Spectral Evolution and Secular Shift of the Z-source GX 340+0: An Interpretation in the Spreading Layer Paradigm
Year: 2026
Type: article
Abstract: We present a comprehensive spectral and timing study of the bright neutron star low-mass X-ray binary and Z-source, GX 340+0, using broadband X-ray observations with AstroSat in 2020 and 2024, focusing on the evolution of physical parameters along the Z-track and the origin of secular shifts of this track across different epochs. The spectra are consistently described by the same model, comprising blackbody emission from a spreading boundary layer (BL) near the neutron star surface, an accretion disk, and a Comptonizing corona. We observe a significant decrease in the blackbody temperature from ∼1.2 keV on the horizontal branch (HB) to ∼1.0 keV on the normal branch (NB), which likely contributes to the harder spectra on the HB and softer spectra on the NB. At the same time, the blackbody normalization varies substantially from ∼440 to 860, indicating changes in the effective emitting area. Within the spreading layer (SL) framework, this evolution reflects changes in the extent of the radiating layer, while the large absolute normalization values suggest an additional contribution from the BL. We also detect variations in the absorption column density, suggesting structural changes in the inner accretion disk, possibly linked to radiation-driven puffing-up. The Comptonized flux shows a branch-dependent behavior, remaining relatively stable along the HB but decreasing along the NB. We interpret these results within a unified framework where the BL, corona, and inner disk evolve coherently with accretion rate, explaining both the Z-track behavior and its secular shifts.
Source: The Astrophysical Journal
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
OpenAlex ID: W7222958600