The Broadband X-ray Spectral Properties during the Rising Phases of the Outburst of the New Black Hole X-ray Binary Candidate Swift J1727.8-1613

Abstract

We report data analysis results about the outburst evolution and spectral properties during the hard state of the recently discovered X-ray transient Swift J1727.8--163 as observed by \emph{Insight}-HXMT. The broadband X-ray spectrum of Swift J1727.8--163 is more complex than the most typical spectral patterns of black hole X-ray binary systems, with not only a comparatively weaker reflection component but also an additional high-energy component, manifesting itself as a hard X-ray tail beyond the thermal Comptonization description detectable below 100 keV. Using reflection models combining thermal ({\tt reflkerr}) and hybrid ({\tt reflkerr\_bb}) Comptonization, we find that the emission arises from a plasma containing both thermal and non-thermal electrons, with the non-thermal tail forming a smooth extension of the thermal continuum. This supports a compact hybrid corona. We find that the inner disk radius remains at the ISCO and the reflection fraction stays low, with no abrupt spectral evolution across epochs. These results highlight the role of hybrid corona in shaping hard-state emission in black hole X-ray binaries.