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

TL;DR

This study used Swift observations to analyze the NUV and X-ray emissions of Cygnus X-2, revealing an anticorrelation that challenges simple reprocessing models and suggests complex scattering or disk thickening effects.

Contribution

It provides the first detailed Swift-based analysis of NUV-X-ray connections in Cygnus X-2 across all Z track states, highlighting the anticorrelation and its possible explanations.

Findings

NUV emission is uncorrelated with soft X-ray flux.

NUV emission is anticorrelated with hard X-ray flux.

Anticorrelation challenges simple reprocessing models.

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.