The Unusual X-ray Spectrum of FU Orionis

Abstract

FU Orionis stars (FUors) are young low-mass eruptive stars that are thought to be accreting at high rates. They could give rise to soft X-ray emission from accretion shocks, but their X-ray properties are largely unknown. We report the first X-ray detection of the prototype star FU Orionis with XMM-Newton. Its X-ray spectrum is unusual compared to those of classical T Tauri stars (cTTS). The cool and hot components typically detected in cTTS are present but are seen through different absorption column densities. A heavily-absorbed hot component at kT > 5 keV accounts for most of the observed flux and a strong Fe K emission line is present. Such high temperatures are characteristic of magnetic processes rather than shock emission. We discuss possible origins of the unusual spectrum in the context of a complex physical environment that likely includes disk accretion, a strong wind, magnetic activity, and close binarity.