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−1), a low and only mildly evolving hardness ratio (≈0.14-$0.16$) 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 (χ2/dof=1.04-$1.15$). The inner-disk temperature cools steadily from 1.021±0.001 to 1.002+0.003−0.002 keV while the disk normalization and hence the derived inner radius, Rin≃27.7 km ≃0.95 RISCO 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 1.35×1037 to 1.15×1037 erg s−1 (0.67% to 0.57 LEdd). 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.
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BibTeX
@misc{hussain2026nustar,
title = {{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}},
author = {Arshad Hussain and Umananda Dev Goswami},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.03624},
eprint = {2609.03624},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.03624},
}
RIS
TY - UNPB
TI - 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
AU - Arshad Hussain
AU - Umananda Dev Goswami
PY - 2026
DA - 2026/09/03
PB - arXiv
DO - 10.48550/arxiv.2609.03624
UR - https://arxiv.org/abs/2609.03624
AB - 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^−1), a low and only mildly evolving hardness ratio (≈0.14-$0.16$) 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/dof=1.04-$1.15$). The inner-disk temperature cools steadily from 1.021±0.001 to 1.002^+0.003_−0.002 keV while the disk normalization and hence the derived inner radius, R_in≃27.7 km ≃0.95 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 1.35×10^37 to 1.15×10^37 erg s^−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.
ER -