NuSTAR observation of the black hole X-ray binary GS 1354-64 in a bright hard state
ATel #17612; Oluwashina K. Adegoke, Javier A. Garcia, John A. Tomsick, James F. Steiner, Fiona A. Harrison, Karl Forster
on 19 Jan 2026; 18:56 UT
Credential Certification: Oluwashina Adegoke (oadegoke@caltech.edu)
Subjects: Binary, Black Hole, Transient
The MAXI/GSC nova alert system triggered on the onset of a new outburst from the black hole X-ray binary GS 1354-64 on 2025 December 25 (ATel #17563). During the trigger observations, the 4-10 keV X-ray flux was reported to be 33+/-6 mCrab (1 sigma error). The source has since brightened rapidly, reaching 174+/-17 mCrab as of today. Follow-up observations with MeerKAT reported the detection of radio emission from the source (ATel #17582). Additional follow-up observations have been reported with Swift (ATel #17583) and the Las Cumbres Observatory (LCO) network (ATel #17586).
We obtained an approximately 30 ks NuSTAR DDT observation of the source on 2026 January 13. The NuSTAR spectrum reveals strong relativistic reflection features, including a broad Fe K emission line and a Compton hump. Fixing the hydrogen column density to NH=1.4 E+22 cm^-2 (ATel #17583), a preliminary relativistic reflection fit yields a photon index of Gamma=1.76^+0.01_-0.02 with high-energy cutoff Ecut=49+/-1 keV. This Ecut is significantly lower than previous NuSTAR measurements for the source from its 2015 outburst. The observed 1-100 keV flux is ~1.2 E-8 erg/cm^2/s. These properties are consistent with the source being in the canonical bright hard state at the time of the NuSTAR observation.
More follow-up observations are encouraged.
