Evolution of the High-energy Excess in Swift J1727.8-1613 during Its 2023 Flare State

Abstract We investigate the evolution of the high-energy excess in Swift J1727.8-1613 during its 2023 flare state using joint Insight-HXMT and INTEGRAL/ISGRI observations covering 2–500 keV. Fits to the broadband spectra with a model including thermal Comptonization and disk reflection leave systematic positive residuals above ∼150 keV. Adding a power-law component improves the fits and yields photon indices of Γ PL = 2.08–2.61, demonstrating that the high-energy excess beyond the thermal Comptonization continuum is present throughout the flare state. We also show that all spectra can be well described by the hybrid Comptonization model eqpair without requiring any additional high-energy component. The fitting results indicate that the energy supply to the electrons transitions from predominantly thermal heating to nonthermal acceleration, and that the electron injection spectrum steepens abruptly near the onset of the flare state. During this evolution, the radio spectral index changes from approximately zero to negative values, accompanied by the ejection of three transient jet knots. Based on this temporal association, we discuss a possible connection between the increase in nonthermal power indicated by our spectral analysis and the ejection of the transient jet.