Swift monitoring of cygnus X-2: Investigating the near-ultraviolet-X-ray connection

Abstract

The neutron star X-ray binary (NSXRB) Cyg X-2 was observed by the Swift satellite 51 times over a 4 month period in 2008 with the X-ray Telescope (XRT), UV/optical telescope, and Burst Alert Telescope (BAT) instruments. During this campaign, we observed Cyg X-2 in all three branches of the Z track (horizontal, normal, and flaring branches). We find that the NUV emission is uncorrelated with the soft X-ray flux detected with the XRT and is anticorrelated with the BAT X-ray flux and the hard X-ray color. The observed anticorrelation is inconsistent with simple models of reprocessing as the source of the NUV emission. The anticorrelation may be a consequence of the high inclination angle of Cyg X-2, where NUV emission is preferentially scattered by a corona that expands as the disk is radiatively heated. Alternatively, if the accretion disk thickens as Cyg X-2 goes down the normal branch toward the flaring branch, this may be able to explain the observed anticorrelation. In these models, the NUV emission may not be a good proxy for ṁ in the system. We also discuss the implications of using Swift/XRT to perform spectral modeling of the continuum emission of NSXRBs. © 2010. The American Astronomical Society. All rights reserved.