The Hard-to-soft Transition of GX 339–4 as Seen by Insight–HXMT

Abstract We present an analysis of the relativistic reflection spectra of GX 339–4 during the hard-to-soft transition of its 2021 outburst observed by Insight–HXMT. The strong relativistic reflection signatures in the data suggest a high black hole spin ( a * > 0.86) and an intermediate disk inclination angle ( i ≈ 35°–43°) of the system. The transition is accompanied by an increasing temperature of the disk and a softening of the corona emission, while the inner disk radius remains stable. Assuming a lamppost geometry, the corona height is also found to stay close to the black hole across the state transition. If we include the Comptonization of the reflection spectrum, the scattering fraction parameter is found to decrease during the state transition. We also perform an analysis with a reflection model designed for hot accretion disks of stellar mass black holes where the surface of the innermost accretion disk is illuminated by emission from the corona and the thermal disk below. Our results support the scenario in which the state transition is associated with variations in the corona properties.

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