Chandra HETG X-ray spectra and variability of π Aqr, a γ Cas-type Be star

High-resolution X-ray spectra of π Aqr, a γ Cas-type star, obtained with the Chandra/HETG spectrometer, revealed emission lines of H-like ions of Mg, Si, S, and Fe; a strong, hard continuum; and a lack of He-like ions, indicating the presence of very hot thermal plasma. The X-ray light curve showed significant fluctuations, with coherent variability at a period of about 3400 s in one observation. The hardness ratio was relatively constant except for one observation in which the spectrum was much harder and more absorbed. We interpret the X-ray emission as arising from accretion onto the secondary, which is likely a magnetic white dwarf, an intermediate polar system

A detailed analysis of X-ray emission-line velocities of Capella from over 20 yr of <i>Chandra</i>/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)

CIAO: Chandra's Data Analysis System

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