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

Abstract

We present a spectral analysis of NuSTAR and NICER observations of the luminous, persistently accreting neutron star (NS) low-mass X-ray binary Cygnus X-2. The data were divided into different branches that the source traces out on 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 was modeled in two different ways that produced a comparable quality fit. The spectra showed clear evidence of a reflection component in the form of a broadened Fe K line, as well as a lower energy emission feature near 1 keV likely due to an ionized plasma located far from the innermost accretion disk. We account for the reflection spectrum with two independent models (relxillns and rdblur*rfxconv). The inferred inclination is in agreement with earlier estimates from optical observations of ellipsoidal light curve modeling (relxillns: $i=67^{\circ }\pm 4^{\circ }$, rdblur*rfxconv: $i=60^{\circ }\pm 10^{\circ }$). The inner disk radius remains close to the NS ($R_{\mathrm{in}}\le 1.15R_{\mathrm{ISCO}}$) regardless of the source position along the Z-track or how the 1 keV feature is modeled. Given the optically determined NS mass of $1.71\pm 0.21M_{\odot }$, this corresponds to a conservative upper limit of $R_{\mathrm{in}}\le 19.5$ km for $M=1.92M_{\odot }$ or $R_{\mathrm{in}}\le 15.3$ km for $M=1.5M_{\odot }$. We compare these radius constraints to those obtained from NS gravitational wave merger events and recent NICER pulsar light curve modeling measurements.

Accepted for publication in ApJ, 13 pages, 5 tables, 5 figures