Exploring the Hard-to-Soft State Transition in GX 339–4: Insights from Multi-Instrument Observations with NuSTAR, NICER, and Insight–HXMT

In RETCO, I will discuss a detailed spectral analysis of the black hole X-ray binary GX 339–4 during its transition from hard to soft states, based on simultaneous observations from NuSTAR, NICER, and Insight-HXMT during the 2021 outburst. Using relativistic reflection models, I examine the evolution of the accretion disk, with particular focus on the inner disk radius ($R_{in}$) across different states. Notably, a single hard Comptonization model reveals an unexpectedly small $R_{in}$ in the hard state (~3 gravitational radii, $r_g$), which then expands to approximately 10 $r_g$ in the soft state. By adding a warm Comptonization component, this inconsistency is resolved, resulting in a more stable hard state $R_{in}$ around 15 $r_g$, exceeding the values observed in the soft states.