NuSTAR detects hard-state decay and weak reflection in GS 1354-64

ATel #17771; Oluwashina Adegoke (Caltech), Javier Garcia (NASA/GSFC, Caltech), Fiona Harrison (Caltech), Karl Forster (Caltech)
on 1 May 2026; 04:43 UT
Credential Certification: Oluwashina Adegoke (oadegoke@caltech.edu)

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The onset of a new outburst from the black hole X-ray binary GS 1354-64 was triggered by the MAXI/GSC nova alert system on 2025 December 25 (ATel #17563). Since then, the source has been followed up extensively across multiple bands (ATel #17582, #17583, #17586, #17611, #17618, #17625, #17650, #17673, #17697, #17724). The first NuSTAR observation of the source during the current outburst was carried out on 2026 January 13 (ATel #17612). At that time, the source was in a bright hard state, revealing strong relativistic reflection features, with Gamma=1.76^+0.01_-0.02 and Ecut=49+/-1 keV. The observed 1-100 keV flux was ~1.2 E-8 erg/cm^2/s. Several additional NuSTAR observations have since been carried out through DDT requests as well as GO/ToO programs.

With the source now fading and approaching the end of its visibility window for NuSTAR, we obtained a ~15 ks NuSTAR DDT observation on 2026 April 29. The 1-100 keV source flux has decreased to ~1 E-9 erg/cm^2/s. The spectrum is hard and can be described by a powerlaw continuum with weak reflection features. Relativistic reflection modeling yields Gamma=1.75^+0.06_-0.17, while the coronal temperature remains unconstrained. These properties suggest that the source is in the low-hard state during the decay phase and is likely to return to quiescence in the near future.