Unveiling Accretion in Ultra Compact X-ray Binaries through Simultaneous NICER and NuSTAR Observations of the Ultra-compact X-ray Binary 4U 0614+091

Ultra-compact X-ray binaries (UCXBs) are a subclass of low-mass X-ray binaries with a very short orbital period (< 80 minutes). This short period implies accretion from a degenerate companion such as a white dwarf. As a result, the reflection spectra of these objects differ from a typical LMXB since the disk is nearly devoid of H and He while overabundant in C and O. Consequently, the spectrum exhibits strong O Lyman emission lines in addition to the standard Fe K component. By studying the unique reflected features present in UCXBs, we can place constraints on various facets of the accretion disk and the compact object itself. The UCXB 4U 0614+091 shows periodic variability in flux on the timescale of days. We obtained four simultaneous NICER and NuSTAR observations that occur several days apart in order to determine how the accretion disk responds to changes in illuminating flux. This is the first time the source has been observed simultaneously with both missions and provides a broad X-ray passband with which we can test the emission radii of each emission line component and condition of the disk (e.g., ionization state of the emitting material, position of the inner disk radius, etc.) and compare to other ultra-compact systems.