Beyond the Coronal Graveyard

TL;DR

Chandra observations confirm Arcturus emits extremely faint X-rays, supporting the idea that some red giants can produce detectable high-energy emissions despite their generally low activity levels.

Contribution

This study provides the first confirmed detection of X-ray emission from Arcturus, demonstrating the star's faint X-ray brightness and validating previous tentative detections.

Findings

Arcturus emits very faint X-rays with an luminosity of about 3×10^{25} erg/s.

The X-ray source's proper motion matches that of Arcturus, confirming the star as the source.

The X-ray flux is consistent with previous tentative detections, strengthening the case for red giants' coronal activity.

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.