U Scorpii 2010 outburst: a new understanding of the binary accretion disk and the secondary star^⋆,⋆⋆

¹ Space Telescope Science Institute, Baltimore, MD, 21218 USA
e-mail: emason@stsci.edu
² Centro de Estudios de Física del Cosmos de Aragón, 41001 Teruel, Spain
³ INAF, Osservatorio Astronomico di Capodimonte, 80131 Napoli, Italy
⁴ Embassy of the Republic of Indonesia, 10557 Berlin, Germany

Received: 7 May 2012
Accepted: 28 June 2012

Abstract

We present optical and near-infrared (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_rem, the mass of accreted material which has not been ejected during the outburst. In addition, most of the He emission lines, and the He ii 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 Ca ii H&K, the G-band and Mg ib absorption from the secondary star. However, no other significant secondary star features have been observed at longer wavelengths and in the NIR band.