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A. L. Fey (USNO), K. M. Desai (NRAO)
We have made multi-frequency VLBA observations of the extragalactic source 2023+336. The line of sight to this source passes through the Cygnus region of the sky. Our new observations confirm the scattered nature of 2023+336 as reported previously by one of us and by other authors. However, the angular size of the source has increased at all observed frequencies by about 50% over observations made just three years prior. We rule out intrinsic structure effects since the source is compact at our highest observing frequency and would contribute negligibly to the measured angular size at our lowest observing frequency (assuming that the intrinsic size scales only as the inverse of the observing frequency). To our knowledge, this is the largest temporal variation of angular broadening ever measured by an interferometer and has no precedence along similar lines of sight through this region of the sky. Interstellar scattering towards Cygnus is believed to be associated with turbulence in the Cygnus superbubble. With an assumed distance of 2~kpc to the Cygnus region, the measured 2.3~GHz angular size of 18~mas projected onto the scattering screen can be used to set a lower limit of about 60~km/s on the bulk velocity of the turbulent media transverse to the line of sight.
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