of U Sco 2010
outburst. From the analysis of lines profiles we identify a broad and a
narrow component and show that the latter originates from the reforming
accretion disk. We show that the accretion resumes shortly after the
outburst, on day +8, roughly when the super-soft (SSS) X-ray phase
starts. Consequently U Sco SSS phase is fueled (in part or fully) by
accretion and should not be used to estimate $m_{\mathrm{rem}}$, the
mass of accreted material which has not been ejected during the
outburst. In addition, most of the He emission lines, and the HeII lies
in particular, form in the accretion flow/disk within the binary and are
optically thick, thus preventing an accurate abundance determination. A
late spectrum taken in quiescence and during eclipse shows CaII H&K,
the G-band and MgI b absorption from the secondary star. However, no
other significant secondary star features have been observed at longer
wavelengths and in the NIR band.