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The neutron star X-ray binary (NSXRB) Cygnus X-2 was observed by the Swiftsatellite 51 times over a 4 month period in 2008 with the XRT, UVOT, and BATinstruments. During this campaign, we observed Cyg X-2 in all three branches ofthe Z track (horizontal, normal, and flaring branches). We find that the NUVemission is uncorrelated with the soft X-ray flux detected with the XRT, and isanticorrelated with the BAT X-ray flux and the hard X-ray color. The observedanticorrelation is inconsistent with simple models of reprocessing as thesource of the NUV emission. The anticorrelation may be a consequence of thehigh inclination angle of Cyg X-2, where NUV emission is preferentiallyscattered by a corona that expands as the disk is radiatively heated.Alternatively, if the accretion disk thickens as Cyg X-2 goes down the normalbranch toward the flaring branch, this may be able to explain the observedanticorrelation. In these models the NUV emission may not be a good proxy for$\dot m$ in the system. We also discuss the implications of using Swift/XRT toperform spectral modeling of the continuum emission of NSXRBs.

Swift monitoring of Cygnus X-2: investigating the NUV-X-ray connection