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, the normal branch, and the vertex between the two. The X-ray continuum spectrum was modeled in two different ways that produced comparable quality fits. 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 lightcurve modeling (relxillns: i = 67° ± 4°; rdblur*rfxconv: i = 60° ± 10°). The inner disk radius remains close to the NS (R_(in) ≤ 1.15 R_(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 ± 0.21 M_⊙, this corresponds to a conservative upper limit of R_(in) ≤ 19.5 km for M = 1.92 M_⊙ or R_(in) ≤ 15.3 km for M = 1.5 M_⊙. We compare these radius constraints to those obtained from NS gravitational wave merger events and recent NICER pulsar lightcurve modeling measurements.