X-ray variability of Capella

¹ SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands e-mail: a.j.j.raassen@sron.nl
² Astronomical Institute Anton Pannekoek, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands

Received: 23 May 2006
Accepted: 21 September 2006

Abstract

We discuss 12 individual observations of stable calibration source Capella over the last five years. The light curves and spectra, derived from LETGS on board CHANDRA are investigated. The spectra cover the wavelength range 1-175 Å. Nine of the twelve observations show comparable count rates, while the three most recent observations between March 2005 and April 2006 show a 40-50% higher count rate. No individual flares were recognised. Line flux ratios between different observations show that the variations are related to the hotter plasma. Variations in line fluxes of highly ionised iron lines show a strong resemblance with earlier line flux variations in the years 1992-1995. A time interval of 10.7(.2) year is determined between two maxima in line fluxes. Emission measure modellings to the total spectra have been made by means of spex in combination with mekal. Observations with higher count rates have a higher emission measure at higher temperature.