X-Ray Doppler Imaging of 44i Bootis with Chandra

Chandra High-Energy Transmission Grating observations of the bright eclipsing contact binary 44i Bootis show X-ray line profiles that are Doppler-shifted by orbital motions. The X-ray emission spectrum contains a multitude of lines superposed on a weak continuum, with strong lines of O VIII, Ne X, Fe XVII, and Mg XII. The profiles of these lines from the total observed spectrum show Doppler-broadened widths of ~550 km s^-1. Line centroids vary with orbital phase, indicating velocity changes of greater than 180 km s^-1. The first-order light curve shows significant variability but no clear evidence for either primary or secondary eclipses. Flares are observed for all spectral ranges; additionally, the light curve constructed near the peak of the emission measure distribution [T_e=(5-8)×10⁶ K] shows quiescent variability as well as flares. The phase dependences of line profiles and light curves together imply that at least half of the emission is localized at high latitude. A simple model with two regions on the primary star at relatively high latitude reproduces the observed line profile shifts and quiescent light curve. These first clear X-ray Doppler shifts of stellar coronal material illustrate the power of Chandra.