Reflecting on Accretion in Neutron Star Low-Mass X-ray Binaries
Renee Ludlum (Wayne State University)
Neutron star (NS) low-mass X-ray binaries (LMXBs) accrete via Roche-lobe overflow from a stellar companion of roughly 1 solar mass. The accretion disk surrounding the NS in these systems can be externally illuminated by X-rays that are reprocessed by the accreting material and re-emitted as the reflection spectrum, which is comprised of emission lines superimposed onto a reprocessed continuum. Due to proximity of the inner region of the disk to the compact accretor, strong gravity and relativistic effects are imparted to the reflection spectrum. Modeling of these effects allows us to infer properties of the NS itself (such as magnetic field strength and limits on the radial extent), as well as other aspects of the system like accretion and illumination source geometry, ionization state and composition of the material, and density of the disk. Much has been learned about these systems using advanced X-ray missions like NICER and NuSTAR, which can obtain spectra from bright sources without distortion. The recent successful operation of a microcalorimeter in space aboard XRISM provides high-energy resolution observations of several persistently accreting NSs. These datasets reveal undeniable structure within the prominent Fe K line region of the reflection spectrum and provide further insights into the accretion geometry of these systems. I will highlight what has been learned about NS LMXBs via reflection modeling over the last decade and prospects for further insights.
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