Optical confirmation of the ongoing outburst in GS 1354-64

ATel #17586; Sandeep K. Rout, David M. Russell, D. M. Bramich, Yi-Jung Yang, Mokhine Motsoaledi, Dalya Akl, Yasmin El Maoued (NYU Abu Dhabi), Kevin Alabarta, M. Cristina Baglio (INAF-OAB), Payaswini Saikia, (Yale), and Fraser Lewis (Faulkes Telescope Project & The Schools' Observatory, LJMU)
on 9 Jan 2026; 12:56 UT
Credential Certification: Sandeep Rout (sandeep.rout@nyu.edu)

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A new X-ray activity was reported in the black hole X-ray binary GS 1354-64 (BW Cir) using MAXI, with a brightening detected on 25 December 2025 (ATel #17563). Detections were also reported by MeerKAT (ATel #17582) and Swift/XRT (ATel #17583) on 04 and 07 January 2026, respectively. GS 1354-64 is a regular target with the 1m and 2m optical telescopes of the Las Cumbres Observatory (LCO) network (see e.g. ATel #7637, #7887, Koljonen et al. 2016), as part of our ongoing monitoring of ~50 LMXBs (Lewis et al. 2008).

Here, we report a steady optical rise of the source during December and into January. The first clear detection above the quiescent level was on 03 December 2026 (MJD 61012.73), about 22 days before the MAXI detection. The magnitudes on this day are g' = 20.64 +/- 0.07, r' = 21.52 +/- 0.12, i' = 19.03 +/- 0.03, and z_s = 18.53 +/- 0.02. The source was more than 1 magnitude brighter than the previous observations made during August 2025, after which it remained Sun constrained until December 2025. To constrain the date of the actual rise, we fitted the lightcurves with a constant plus linear model and found the outburst to have begun on 21 November 2025 (~MJD 61000.6) in all filters. As of the latest observation on 08 January 2026 (MJD 61048.05), the magnitudes are g' = 17.77 +/- 0.02, r' = 17.14 +/- 0.01, i' = 16.45 +/- 0.01, and z_s = 16.22 +/- 0.01. These values are near the historic peaks of the previous outbursts. We will continue to monitor the source with LCO and encourage other multi-wavelength observations.

All the obtained images are analysed with the real-time data analysis pipeline "X-ray Binary New Early Warning System" developed in-house (XB-NEWS; see Russell et al. 2019, Goodwin et al. 2020, Alabarta et al., in prep, and ATel #13451 for details).

A link to the optical light curve.