A method for internal calibration of optical interferometer data and application to the circumstellar envelope of γ Cassiopeiae

We present a technique for calibrating optical long-baseline interferometric observations in which both the calibration corrections and the source characteristics are obtained from the observations of a program star. This calibration can only be applied to certain classes of objects, such as emission-line sources or binary systems, in which the parameters describing the characteristics of the source are orthogonal to the calibration parameters. The technique is applied to observations of γ Cassiopeiae, obtained on four different nights with the Navy Prototype Optical Interferometer, and utilizes measurements obtained simultaneously in many spectral channels covering a wide spectral range, of which only two channels contain a strong signal due to the circumstellar envelope in the Hα emission line. The calibrated observations in Hα show a clearly resolved circumstellar structure. The best-fit elliptical Gaussian model fitted to our observations has ensemble average parameters of 3.67 ± 0.09 mas for the angular size of the major axis, 0.79 ± 0.03 for the axial ratio, and 32° ± 5° for the position angle, all in good agreement with values reported by previous investigations.