X-ray Spectra and Light Curves of AR Lac: Temperature Structure, Abundances, and Variability

We observed AR Lac, an eclipsing RS CVn binary star, with the Chandra High Energy Grating Spectrometer for a total of 100 ks divided into six intervals covering both quadratures and eclipses. We repeated observations at each phase. At least two flares were seen, in which the flux increased by factors of two and four. The flares occurred near eclipse phases and compromised detection of eclipse modulation. The quadrature fluxes were the most stable, but they also show non-repeating trends. Quadrature line profiles are broadened relative to eclipse profiles, presumably due to orbital velocity Doppler shifted emission from each binary component. The spectrum appears to be iron poor and neon rich, similar to HR 1099, but not as extreme as II Pegasi. We will examine line strengths, widths, and positions vs. phase, and present preliminary differential emission measure models and abundance determinations using the APED emissivity database. This work is supported by NASA contract NAS8-38249 (HETG) and SAO SV1-61010 (CXC) to MIT.