A 33 yr CONSTANCY OF THE X-RAY CORONAE OF AR Lac AND ECLIPSE DIAGNOSIS OF SCALE HEIGHT

Abstract

Extensive X-ray and extreme ultraviolet (EUV) photometric observations of the eclipsing RS CVn system AR Lac were obtained over the years 1997 to 2013 with the Chandra X-ray Observatory Extreme Ultraviolet Explorer. During primary eclipse, HRC count rates decrease by ∼40%. A similar minimum is seen during one primary eclipse observed by EUVE but not in others owing to intrinsic source variability. Little evidence for secondary eclipses is present in either the X-ray or EUV data, reminiscent of earlier X-ray and EUV observations. Primary eclipses allow us to estimate the extent of a spherically symmetric corona on the primary G star of about 1.3R , or 0.86R , and indicate the G star is likely brighter than the K component by a factor of 2-5. Brightness changes not attributable to eclipses appear to be dominated by stochastic variability and are generally non-repeating. X-ray and EUV light curves cannot therefore be reliably used to reconstruct the spatial distribution of emission assuming only eclipses and rotational modulation are at work. Moderate flaring is observed, where count rates increase by up to a factor of three above quiescence. Combined with older ASCA, Einstein, EXOSAT, ROSAT and Beppo-SAX observations, the data show that the level of quiescent coronal emission at X-ray wavelengths has remained remarkably constant over 33 years, with no sign of variation due to magnetic cycles. Variations in base level X-ray emission seen by Chandra over 13 years are only ∼ 10%, while variations back to pioneering Einstein observations in 1980 amount to a maximum of 45% and more typically about 15%.

Giant Coronal Loops Dominate the Quiescent X-Ray Emission in Rapidly Rotating M Stars

“…This is similar to the results for systems with higher mass secondaries (eg. σ 2 Cor Bor, AR LAc, V711 Tau, Osten et al 2003;Pandey & Singh 2012;Drake et al 2014). The scale height of the corona of the G star in AR Lac is about 1.3 solar radii (Drake et al 2014).…”

“…σ 2 Cor Bor, AR LAc, V711 Tau, Osten et al 2003;Pandey & Singh 2012;Drake et al 2014). The scale height of the corona of the G star in AR Lac is about 1.3 solar radii (Drake et al 2014). Those authors warn that symmetrical coronal eclipses that can easily be interpreted in terms of spherical emitting geometry are fairly rare.…”

Optical, UV, and X-ray evidence for a 7-yr stellar cycle in Proxima Centauri

“…X-ray intensity varied by about a factor of two over the 14 XMM-Newton observations that spanned 21 months in 2011-2013. Although not all stars will exhibit X-ray cycles (e.g., Hoffman, Günther & Wright 2012;Drake et al 2014), the above examples illustrate the potential of detecting cycles using X-ray monitoring. Prox Cen is however, a more challenging case because it flares more often and its X-ray cycle amplitude appears to be smaller than for these other stars.…”

X-ray emission in the enigmatic CVSO 30 system

“…This finding is in line with the results reported by Telleschi et al (2007), who find a similar variation between ROSAT and XMM-Newton observations in their study of nine T Tauri stars. Other highly active stars such as the 50 Myr old AB Dor and the RS CVn binaries AR Lac and HR 1099 also show similarly constant levels of X-ray emission, disregarding strong short-term variability due to flares (Lalitha & Schmitt 2013;Drake et al 2014;Perdelwitz et al 2018).…”