2022/01/27 by R. M. Ludlam, Edward M. Cackett, E. M. Cackett +15
Earth and Planetary Sciences · Physics and Astronomy · #Astronomy #Astrophysical Phenomena and Observations #Astrophysics #Emission spectrum #High-pressure geophysics and materials #Light curve #Neutron star #Physics #Pulsars and Gravitational Waves Research #RADIUS #Reflection (computer programming) #Spectral line #astro-ph.HE
paper · pdf · doi:10.3847/1538-4357/ac5028
Accepted for publication in ApJ, 13 pages, 5 tables, 5 figures
arxiv created 2022/01/27 · openalex publication_date 2022/03/01 · arxiv updated 2022/03/14 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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∘±4∘, rdblur*rfxconv: i=60∘±10∘). The inner disk radius remains close to the NS (R\rm in≤1.15 RISCO) 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\odot, this corresponds to a conservative upper limit of R\rm in≤19.5 km for M=1.92 M\odot or R\rm in≤15.3 km for M=1.5 M\odot. We compare these radius constraints to those obtained from NS gravitational wave merger events and recent NICER pulsar light curve modeling measurements.