X-ray Eclipse Diagnosis of the Evolving Mass Loss in the Recurrent Nova U Scorpii 2010

TL;DR

This study reports the detection of the earliest X-ray eclipse in the recurrent nova U Scorpii 2010, revealing insights into the evolving mass loss and the size of the X-ray emitting region during the outburst.

Contribution

First detection of early X-ray eclipse in U Scorpii 2010, providing constraints on ejecta mass and mass-loss rate evolution.

Findings

X-ray eclipse observed 15 days post-outburst with 27% dimming

Source region shrank by 10-20% between days 15 and 35

Estimated ejecta mass of 10^{-7} to 10^{-6} solar masses

Abstract

We report the Suzaku detection of the earliest X-ray eclipse seen in the recurrent nova U Scorpii 2010. A target-of-opportunity observation 15 days after the outburst found a 27+/-5% dimming in the 0.2-1.0 keV energy band at the predicted center of an eclipse. In comparison with the X-ray eclipse depths seen at two later epochs by XMM-Newton, the source region shrank by about 10-20% between days 15 and 35 after the outburst. The X-ray eclipses appear to be deeper than or similar to contemporaneous optical eclipses, suggesting the X-ray and optical source region extents are comparable on day 15. We raise the possibility of the energy dependency in the photon escape regions, and that this would be a result of the supersoft X-ray opacity being higher than the Thomson scattering optical opacity at the photosphere due to bound-free transitions in abundant metals that are not fully ionized. Assuming a spherically symmetric explosion model, we constrain the mass-loss rate as a function of time. For a ratio of actual to Thomson opacity of 10-100 in supersoft X-rays, we find a total ejecta mass of about 10^{-7}-10^{-6} M_{\odot}.