A detailed analysis of X-ray emission-line velocities of Capella from over 20 yr of Chandra /HETG spectroscopy

Capella is the brightest chromospherically active binary in the sky, hosting a cooler G8III giant (Aa) and a hotter G1III companion (Ab). The source has been extensively observed in the X-rays in the past decades not only for its astrophysical interest in the field of corona sources, but also for in-flight calibrations of space-based X-ray instruments. In 2006, it was demonstrated using Chandra/High Energy Transmission Grating (HETG) observations that Aa is the main contributor to Capella’s X-ray emission, as the centroid energies of the emission lines are Doppler shifted along the orbit of the G8III giant (an aspect that has to be taken into consideration for calibration activities of X-ray instruments). In this paper, we extend the previous analysis performed in 2006 by re-analysing in a homogeneous way all Chandra/HETG observations performed in the direction of Capella. By doubling the amount of data available, we strengthened the conclusion that Capella Aa is the dominant emitter in soft X-rays. We did not find any evidence of a statistically significant contribution to this emission by the Ab giant. Our findings are discussed also in light of the incoming launch of the X-Ray Imaging and Spectroscopy Mission (XRISM) mission (spring 2023).