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

Abstract

The neutron star X-ray binary (NSXRB) Cygnus X-2 was observed by the Swift satellite 51 times over a 4 month period in 2008 with the XRT, UVOT, and 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 $\dot{m}$ in the system. We also discuss the implications of using Swift/XRT to perform spectral modeling of the continuum emission of NSXRBs.

10 pages, 8 figures. ApJ Accepted

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• The connection between the UV/optical and X-ray emission in the neutron star low-mass X-ray binary Aql X-1
• Thermonuclear X-ray bursts detected in Cyg X-2 using AstroSat/LAXPC
• The origin of UV/optical emission in the black hole low-mass X-ray binary Swift J1753.5-0127