Beyond the Coronal Graveyard

Abstract: New Chandra High Resolution Camera pointings on the "non-coronal" red giant Arcturus (HD124897; $\alpha$ Boo: K1.5III) corroborate a tentative soft X-ray detection in a shorter exploratory exposure sixteen years earlier. The apparent source followed the (large) proper motion of the nearby bright star over the intervening years, and there were null detections at the previous location in the current epoch, as well as at the future location in the earlier epoch, reducing the possibility of chance coincidences with unrelated high-energy objects. The apparent X-ray brightness at Earth, averaged over the 98ks of total exposure and accounting for absorption in the red giant's wind, is $2\times10^{-15}$ erg cm$^{-2}$ s$^{-1}$ (0.2-2 keV). Systematic errors in the energy conversion factor, devolving from the unknown spectrum, amount to only about 10%, smaller than the 30% statistical uncertainties in the count rates. The X-ray luminosity is only $3\times10^{25}$ erg s$^{-1}$, confirming Arcturus as one of Chandra's darkest bright stars.