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 gamma 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 Halpha emission line. The calibrated observations in Halpha 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 32degrees +/- 5degrees for the position angle, all in good agreement with values reported by previous investigations.