The Coronae of AR Lacertae

We observed the coronally active eclipsing binary, AR Lac, with the HighEnergy Transmission Grating on Chandra for a total of 97 ks, spaced over fiveorbits, at quadratures and conjunctions. Contemporaneous and simultaneous EUVspectra and photometry were also obtained with the Extreme UltravioletExplorer. Significant variability in both X-ray and EUV fluxes were observed,dominated by at least one X-ray flare and one EUV flare. We saw no evidence ofprimary or secondary eclipses. X-ray flux modulation was largest at hightemperature, indicative of flare heating of coronal plasma. Line widthsinterpreted in terms of Doppler broadening suggest that both binary stellarcomponents are active. From line fluxes obtained from total integrated spectra,we have modeled the emission measure and abundance distributions. A strongmaximum was found in the differential emission measure, characterized by peaksat log T = 6.9 and 7.4, together with a weak but significant cooler maximumnear log T=6.2, and a moderately strong hot tail from log T= 7.6-8.2. Coronalabundances have a broad distribution and show no simple correlation with firstionization potential. While the resulting model spectrum generally agrees verywell with the observed spectrum, there are some significant discrepancies,especially among the many Fe L-lines. Both the emission measure and abundancedistributions are qualitatively similar to prior determinations from otherX-ray and ultraviolet spectra, indicating some long-term stability in theoverall coronal structure.Comment: 31 pages, 8 figures, 3 tables; Accepted for publication in the Astrophysical Journal (tentatively October 1, 2003