Spectral-Timing Evolution of the Black Hole X-ray Binary LMC X-3: An XPoSat View

Abstract

LMC X-3 is a persistent black hole high-mass X-ray binary and serves as an important laboratory for studying accretion disk evolution across different spectral states. We present a spectral-timing analysis of XPoSat/XSPECT observations of LMC X--3 obtained during 2024 and 2025. Using broadband X-ray light curves, hardness diagnostics, power spectral analysis, and spectral modeling, we investigate the long-term evolution of the source across multiple epochs. LMC X-3 exhibits significant changes in both its spectral and timing properties between the two observing campaigns. Hardness-intensity and hardness-rms diagrams reveal a clear separation between the 2024 and 2025 observations and even within the 2024 observations. Overall, it indicates a transition from a relatively harder and more variable state to a brighter, softer, and less variable state. Power spectral analysis shows enhanced broadband variability during the 2024 observations, while the 2025 observations are characterized by weaker variability. Spectral modeling of the combined XSPECT observations indicates an increase in the thermal disk temperature from 0.8 to 1.17 keV, accompanied by a ~5 times increase in X-ray flux between the two epochs. The observed reduction in fractional rms variability with increasing source brightness and decreasing spectral hardness is consistent with more contribution from a stable thermal accretion disk. Taken together, the spectral and timing diagnostics provide a self-consistent picture of accretion-state evolution in LMC X-3, demonstrating the capability of XSPECT to probe long-term variability and accretion processes in bright black hole X-ray binaries.

arXiv comment12 pages, 5 figures, 2 tables. Published in ApJ

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