X-ray spectroscopy of UZ Librae

Space Science Department, European Space Agency – Postbus 299, 2200 AG Noordwijk, The Netherlands

Corresponding author: pgondoin@rssd.esa.int

Received: 11 November 2002
Accepted: 16 December 2002

Abstract

UZ Librae is the giant primary of a close binary system with extremely fast rotation and revolution. Its distinctive characteristics include an optical light curve with a large amplitude and a high X-ray luminosity. UZ Librae was observed twice at five months interval in August 2000 and January 2001 by the XMM–Newton space observatory. Serie of lines of highly ionized Fe and several lines of the Ly and He series are visible in the reflection grating spectra, most notably from C, O and Ne. The corresponding ions are associated with plasma components with temperature in the range 3–8 MK. Bremstrahlung emission from plasma at high temperature ( K) indicates an intense flaring activity on UZ Librae. This is also supported by a Ne abundance enhancement relative to oxygen reminiscent of abundance anomalies observed during stellar and solar flares. In contrast to oxygen and neon, the abundance of Fe in the corona of UZ Librae is determined to be a tenth of the solar photospheric value. We conjecture that the large fluid kinetic helicity induced by the rapid rotation of the star currently generates magnetic fields with two characteristic scales comparable with compact loops within solar active regions and with larger cooler loops interconnecting solar active regions. The X-ray emission of UZ Librae would be strongly enhanced not only due to the occurrence of these magnetic structures, but mainly due to their large surface coverage and to their permanent interactions responsible for heating UZ Librae plasma to high temperatures. A rotational modulation effect could have contributed to the luminosity variation between the two XMM–Newton observations since a large fraction of the X-ray emitting material might be located above large photospheric spots detected on Doppler images of the star.