Chandra LETGS observation of the active binary Algol

¹ Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
² Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1603, 85740 Garching, Germany
³ Space Research Organization Netherlands (SRON), Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

Corresponding author: J.-U. Ness, jness@hs.uni-hamburg.de

Received: 17 December 2001
Accepted: 22 March 2002

Abstract

A high-resolution spectrum obtained with the low-energy transmission grating onboard the Chandra observatory is presented and analyzed. Our analysis indicates very hot plasma with temperatures up to  MK from the continuum and from ratios of hydrogen-like and helium-like ions of Si, Mg, and Ne. In addition lower temperature material is present since O vii and N vi are detected. Two methods for density diagnostics are applied. The He-like triplets from N vii to Si xiii are used and densities around 10¹¹ cm^-3 are found for the low temperature ions. Taking the UV radiation field from the B star companion into account, we find that the low-Z ions can be affected by the radiation field quite strongly, such that densities of  cm^-3 are also possible, but only assuming that the emitting plasma is immersed in the radiation field. For the high temperature He-like ions only low density limits are found. Using ratios of Fe xxi lines produced at similar temperatures are sensitive to lower densities but again yield only low density limits. We thus conclude that the hot plasma has densities below 10¹² cm^-3. Assuming a constant pressure corona we show that the characteristic loop sizes must be small compared to the stellar radius and that filling factors below 0.1 are unlikely.