Navy Prototype Optical Interferometer Imaging of Line Emission Regions of beta Lyrae Using Differential Phase Referencing

We present the results of an experiment to image the interacting binary star β Lyrae with data from the Navy Prototype Optical Interferometer using a differential phase technique to correct for the effects of the instrument and atmosphere on the interferometer phases. We take advantage of the fact that the visual primary of β Lyrae and the visibility calibrator we used are both nearly unresolved and nearly centrally symmetric, and consequently have interferometric phases near zero. We used this property to correct for the effects of the instrument and atmosphere on the phases of β Lyrae and to obtain differential phases in the channel containing the Hα emission line. Combining the Hα-channel phases with information about the line strength, we recovered complex visibilities and imaged the Hα emission using standard radio interferometry methods. Our images show the position of the Hα-emitting regions relative to the continuum photocenter as a function of orbital phase, indicating a major axis line of nodes along Ω = 249° ± 4°. The orbit is smaller than previously predicted, a discrepancy that can be alleviated if we assume that the system is at a larger distance, or if the stellar continuum contribution to the Hα channel was underestimated. We do not detect a jet in the Hα images, which may be due to the limited resolution of the observations along the direction perpendicular to the orbital plane. We find that the differential phase results are consistent with those obtained from a more standard analysis using squared visibilities (V 2s) and closure phases, which also indicate an Hα disk radius of 0.6 ± 0.1 mas, and ΔV = 1.30 ± 0.1 and ΔR = 1.20 ± 0.1 mag for the magnitude difference between the stars.