The black hole X-ray binary GS 1354-64 in transition from the hard to the soft state

ATel #17638; Chiara Salvaggio, Sara E. Motta, M. Cristina Baglio (INAF-OAB), Rob Fender (University of Oxford) on behalf of the X-KAT collaboration, Melania Del Santo Carlotta Miceli (INAF-IASF Palermo), Adam Ingram, Melissa Ewing (Newcastle University), Aru Beri (IIA Bangalore, University of Southampton)
on 29 Jan 2026; 14:23 UT
Credential Certification: Chiara Salvaggio (chiara.salvaggio@inaf.it)

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The MAXI instrument detected a new outburst of the black hole X-ray binary GS 1354-64 in December 2025 (ATel #17563). Following the X-ray trigger, we initiated both X-ray and radio follow-up observations with the Swift satellite and the MeerKAT radio telescope as part of the SwiftKAT and X-KAT programmes. The latest MeerKAT observation was taken on 2026-01-26T03:28:52 UTC. GS 1354-64 was observed for 10 minutes at a central frequency of 1.28 GHz, with a total bandwidth of 856 MHz. We used J1939-6342 for flux and bandpass calibration, and J1424-4913 for complex gain calibration. The source remains compact (ATel #17582) and is detected at a flux density of (13 ± 1.3) mJy. Preliminary analysis shows that the radio spectrum is consistent with being flat (alpha ~ -0.2).

We have been monitoring the current outburst of GS 1354-64 at near infrared (NIR) frequencies with the Robotic Eye Mount (REM) 60-cm telescope in La Silla (Cile), using the J (0.26 micron), H (0.29 micron), K (0.41 micron) filters. The last observations took place on January 27th, 2026 (MJD 61067.2). After calibrating with 2MASS stars in the field, we measure the following magnitudes: J=14.45+/-0.23 mag, H=13.53 +/- 0.08 mag, K=13.22 +/- 0.22 mag. Once reddening is taken into account (Av~2.6 mag; Koljonen et al. 2016), these magnitudes translate into the following de-reddened flux densities: 5.09 +\- 1.09 mJy, 5.93 +/- 0.45 mJy, 4.22 +/- 0.88 mJy in the J, H, K bands, respectively, consistent with a flat NIR spectrum.

The latest Swift observation was obtained in the Windowed Timing mode on 2026-01-28T08:57:57, for a total exposure of 1.5 ks. The X-ray spectrum is dominated by powerlaw emission with photon index ~2.1, and the 0.3-10 keV unabsorbed flux is ~4 E-8 erg/cm^2/s for an nH of nH = 1.4 E+22 cm^-2 (Atel #17583). The X-ray spectrum is well described by a thermal disk component plus a power-law continuum. The fit yields a photon index of ~1.5 and a disk temperature of ~1.0 keV, for an absorbing column density of nH = 1.2 E+22 cm^-2. The unabsorbed flux in the 1–10 keV band is ~1.9 E-8 erg/cm^2/s. The power density spectrum shows a clear Type-C QPO with a centroid frequency of ~3.5 Hz, typical of the intermediate state. The MAXI and Swift/BAT lightcurves combined show that the source is currently slowly softening. Compared to the previous (hard-only) outburst observed from this source in 2006, the current outburst appears significantly brighter.

The above suggests that GS 1354-64 is likely crossing the hard-intermediate state, and is approaching the hard-to-soft transition, where the launch of relativistic ejecta may be expected.

The MeerKAT radio telescope will continue monitoring the target weekly for the entire duration of the outburst. A Swift monitoring of the system is planned. REM will continue to monitor the system daily during its outburst. Multi-wavelength observations are encouraged to study the evolution of the outburst.

X-KAT is a large MeerKAT open-time programme to observe X-ray binaries in the radio band, performing weekly monitoring of bright, active systems, with capacity for higher cadence observations, and in coordination with large X-ray and optical monitoring programmes. For further information on this programme contact Rob Fender. The Swift follow-up of X-ray binaries is largely performed as part of the SwiftKAT program, which provides quasi simultaneous X-ray coverage of the X-KAT targets.