NuSTAR View of the 2025 Mini-Outburst of the Black Hole X-ray Binary GRS 1739-278: Spectral and Timing Evolution in the Soft State

Abstract: We present a dedicated timing and spectral analysis of three NuSTAR observations of the Galactic black hole candidate GRS 1739-278 obtained during the decay of its 2025 mini-outburst (September 18-30, 2025). The source is found in a soft, disk-dominated state throughout, with a declining $3$-$79$ keV count rate (from $112.4$ to $99.4$ cts s$<sup>{-1}$), a low and only mildly evolving hardness ratio ($\approx0.14$-$0.16$) and a fractional variability that drops from $3.2\%$ to $1.1\%$; no periodic or quasi-periodic signal is detected in the power spectrum or the Lomb-Scargle periodogram of any observation. Joint FPMA+FPMB spectral fits with an absorbed disk-blackbody plus thermal-Comptonization continuum, together with a broad, high-significance ($11.7$-$15.0σ$) curvature feature near $10$-$12$ keV (an empirical proxy for a strong disk-reflection/returning-radiation component) describe the $4$-$30$ keV spectra well ($χ^2/{\rm dof}=1.04$-$1.15$). The inner-disk temperature cools steadily from $1.021\pm0.001$ to $1.002<sup>{+0.003}_{-0.002}$ keV while the disk normalization and hence the derived inner radius, $R_{\rm in}\simeq27.7$ km $\simeq0.95\,R_{ISCO}$ remains constant to within $0.3\%$, consistent with a disk anchored at the innermost stable circular orbit throughout. The unabsorbed $4$-$30$ keV luminosity declines from $1.35\times10<sup>{37}$ to $1.15\times10<sup>{37}$ erg s$<sup>{-1}$ ($0.67\%$ to $0.57 L_{Edd}$). These results of the spectral-timing characterization of the 2025 mini-outburst are broadly consistent with a high-spin black hole accreting at a low, steadily declining rate in the canonical soft state.