U Sco 2010 outburst: a new understanding of the binary accretion disk and the secondary star

TL;DR

This study analyzes optical and NIR spectra of U Sco's 2010 outburst, revealing the rapid reformation of the accretion disk and its role in fueling the super-soft X-ray phase, challenging previous assumptions about mass estimates.

Contribution

It provides new insights into the timing of accretion disk reformation and its influence on the super-soft X-ray phase in U Sco, highlighting the importance of accretion in outburst evolution.

Findings

Accretion resumes around day +8 post-outburst.

The super-soft X-ray phase is fueled by accretion, not just ejected mass.

He emission lines form in the accretion flow, complicating abundance analysis.

Abstract

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.