{
  "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 ($\\approx0.14$-$0.16$) 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 $1.002^{+0.003}_{-0.002}$ 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 $1.35\\times10^{37}$ to $1.15\\times10^{37}$ 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.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.03624",
  "author_total": 2,
  "authors": [
    {
      "name": "Arshad Hussain"
    },
    {
      "name": "Umananda Dev Goswami"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/e4jrq02y0j56g.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.03624",
  "fulltext_url": null,
  "id": "e4jrq02y0j56g",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.03624",
  "language": null,
  "licence": "LicenseRef-arXiv-nonexclusive-1.0",
  "licence_label": "arXiv licence",
  "licence_source": "article",
  "licence_url": "http://arxiv.org/licenses/nonexclusive-distrib/1.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.HE"
    ],
    [
      "openalex",
      "3103|Astrophysical Phenomena and Observations"
    ]
  ],
  "openalex": "W7208743588",
  "pdf_url": "https://arxiv.org/pdf/2609.03624",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-03",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/e4jrq02y0j56g.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:31Z",
      "native_id": "2609.03624",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.03624"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.HE",
      "subfield": "high-energy-astrophysics"
    }
  ],
  "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",
  "title_text": "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",
  "type": "preprint",
  "updated_at": "2026-10-30T12:18:31Z",
  "url": "https://research.draconitic.com/paper/e4jrq02y0j56g",
  "venue": "arXiv",
  "versions": []
}