A View of the Long-Term Spectral Behavior of Ultra Compact X-Ray Binary 4U 0614+091

TL;DR

This study analyzes long-term spectral data of the ultra-compact X-ray binary 4U 0614+091, revealing how system components vary with flux and state, and suggesting changes in coronal geometry during different spectral states.

Contribution

First detailed long-term spectral analysis of 4U 0614+091 using reflection models with overabundant elements, revealing flux-dependent component behavior and state-related disk geometry changes.

Findings

Most spectral components scale linearly with flux.

Coronal emission is anti-correlated with overall flux.

Disk emissivity becomes steeper in high-soft states.

Abstract

In this study, we examine 51 archival NICER observations and 6 archival NuSTAR observations of the neutron star (NS) ultra-compact X-ray binary (UCXB) 4U 0614+091, which span over 5 years. The source displays persistent reflection features, so we use a reflection model designed for UCXBs, with overabundant carbon and oxygen ({\sc xillverCO}) to study how various components of the system vary over time. The flux of this source is known to vary quasi-periodically 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 anti-correlated 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 {\sc xillverCO} may be inadequate to describe the illumination of the disk.