X-ray Doppler Imaging of 44i Boo with Chandra
Abstract
Chandra High-Energy Transmission Grating observations of the bright eclipsing contact binary 44i Boo show X-ray line profiles which are Doppler-shifted by orbital motions. The X-ray emission spectrum contains a multitude of lines superimposed 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 > 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 (Te = 5 to 8 X 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.
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