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We present a spectral analysis of NuSTAR and NICER observations of theluminous, persistently accreting neutron star (NS) low-mass X-ray binary CygnusX-2. The data were divided into different branches that the source traces outon the Z-track of the X-ray color-color diagram; namely the horizontal branch,normal branch, and the vertex between the two. The X-ray continuum spectrum wasmodeled in two different ways that produced a comparable quality fit. Thespectra showed clear evidence of a reflection component in the form of abroadened Fe K line, as well as a lower energy emission feature near 1 keVlikely due to an ionized plasma located far from the innermost accretion disk.We account for the reflection spectrum with two independent models (relxillnsand rdblur*rfxconv). The inferred inclination is in agreement with earlierestimates from optical observations of ellipsoidal light curve modeling(relxillns: $i=67^{\circ}\pm4^{\circ}$, rdblur*rfxconv:$i=60^{\circ}\pm10^{\circ}$). The inner disk radius remains close to the NS($R_{\rm in}\leq1.15\ R_{\mathrm{ISCO}}$) regardless of the source positionalong the Z-track or how the 1 keV feature is modeled. Given the opticallydetermined NS mass of $1.71\pm0.21\ M_{\odot}$, this corresponds to aconservative upper limit of $R_{\rm in}\leq19.5$ km for $M=1.92\ M_{\odot}$ or$R_{\rm in}\leq15.3$ km for $M=1.5\ M_{\odot}$. We compare these radiusconstraints to those obtained from NS gravitational wave merger events andrecent NICER pulsar light curve modeling measurements.

Radius Constraints from Reflection Modeling of Cygnus X-2 with NuSTAR and NICER