X-ray spectroscopy of the W UMa-type binary 44 Bootis

European Space Agency, ESTEC – Postbus 299, 2200 AG Noordwijk, The Netherlands e-mail: pgondoin@rssd.esa.int

Received: 22 March 2004
Accepted: 1 July 2004

Abstract

44 Boo B, a W UMa-type binary system, was observed in June 2001 during one entire revolution period with the observatory. The count rate in the 0.3 to 2 keV band is constant in average with 5 to 20% count rate increases reminiscent of flares. Spectral fitting of the EPIC spectra indicates a corona configuration with little contribution from quiet regions, similar to the Sun. On the contrary, the (2–9)  10⁶ K temperature range of the “cool” plasma suggests that the active corona around the two companions is densely filled with low-lying loops similar to those found in solar-type active regions. The 44 Boo O VII He-like triplet constrains the electron density to an upper limit n_e < 8.6  10¹⁰ cm^-3. We argue that this low-lying loop system may be overlaid by larger loops. Magnetic reconnection phenomena in this large loops system may explain the characteristic flare decay time in the light curve that implies loop lengths of about 16  10⁹ cm. An extended corona around 44 Boo would explain the absence of eclipses in its X-ray light curve. The average element abundance in 44 Boo corona is found to be lower than the solar photospheric value. The spectral analysis indicates enhanced abundances of oxygen and neon relative to iron which suggest an inverse FIP effect. Compared with other active binary systems such as RSCVn or BY Dra, 44 Boo has relatively less material at temperatures higher than 10⁷ K and the temperature of its hottest plasma component appears to be lower.