EUVE Photometric Eclipse Observations of AR Lacertae

doi:10.1086/118050

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Title:		EUVE Photometric Eclipse Observations of AR Lacertae
Authors:		Christian, D. J.; Drake, J. J.; Patterer, R. J.; Vedder, P. W.; Bowyer, S.
Publication:		Astronomical Journal v.112, p.751 (AJ Homepage)
Publication Date:		08/1996
Origin:		AJ
AJ/ApJ Keywords:		BINARIES: ECLIPSE, STARS: INDIVIDUAL: AR LACERTAE
DOI:		10.1086/118050
Bibliographic Code:		1996AJ....112..751C

Abstract

We report on observations of the eclipsing RS CVn system AR Lac with the Extreme Ultraviolet Explorer (EUVE) scanning and deep survey (DS) telescopes. This system consists of a G2 IV star and a K0 IV star separated by 9.2R_sun with an orbital period of about 1.98 days. The scanner observations consisted of a single pointing over a period of 25 hours, providing some coverage of both primary and secondary eclipses, and subsequent scanning during the all-sky survey. During the pointed observation AR Lac was detected in the EUVE Lexan/boron (Lexan/B, 50-180 Å) and Al/Ti/C (160-240 Å) bands at average count rates of 0.12 and 0.016 count s^-1 respectively. During primary eclipse, the Lexan/B count rate was at about one-third of its subsequent value at quadrature, suggesting that, in the case of a compact and relatively homogeneous corona, a minimum of 60%-70% of the extreme ultraviolet (EUV) emission originates on the G star. There was no detectable modulation corresponding to secondary eclipse in either Lexan/B or Al/Ti/C bands. Data taken several months later during the EUVE all-sky survey yield the same Lexan/B count rate as the pointed data but only an upper limit in the Al/Ti/C bandpass. AR Lac was observed again with the DS as part of a Guest Observer pointed observation. This single pointing lasted slightly more than three days and included both primary and secondary eclipses. AR Lac was detected in the DS Lexan/B band with an average count rate of 0.31 count s^-1. The primary eclipse is easily visible as a ˜40% decrease in intensity. A dip in the light curve is present at the time of secondary eclipse, but is not readily discernible from the average level of variability. A flare corresponding to a factor of 2 increase over the quiescent emission count rate was observed during the next expected secondary eclipse. We have performed an elementary emission measure analysis and compared isothermal models that fit the EUVE data to earlier Einstein, EXOSAT, ASCA, and ROSAT two-temperature models culled from the literature. In general, the EUVE data indicate very similar emission measures to those derived in previous work, indicating that the corona of AR Lac does not vary in average x-ray or EUV luminosity over periods of years up to a decade. Assuming an isothermal model with T≍10⁷ K, as indicated by the emission measure analysis, the total Lexan/B count rate at quadrature implies an EUV luminosity of 1.1×10³⁰ erg s^-1 in the 50-180 A bandpass.

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