Presentation #110.45 in the session “Stellar/Compact (Poster)”.
Neutron star (NS) low-mass X-ray binaries (LMXBs) accrete via Roche-lobe overflow from a stellar companion that is roughly the mass of the Sun. X-rays that originate from a hot electron corona or emission from the surface of the NS can irradiate the surrounding accretion disk where they are reprocessed and re-emitted as a series of discrete emission lines superimposed on a reprocessed continuum, which is referred to as “reflection”. These narrow emission lines are broadened due to Doppler, general, and special relativistic effects that provide a diagnostic of the location and conditions of the innermost region of the accretion disk. Since the accretion disk must terminate at or prior to the NS surface, determining the inner disk radius can provide a limit on the radial extent of the NS. Cygnus X-2 is a prime source for demonstrating the power of reflection studies to obtain NS radius constraints since it has an optically determined mass estimate and shows broadened emission lines. Reflection modeling of X-ray spectra from the combined bandpass and sensitivity of NICER and NuSTAR indicates that the disk remains close to the NS even as the mass accretion rate changes. These radius constraints are compared to NS radius measurements from pulsar light curve modeling and gravitational wave events due to NS-NS mergers.