Simultaneous NICER and NuSTAR Observations of the Ultra-compact X-ray Binary 4U 0614+091

TL;DR

This study presents joint NuSTAR and NICER observations of the ultra-compact X-ray binary 4U 0614+091, revealing flux variations, reflection modeling insights, and evidence of accretion disk truncation during low flux states.

Contribution

First combined NuSTAR and NICER observations of 4U 0614+091, analyzing flux variability and accretion disk behavior using reflection modeling techniques.

Findings

Reflected and thermal flux components track overall flux variations.

Coronal flux increases as total flux decreases.

Evidence of accretion disk truncation at low flux levels.

Abstract

We present the first joint NuSTAR and NICER observations of the ultra-compact X-ray binary (UCXB) 4U 0614+091. This source shows quasi-periodic flux variations on the timescale of ~days. We use reflection modeling techniques to study various components of the accretion system as the flux varies. We find that the flux of the reflected emission and the thermal components representing the disk and the compact object trend closely with the overall flux. However, the flux of the power-law component representing the illuminating X-ray corona scales in the opposite direction, increasing as the total flux decreases. During the lowest flux observation, we see evidence of accretion disk truncation from roughly 6 gravitational radii to 11.5 gravitational radii. This is potentially analogous to the truncation seen in black hole low-mass X-ray binaries, which tends to occur during the low/hard state at sufficiently low Eddington ratios.