XB-NEWS detects a new outburst of GX 339-4

ATel #17327; Payaswini Saikia, David M. Russell, Kevin Alabarta, D. M. Bramich, Sandeep K. Rout (NYU Abu Dhabi), M. Cristina Baglio (INAF- OAB), Thomas Russell (INAF- Palermo) and Fraser Lewis (Faulkes Telescope Project & The Schools’ Observatory, LJMU)
on 11 Aug 2025; 00:01 UT
Credential Certification: Payaswini Saikia (ps164@nyu.edu)

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GX 339–4 is a well-known black hole low-mass X-ray binary that undergoes recurrent outbursts approximately every 1-2 years. The most recent outburst prior to this report started in 2023 September (MJD 60203.43), detected in the optical with the Las Cumbres Observatory (LCO) network (ATel #16260) and subsequently confirmed in X-rays with Swift/XRT (ATel #16260), MAXI/GSC (ATel #16302), NuSTAR (ATel #16425), and AstroSat (ATel #16570). Infrared observations with REM (ATel #16460) and radio detections with MeerKAT (ATel #16421) and ATCA (ATel #16621) were also obtained.

We report the onset of a new outburst from GX 339-4, detected in our latest observations as part of an ongoing optical monitoring with the 1 m and 2 m telescopes of the LCO network. This monitoring is part of a long-term program targeting around 50 low-mass X-ray binaries (Lewis et al. 2008). LCO images are processed in real time and photometrically calibrated using the "X-ray Binary New Early Warning System" (XB-NEWS; see Russell et al. 2019 and Goodwin et al. 2020 for details).

The source has remained in quiescence since around 2025 March 1 (MJD 60735). Our latest LCO observations, obtained on 2025 August 9 (MJD 60896), yield optical magnitudes of g' = 18.28+/-0.03, V = 17.86+/-0.02, r' = 17.57+/-0.01, R = 16.99+/-0.014, and i' = 17.02+/-0.02. For comparison, the most recent LCO measurements prior to this were consistent with quiescence: on July 12 (MJD 60868), R = 18.93+/-0.05 and V = 19.69+/-0.08; and on July 16 (MJD 60872), r' = 19.36+/-0.03, g' = 20.55+/-0.13, and i' = 18.71+/-0.028. These values mark the final quiescent state available prior to the current optical rise. The recent data show a brightening of approximately 1.7-2.3 magnitudes across all bands relative to quiescence, with an average increase of about 1.9 magnitudes, confirming the onset of the outburst. We encourage coordinated multi-wavelength observations to track the rise of the outburst.

Optical light-curve of GX 339-4