A View of the Long-term Spectral Behavior of Ultracompact X-Ray Binary 4U 0614+091

Abstract In this study, we examine 51 archival NICER observations and six archival NuSTAR observations of the neutron star ultracompact X-ray binary (UCXB) 4U 0614+091, which span over 5 yr. The source displays persistent reflection features, so we use a reflection model designed for UCXBs, with overabundant carbon and oxygen ( xillverCO ) to study how various components of the system vary over time. The flux of this source is known to vary quasiperiodically on a timescale of a few days, so we study how the various model components change as the overall flux varies. The flux of most components scales linearly with the overall flux, while the power law, representing coronal emission, is anticorrelated, as expected. This is consistent with previous studies of the source. We also find that during observations of the high-soft state, the disk emissivity profile as a function of radius becomes steeper. We interpret this as the corona receding to be closer to the compact object during these states, at which point the assumed power law illumination of xillverCO may be inadequate to describe the illumination of the disk.