NuSTAR follow up of the outburst of GRS 1739-278

ATel #17430; Honghui Liu (Uni. of Tuebingen), Jiachen Jiang (Uni. of Warwick), Adam Ingram (Newcastle Uni.), Javier Garcia (GSFC), John Tomsick (UCB), James Steiner (CfA), Andrea Santangelo (Uni. of Tuebingen), Zuobin Zhang (Uni. of Oxford)
on 3 Oct 2025; 09:11 UT
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Credential Certification: Honghui Liu (honghui.liu@uni-tuebingen.de)

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GRS 1739-278 is an X-ray binary system with a black hole candidate discovered in 1996 by the Granat satellite (Vargas et al. 1997, ApJ, 476, 23). It went into a new outburst in 2025. We triggered a NuSTAR ToO observation, which was conducted on 18 September 2025. The observation captured the source in the soft state, with its spectrum dominated by thermal disk emission. The inner disk temperature measured with the DISKBB model is approximately 0.9 keV and the normalization is around 1300. The spectrum also includes a power-law component with a photon index of ~2.8, a high-energy cutoff at ~40 keV and a normlization of around 0.39 (with CUTOFFPL). A broad iron line and a Compton hump are also present, indicating the existence of a reflection component. A quick fitting with the RELXILL model finds a similar spin and inclination angle as in Draghis et al. 2024, ApJ, 969, 40. After fitting these components, the residuals reveal an absorption line feature at 7 keV, which might indicate absorption by a disk wind. Previous Chandra/HETG observations only found an upper limit for such an absorption line (Parra et al. 2024, A&A, 681, 49), but it appears quite significant in the NuSTAR spectrum from this outburst. The 3-20 keV flux during the observation was 3.6e-9 erg/s/cm^2. Follow-up observations could help confirm the presence of the disk wind. We thank the NuSTAR team for promptly scheduling the observation.